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Laboratory studies of photochemistry and gas phase radical reaction kinetics relevant to planetary atmospheresw M. A. Blitz and P. W. Seakins Received 6th June 2012 DOI: 10.1039/c2cs35204d This review seeks to bring together a selection of recent laboratory work on gas phase photochemistry, kinetics and reaction dynamics of radical species relevant to the understanding of planetary atmospheres other than that of Earth. A majority of work focuses on the rich organic chemistry associated with photochemically initiated reactions in the upper atmospheres of the giant planets. Reactions relevant to Titan, the largest moon of Saturn and with a nitrogen/ methane dominated atmosphere, have also received much focus due to potential to explain the chemistry of Earth’s prebiotic atmosphere. Analogies are drawn between the approaches of terrestrial and non-terrestrial atmospheric chemistry.

School of Chemistry, University of Leeds, Leeds, LS2 9JT, UK. E-mail: [email protected], [email protected] w Part of the atmospheric chemistry themed issue.

The terrestrial planets (Venus, Earth and Mars) are often considered together but there are both important similarities and differences.1–4 With its significant greenhouse warming (surface temperature = 732 K), the Venusian atmosphere5 has been subject to extensive investigation, most recently from the productive Venus Express Mission,6 see for example the recent special issue on Venus in Icarus where many of the papers have been inspired by Venus Express data.7 Although similar in original composition, Venus and Earth have evolved in different ways,1 perhaps most obviously in the loss of water on Venus due to the Runaway Greenhouse Effect.8 Additionally, in contrast to Earth, chemistry in the troposphere, below the extensive cloud cover, is characterised by non-radical chemistry. Photochemistry above the clouds is driven by CO2 photolysis and photolysis of sulphur species as illustrated in Fig. 1.9,10 The coupling of sulphur and nitrogen chemistry (with links to reactions involved in combustion chemistry) may also be important (Section 3.1). Mars is another planet dominated by CO2 based chemistry, but the tenuous atmosphere, means that photochemistry dominates albeit at much lower temperatures and pressures compared to Venus.11

Dr Mark Blitz, obtained his PhD in 1990 studying the kinetics of silicon radicals at the University of Reading with Profs Monty Frey, Robin Walsh and Dr Jim Baggott. He then worked as a post-doctoral fellow at the National Research Council Canada, Ottawa, with Drs Peter Hackett and Steven Mitchell, studying the kinetics of transition metal atoms. Since coming to Leeds in 1995 he has worked on a range of kinetics/photochemistry problems, in particular, developing techniques that allow the products of reaction to be monitored. He is currently an NCAS research scientist.

Prof. Paul Seakins, obtained his DPhil from the University of Oxford with Prof. Michael Pilling, before working as an SERC/ NATO Fellow at the Joint Institute for Laboratory Astrophysics (JILA) at the University of Colorado, Boulder, with Prof. Stephen Leone. Since coming to Leeds in 1992 he has worked on a variety projects related to kinetics including combustion chemistry, atmospheric chemistry and planetary atmospheres. He has held visiting fellowships with Dr Lou Stief at NASA Goddard and Dr Geoff Tyndall at NCAR. He has been Professor of Reaction Kinetics since 2008.

1. Introduction The focus of this review is to discuss recent developments in laboratory studies of photochemistry, reaction kinetics and dynamics relevant to the atmospheres of planetary atmospheres other than that of Earth. The chemical composition of planetary atmospheres can be broken down into two regimes: equilibrium (e.g. deep in the atmospheres of gas giant planets or below the Venusian cloud layer) where the distribution of species can be calculated from thermodynamics, and non-equilibrium, where composition is determined by kinetics and photochemistry of neutral and ion species. The latter case corresponds to the Earth’s atmosphere and the main focus of this review will focus on this non-equilibrium composition, with particular emphasis on neutral chemistry. Table 1 highlights the diverse conditions present in the tropospheres and upper atmospheres of the planets of our solar system and this diversity has significantly increased with the increasing catalogue of exoplanets (Table 2).

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Table 1

Summary of conditions is the atmospheres of planets in the solar system

Planet

Troposphere

Stratosphere

Earth

O2/N2 dominated atmosphere. Photochemistry of minor constituents driven by light of l > 300 nm. Pressure 1–0.1 bar. Temperature 310–190 K N2 dominated atmosphere with CO2 and/or CH4. H2O photochemistry drives hydrocarbon chemistry CO2/N2 dominated atmosphere. Limited photochemistry below thick clouds. High surface temperatures (730 K) and pressures (90 bar) Low pressure CO2 atmosphere. Surface temperature 223 K, pressure 0.006 bar H2/He dominated atmosphere. Limited photochemistry below thick cloud layers. Low temperatures (60–170 K at 1 bar equivalent) N2/CH4 dominated atmosphere. Limited photochemistry below aerosol (tholin) layer. Surface temperature 94 K, pressure 1.5 bar

O2 and O3 photochemistry, l > 200 nm. ClOx, HOx and NOx cycles. Temperature 190–250 K

Prebiotic Earth Venus


Mars Giant planets Titan

Table 2

Significantly reduced trapping of UV radiation CO2 and sulphur photochemistry Metal atom and ion chemistry in upper atmosphere Rich hydrocarbon photochemistry in a reducing atmosphere Rich CH4/N2 coupled photochemistry. Formation of larger molecular species and aerosol production

Summary of exoplanet classifications and conditions

Type

Conditions

Hot Jupiters

Sufficiently massive to retain H2 and He in line with composition of protoplanetary nebula. In contrast to our own giant planets, temperatures can be up to 2000 K. Hot Super Earths can have temperatures up to 1500 K sufficient not only to remove primordial H2 and He, but all that in the solar system would be considered as volatiles. Atmospheres composed of silicates. Earths and Super Earths are characterised by a rocky composition. Atmospheres dominated by outgassed material, but Super Earths may be sufficient large/cold to retain hydrogen, providing a different atmosphere from Solar System terrestrial planets.

Hot Super Earths Earths and Super Earths

Fig. 2 The coupling of CO2 and HOx chemical cycles. CO is recycled back to CO2 in the top left hand corner of the plot, whereas the right hand side of the diagram describes OH generation. Reproduced and adapted from Atreya and Gu.12

Fig. 1 The complex chemistry and photochemistry of sulphur containing species relevant to the upper atmosphere of Venus. Reproduced from Zhang et al.9 (Copyright 2012, with permission from Elsevier).

For both Mars and Venus, mechanisms regenerating CO2 following photolysis to CO and O: CO2 + hn - CO + O

(P1)

must be present to maintain the observed CO2, CO and O2 concentrations as the simple recombination of CO (X 1S+) and O (3P) is spin-forbidden. A variety of catalytic mechanisms have been proposed to explain CO oxidation in the Venusian atmosphere; a mechanism involving ClC(O)O2 is discussed in Section 2.4. On Mars, with higher water vapour concentrations, it is HOx chemistry, similar to cycles in the Earth’s atmosphere that regenerate CO2 as shown below in Fig. 2 (see also Section 3.4). This journal is

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Reported observations of methane with potential implications for biological production have stimulated further investigations.13–15 Beyond Mars, the four gas-giant planets Jupiter,16 Saturn,17 Uranus18 and Neptune18,19 provide very contrasting chemistry. The high masses and low temperatures of these planets mean that they have retained atmospheres of hydrogen and helium and therefore chemistry occurs under reducing conditions. However, there are differences in the H2 : He ratio between the planets and from the solar value. Additionally Uranus and Neptune are significantly smaller than Jupiter or Saturn, with their higher densities suggesting a rocky or icy core. The giant planets have extensive families of satellites, some of which are large enough to maintain an atmosphere. The most studied of these moons is Titan, the largest moon of Saturn. Titan contains a nitrogen/methane atmosphere and therefore potential analogies with the atmosphere of pre-biotic Earth20 although there is debate as to actually how reducing the early Earth atmosphere was.21 There has been particular interest, further stimulated by results from the Cassini– Huygens mission,22 in how more complex organic molecules Chem. Soc. Rev., 2012, 41, 6318–6347

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Fig. 3 The methane–ethane cycle on Titan from Raulin et al.20 and Poch et al.31 (Copyright 2011, with permission from Elsevier).

are formed and in the formation of the tholin (from the Greek ‘tholus’ – muddy) cloud layers.23–27 Some relevant issues to the atmospheric community were addressed in a special issue of the Journal of Physical Chemistry A in 2009.28 A major question is the source of methane into Titan’s atmosphere and the fate of ethane, the primary product. Without replenishment from the surface, methane would have been removed over geological time and models predict that less volatile ethane should have built up as liquid on the surface.15,29,30 A summary of the ‘hydrological’ cycle is shown in Fig. 3 and relevant studies can be found in Sections 4.1 and 5.2. The pre-biotic atmosphere of Earth was thought to have been dominated by nitrogen and methane, but in addition there were likely to have been high concentrations of water vapour and, in the absence of a screening ozone layer, significant chemistry driven by water photolysis.32 The evolution of oxygen in the Earth’s atmosphere at approximately two billion years before present, has been reviewed by Canfield.33 However, the nature of the Earth’s pre-biotic atmosphere is a topic of some debate.21 A highly reducing N2/CH4 atmosphere, which further promotes the importance of the pioneering simulation experiments of Miller34 and Urey, may not have existed; however Zahnle et al.35 argue that both reduced and oxidised forms of carbon could have been present in the Earth’s pre-biotic atmosphere at various points. A themed issue on pre-biotic chemistry will be available shortly from Chem. Soc. Reviews. Exoplanets are a new, exciting and developing topic. Since the discovery of the first exoplanet in 199536 new exoplanets have been discovered at an increasing rate. Information on the atmospheric composition can be obtained either from direct observation or more often from transit information looking at the difference in the star spectrum as the planet passes in front and behind the star. Updates on the number and nature of exoplanets can be found on websites such as the Interactive Extra-Solar Planets Catalogue, http://exoplanet.eu/catalog.php (763 planets 23rd March 2012!) or the NASA Exoplanet Archive (http://exoplanetarchive.ipac.caltech.edu/). Various types of exoplanets37–39 have been discovered and a summary of those with atmospheres are given in Table 2. Information on ‘hot Jupiters’, large gas planets dominated by H2 and He with masses that can approach solar masses and temperatures up to 2000 K, dominate the catalogue, but potentially more interesting are the Super Earth and Earth type planets. These are rocky planets with secondary atmospheres, but with Super Earths, their larger gravitational 6320

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attraction means that they may retain significant hydrogen, either as molecular hydrogen or derived forms. Unfortunately the composition of Earth type planets are harder to determine and theory and models are likely to lead observations for some time. Observed molecules listed on the Extra-Solar Planets Catalogue are: CH4, CO, CO2, H, H2, H2O, K, Na, TiO and VO. The latter two compounds may be important in producing a temperature inversion in the upper atmosphere of hot Jupiter planets,40,41 although photochemical haze may be another explanation.42 As with the planets in our Solar System, the chemistry of the atmosphere can be controlled by equilibria, or for thinner atmospheres or regions of atmospheres, by photochemistry. Part of the driving force for the study of exoplanets is the discovery of habitable planets.43 Currently such planets are not yet observable, but much work has gone into the observation of Earthshine as a template for signatures of habitability and life.44 The discovery of hot Jupiters and Super Earth type planets greatly extends the range of conditions under which experimental data are required as has been demonstrated in detailed chemical modelling studies.45 Fig. 4 compares the physical conditions of the giant planets of our own solar system with hot-Jupiters and a brown dwarf star. The conditions are quite different in the exoplanets with water being in the vapour phase, CO rather than methane being the stable form of carbon and at the highest temperatures vapourization of silicates and even iron. As with the Earth’s atmosphere our understanding of the atmospheric chemistry of other planetary atmospheres is based on observation of composition and the development of chemical models of varying complexity which may be coupled with planetary circulation models. Clearly observations of other planetary atmospheres are considerably less comprehensive than those of our own atmosphere and there is

Fig. 4 Temperature vs. pressure for the atmospheres of Saturn, Jupiter and the hot Jupiter planet HD189733b. The dotted lines represent the condensation curves of ice, MgSiO3 and Fe (with increasing temperature) and the dashed line represents the equilibrium between CO and CH4 (with higher temperatures favouring CO) emphasising the potentially unusual atmospheres of bodies such as hot Jupiters and brown dwarves (GI229b). Figure with permission taken from Baraffe et al.37 (Copyright 2010, IOP Publishing).

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therefore much greater uncertainty in our understanding. As with terrestrial models, the inputs include photochemistry, reaction kinetics and partitioning of molecules into the aerosol phase. The focus of this review is on laboratory studies that have contributed to these inputs, concentrating primarily on photochemistry and neutral species reaction kinetics, and how such studies contrast with their terrestrial counterparts. We begin our review with a consideration of photochemistry focusing primarily on methane photochemistry. Despite extensive study, many uncertainties still exist in the primary processes at important wavelengths. In Section 3 we consider kinetic studies of radical species, contrasting methods of radical generation and experimental conditions with those for terrestrial atmospheric chemistry and considering in more detail the study of radical–radical reactions and some metal ion chemistry. Knowledge of the products of chemical reactions is as important (if not more important) as determining the overall rate coefficient and in Sections 4 and 6 we consider product measurement under ‘bulb’ and ‘molecular beam’ conditions. Generation of the low temperatures relevant for many planetary atmospheres is vital and Section 5 describes the Laval expansion method for generating low temperatures in what is effectively a wall-less flow tube. This methodology has generated a large amount of kinetic data and further advances in experimental design is at last allowing for product study. Theoretical study, simulations and modelling are important components of terrestrial atmospheric chemistry and in Section 7, we briefly consider particular issues in the application of these techniques to other planetary conditions.

2. Photochemistry As with the Earth’s atmosphere, photochemical studies play an important role in laboratory studies of other planetary atmospheres. Photochemistry is of limited importance in the lower Venusian atmosphere. Below the cloud tops only photolysis of higher sulphur species (Sx, where x > 3) is relevant,5,46 but above the clouds photolysis is more pronounced and SO2

photolysis (Section 2.3) is important. On Mars, photochemical processes are dominated by the well established photolysis of CO2 to CO and O at l o 204 nm and the interest is in mechanisms to catalyse the reformation of CO2 to explain the observed levels of CO and CO2. Photochemistry for the outer planets and Titan occurs in the stratosphere and, in the absence of oxygen, it is the photolysis of hydrocarbons, particularly methane, that is the driving force of atmospheric photochemistry. Hydrocarbon photolysis poses significant challenges for laboratory studies requiring the generation of VUV wavelengths (particularly Lyman-a, 121.6 nm); such high energy photons mean that many product channels, often involving reactive species, and hence complex secondary chemistry, can be present. In this section of the review we focus on methane and ammonia photolysis as important case studies which illustrate developments in instrumentation and approaches. CO2 and SO2 photolysis are also considered and we conclude this section with a brief discussion on the spectroscopy and UV cross section of ClC(O)O2, an important intermediate in CO oxidation processes. 2.1 Methane photolysis A recent study by Gans et al.47 on methane photolysis illustrates many of the issues associated with hydrocarbon photolysis. Methane photolysis is the primary source of radical chemistry and the production of higher hydrocarbons in the atmospheres of Titan and the giant planets. The importance of the process is illustrated in a number of reviews; examples include Wilson and Atreya48 and Romanzin et al.49 The sensitivity of radical concentrations and hydrocarbon products on methane photolysis, particularly the branching ratios between the various products, has been investigated by several authors with reference to Titan48–50 and the giant planets.51,52 An example is shown in Fig. 5 of the sensitivity of methyl radical concentrations at different altitudes in the Jovian atmosphere to the photolysis rate and branching ratio.

Fig. 5 Photolysis sensitivities for methyl radical concentrations (d(ln[CH3])/d(ln k)). CH2* refers to the first excited singlet state of methylene, CH2 (a 1A1). Reproduced from Smith and Nash.51 (Copyright 2006, with permission from Elsevier).

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The main channels from Lyman-a photolysis are thought to be: CH4 + hv - CH3 + H 4.47 eV

(P2)

- CH2 (a 1A1) + H2 5.01 eV

(P3)

- CH2 (X 3B1) + 2H 9.14 eV

(P4)

- CH2 (a 1A1) + 2H 9.53 eV

(P5)

- CH (X P) + H + H2 9.06 eV

(P6)

2

The importance of Lyman-a methane photolysis has long been recognised and there have been several previous studies discussed in Gans et al. A number of studies have focused on measuring the quantum yield for H atom production, inferring the nature of the co-product from the velocity distribution of the H atoms. However, the complexity of the system is illustrated by the fact that H atom quantum yields range from 1.0 0.553 to 0.31 0.05.54 Gans et al. utilized mass spectrometry with VUV photoionization to directly detect the CH3 and CH2 products, however, such studies require careful determination of ionization cross sections and investigations of whether the internal excitation of the fragments alter the efficiency of the photoionization process (not a complication for the H atom velocity technique). Gans et al. used VUV radiation at 121.6 (generated by tripling 364.8 nm radiation from a Nd:YAG pumped dye laser in Kr–Ar mixture) and 118.2 nm (tripling the 355 nm tripled output of a Nd:YAG laser in xenon) to both photolyse CH4 and to photoionize the methyl and methylene fragments. Fig. 6 shows the relative CH3 and CH2 peaks for the various pump and probe wavelengths. Combining the measured CH3 : CH2 ratios with carefully determined photoionization cross sections and elegant, but indirect, measurements of the CH channel from Rebbert and Ausloos,55 allowed Gans et al. to extract quantum yields for the various channels at 121.6 and 118.2 nm. Values from this work and other literature data are summarized in Table 3. A surprising observation is that the product distributions at 121.6 and 118.2 are dramatically different despite the small

difference in excitation energy (10.2 vs. 10.5 eV) and the unstructured absorption spectrum in this region. Gans et al. comment on this, noting that there is no quantitative explanation for the difference, but the absorption cross sections in this region are complicated by Jahn–Teller distortions in the excited state.56 Support for the variation in product distribution with photolysis wavelength comes from an earlier comprehensive study on methane photolysis by Wang et al.57 The H : D ratio from the photolysis of CH3D, CH2D2 and CHD3 at 121.6 nm was consistent at B1.5 for each isotopologue once normalized for the H : D ratio in the parent methane. This expected result contrasts distinctly with the H : D ratio following photolysis at 118.2 nm where the normalized H : D ratio increases from 0.94, 1.68 and 3.03 for CH3D, CH2D2 and CHD3. Note however, that whilst both Wang et al. and Gans et al. suggest that there are significant differences in the mechanism of methane photolysis between 121.6 and 118.2, the earlier work of Wang et al. does not support the production of a significant yield of ground state methylene (3CH2) as suggested by Gans et al. (Table 3, column 2). Wang et al. show that when CH4 is photolysed at 118 nm in the presence of excess nitrogen a significant ethene signal is observed due to rapid relaxation of 1 CH2 by nitrogen,58 followed by 3CH2 recombination. However, replacing N2 with H2 removes the ethene signal as 1CH2 reacts efficiently with hydrogen to produce CH3 + H followed by CH3 recombination to produce ethane.59 Theoretical studies emphasise the complexity of methane photolysis with dissociations occurring via internal conversions and intersystem crossings.60,61 Lodriguito et al.60 calculated branching ratios (Table 3) which confirm CH3 + H and 1 CH2 + H2 as the major products. Lodriguito et al. noted that approximately 5% of trajectories occur via a ‘roaming atom mechanism’ where the H atom moves to long separations, subsequently abstracting a hydrogen atom to form the final products. Roaming processes62 have been investigated extensively in dynamical and combustion related studies and are of relevance in the photolysis of atmospherically relevant species such as acetaldehyde.63 Most experimental studies agree that CH is a small but significant product in the photolysis, unfortunately there are no direct determinations of CH, with the quantum yield for channel (5) being determined by end-product analysis55,64 or inferred from the H atom velocity distributions. Considering the importance of CH chemistry (see Sections 4.1 and 6.1), a direct observation of CH from methane photolysis would be very helpful. Gans et al. emphasise the importance of carrying out photolysis studies at a range of wavelengths as product distributions can change significantly and, as with the Earth’s atmosphere, relatively long wavelength radiation penetrates more efficiently to lower levels. Synchrotron radiation should be ideal for wavelength dependent studies. 2.2 Ammonia photolysis

Fig. 6 Mass spectra obtained for the different photolysis–ionization combinations. The signals have been normalized to unity for CH3+. Reproduced from ref. 46 with permission from the PCCP Owner Societies.

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A study by Cheng et al.67 on ammonia photolysis illustrates the advantages of synchrotron photolysis. Ammonia has a very structured spectrum in its first two electronic bands (210–170 nm and 160–140 nm). Ammonia is an important source of atomic hydrogen in the lower stratosphere and upper This journal is

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Table 3

Summary of methane photolysis studies Mordaunt et al.53

Heck et al.65

Brownsword et al.66

Ref.

Gans et al.47

Gans et al.47

Park et al.54

Method

Direct determination of CH2 and CH3

Direct determination of CH2 and CH3

ToF H atom kinetic energy spectroscopy

Photofragment imaging

Date l/nm

2011 118.2

2011 121.6

Simultaneous photolysis and detection of H atoms by LIF 2008 121.6

1993 121.6

2000 118.2 and 121.6

CH3 + H CH2 (a 1A1) + H2 CH2 (X 3B1) + 2H CH + H + H2 Total H Total H2

0.26 0.04a 0.17 0.05 0.48 0.06

0.42 0.05a 0.48 0.05 0.03 0.08

0.31 0.05c 0.69c —

0.49 0 0

1996 121.6 nm H atom 105–115 nm H2 0.66 — 0.22 — — —

0.09b 1.31 0.13 0.26 0.05

0.07b 0.55 0.17 0.55 0.05

— 0.31 0.05 0.69c

0.51 1.0 0.5 0.51

0.11

0.068 0.013 0.47 0.10 0.65 0.10

Photolysis and H atom detection (vuvLIF) at Lyman a 1997 121.6

— 0.47 0.11

Wang et al.57

Lodriguito et al.60

Determination Trajectory of H and calculations molecular products 2009 121.6

0.291 0.068 0.39 0.03 0.585 0.098 0.50 0.06e 0.066 0.012d 0.10 0.02f 0.015 0.008 0.60 0.10 0.51 0.06

a Error is 1s. b From Rebbert and Ausloos.55 c H atom yield attributed to CH3 + H, assumed that (a 1A1)CH2 + H2 is the only other significant channel. d A majority of the CH2 formed is thought to be in the first excited state. e CH2 calculated to be in both the first and second excited singlet states. f CH2 dominated by ground state triplet state.

troposphere of the giant planets (note that whilst still in the VUV, photolysis occurs at longer wavelengths than for hydrocarbons).68 Cheng et al. used the national synchrotron facility in Taiwan (see also Section 6.2) to determine the absorption cross sections of all the isotopologues of ammonia at a resolution of 0.02 nm and examples of transitions in the B ’ X band are shown in Fig. 7. Note the significant decrease in absorption cross section with increasing isotopic substitution and the additional peaks in the NH2D and NHD2 spectra due to the degradation of the symmetry to Cs. Differences in cross section with isotopic substitution are important as modelling has shown that the isotopic ratios of ammonia are useful tracers of the meteorology of the Jovian troposphere.69 An example of how temperature dependent cross sections can be obtained is given in Cheng et al.70 where they have coupled the synchrotron to a jet expansion system, determining the absorption cross sections of acetylene at 298 and 85 K allowing investigation of the temperature dependence of absorption cross sections with the flexibility and resolution of the synchrotron source. 2.3

CO2 and SO2 VUV absorption

The importance of CO2 photolysis in the atmospheric chemistry of Venus and Mars was introduced in Section 1. CO2 absorption cross sections are considered to be well determined above 120 nm (see for example Parkinson et al.71), but crosssections at lower wavelengths relevant to atmospheric VUV airglow features and planetary models are less certain. A particular issue is the variation in cross section with temperature. Stark et al.72 have measured the VUV cross section of CO2 at high resolution (0.005 nm) using synchrotron radiation from the Photon Factory synchrotron facility in Tsukuba, Japan. The relatively small size of the absorption cell (12 cm length) meant that it could be immersed in a dry ice–ethanol slush bath, allowing determination of cross-sections at 195 K. As can be seen from Fig. 8, absorption cross sections vary significantly over the observed wavelength range of 106–119 nm This journal is

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Fig. 7 Absorption cross sections (Mb = 1 1018 cm2) of ammonia isotopologues between 165 and 220 nm.67 Reproduced with permission.

and therefore varying concentrations of CO2 were used in different spectral regions. In each region measurements were made at several total pressures of CO2. The log scale of Fig. 8 clearly demonstrates the significant changes in absorption cross-sections in the 112–119 nm region Chem. Soc. Rev., 2012, 41, 6318–6347

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Fig. 8 Comparison of absorption cross-sections at 295 K (solid line) and 195 K (dotted line). Reproduced from Stark et al.72 (Copyright 2007, with permission from Elsevier).

of the spectrum. The temperature dependence of the shorter wavelength region was less pronounced. SO2 photolysis is important in the upper atmosphere of Venus and in the sulphur dominated chemistry of Io.73 The UV and VUV spectra of SO2 have been examined in detail in a series of papers involving a collaboration between Imperial College and Wellesley College.74–77 In contrast to the high resolution CO2 data described above, these studies have used UV Fourier Transform spectroscopy to obtain the appropriate resolution.78,79 Rufus et al.75 emphasise the importance of obtaining high resolution spectra for accurate determination of absorption cross-sections for species such as SO2 where the spectra are rotationally resolved; low resolution spectra tend to underestimate cross sections at narrow line centres and overestimate in the wing regions. As with the CO2 studies, spectra have been obtained over a range of temperatures o300 K to facilitate application to cold environments such as Io or the upper Venusian atmosphere. The temperature dependence of CO2 and SO2 cross sections is significant, however, Wang et al. report virtually no difference in the VUV absorption spectra of CH4 and CH3D between room temperature and 75 K.57 2.4 IR spectra and UV cross-sections for the ClC(O)O2 radical CO2 photolysis in the upper atmosphere of Venus would lead to much higher CO : CO2 than are observed without some mechanism for the oxidation of CO. A variety of mechanisms have been proposed included coupled mechanisms with sulphur chemistry.80 An important intermediate in several cycles is ClC(O)O2, e.g. Cl + CO + M - ClCO + M

(R1)

ClCO + O2 + M - ClC(O)O2 + M

(R2)

ClC(O)O2 + Cl - CO2 + ClO + Cl

(R3)

ClO + O - Cl + O2

(R4)

NET:

CO + O - CO2

In sulphur coupled schemes ClO can be converted back to Cl via the reaction with SO: SO + ClO - SO2 + Cl 6324

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(R5)

Fig. 9 IR spectra of thermolysis products of ClC(O)O2NO2 at various temperatures, trapped in an argon matrix. The expected thermal threshold for ClC(O)O2 decomposition is around 200 1C consistent with signatures of ClC(O)O2 decreasing at higher temperatures and the appearance of ClCO. Reproduced with permission from PNAS from Pernice et al.82

This radical–radical reaction was studied by Brunning and Stief81 using techniques described in Section 3.4. Prior to 2004, catalytic cycles such as those outline above had been postulated without any experimental evidence for the ClC(O)O2 intermediate, however, in 2004 Pernice et al.82 published a matrix isolation study following the thermolysis of ClC(O)O2NO2: ClC(O)O2NO2 - ClC(O)O2 + NO2

(R6)

Fig. 9 shows the IR spectra following the thermolysis. Spectral features associated with ClC(O)O2 decrease with increasing thermolysis temperature giving information on the thermal stability of the radical. Comparison of the modelled O2 column with ClC(O)O2 kinetics provided much better agreement with observation.82 In addition UV cross sections were obtained; these are important as photolysis potentially provides an alternative method for CO2 generation: ClC(O)O2 + hn - ClO + CO2

(P7)

However, the measured UV cross sections provide little change in model outputs for the O2 column.82

3. Basic radical kinetics studies 3.1 Discharge flow and flash photolysis As with the Earth’s atmosphere, much of the important gas phase chemistry of other planetary atmospheres is driven by solar produced, radical chemistry. The reactions of radicals with many molecules have been shown to have negative temperature dependencies83 and hence, despite the low temperatures of the outer planets, radical–molecule, as well as radical–radical chemistry is important. Discharge flow systems and laser flash photolysis coupled to a variety of detection techniques have provided much of the kinetic and product branching data relevant for planetary atmospheres as they have for the Earth’s atmosphere. In flash photolysis the radical precursor, substrate and bath gas are premixed and flowed into the reaction cell. A short This journal is

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pulse of light, usually from a laser, dissociates the radical precursor generating a homogeneous distribution of radicals with an excess concentration of substrate reagent. Under these conditions the radical decays via pseudo-first-order kinetics and a variety of techniques can be used to follow the exponential decay of the reagent, extracting the pseudofirst-order rate coefficient, k 0 , where k 0 is the product of the bimolecular rate coefficient and the concentration of the reagent in excess (k 0 = kbi[reagent]). The experiment is then repeated with varying substrate reagent concentrations, and the bimolecular rate coefficient is extracted from a plot of k 0 vs. [reagent]. In a discharge flow study (see Fig. 14) a steady state of radicals is generated from the microwave discharge of a suitable precursor. An excess of the stable reagent is introduced through a moveable injector allowing variation of the contact time between the point of mixing and detection. The position of the flow tube is changed to build up the radical profile as a function of distance from the detector (which can be converted to time from the measured linear flow rate). Analysis of such a plot yields the pseudofirst-order rate coefficient and the bimolecular rate coefficient is again obtained by varying the concentration of the stable reagent. Major advantages of the discharge flow system are the flexibility of radical generation (see below) and that the detector does not need to monitor changes in concentration in real time. Conventional flow tubes rely on low pressures to ensure rapid mixing at the injection point, but more recently high pressure turbulent flow systems have become common.84 Mixing times are relatively long, limiting the value of k 0 that can be determined. In flash photolysis, the mixing time is effectively the laser pulse duration, and reactions can be studied over a wide range of pressures. Both methods are subject to different potential systematic errors and are suitable for different detection systems and therefore are complementary in many ways. Mass spectrometric detection of reagents and/or products is an ideal technique to monitor reactions. Mass spectrometry is potentially a universal detection system and observation of both reagents and products open up the potential for easily quantifying branching ratios of multi-channel reactions (Section 4). The major disadvantage of the method is the need to physically sample the analyte from the reactor introducing a delay time. Additionally the low operating pressures of mass spectrometers can limit the reactor pressure. For flow tubes any loss of time resolution is irrelevant. Laser flash photolysis can be coupled with mass spectrometric sampling; Gutman and co-workers were amongst the first to demonstrate the utility and flexibility of this method.85 A uniform concentration of radicals is generated along a flow tube by photolysis along the axis of the tube. The reaction mixture is sampled through a pinhole at the side of the reactor, photoionized by discharge resonance lamps and the selected species monitored via a quadrupole mass spectrometer. Examples of work on radical–radical reactions are described below in Section 3.4. A development of the technique was to couple the flow tube to a time-of-flight mass spectrometer to obtain a range of masses86 and more recently tuneable VUV laser photoionization has This journal is

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Fig. 10 Schematic of the University of Leeds vuv photoionization apparatus to study kinetic and photochemical processes. Figure reproduced with permission from Baeza-Romero et al.88

been coupled to the flow tube giving much greater ionization efficiency and the potential to vary the ionization energy to provide greater selectivity.87 A schematic of the apparatus is shown in Fig. 10. The time taken for radicals to be sampled has limited the (pseudo-)first-order rate coefficient for study to o1000 s1. Recently Baeza-Romero et al.88 demonstrated that the sampling time can be accounted for and allowing the study of much faster (pseudo-)first-order rate coefficients. The advantage is that higher radical concentrations can be used with pseudo-first-order conditions maintained. Fig. 11 shows the resultant biexponential profile of SO in the reaction of SO with NO2: SO + NO2 - SO2 + NO

(R7)

Reaction (7) has been shown by Krasnopolsky to be relevant in the sulphur chemistry of the Venusian atmosphere.89 The use of synchrotron radiation to photoionize reagents or products provides significantly greater selectivity.90 As demonstrated in Section 4.1, tuneable synchrotron radiation can be

Fig. 11 Time profile of SO in the reaction of SO + NO2. SO is generated from laser flash photolysis of Cl2SO at 248 nm. Figure reproduced with permission from Baeza-Romero et al.88

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used to detect different product isomers providing site selective information. The major challenges for kinetic studies relevant to planetary atmospheres is generation of the predominantly hydrocarbon based radicals (Section 3.2) and the production of low temperatures (Section 3.3). The Laval nozzle technique, an alternative method for generating low temperatures is described in a separate section. Particular issues around radical–radical reactions are discussed in Section 3.4 and we conclude Section 3 with a brief discussion of metal atom and ion chemistry in Section 3.5. 3.2

Radical production

Table 4 lists some of the common radicals and possible precursors using laser flash photolysis. The generation of highly endothermic radicals such as CH2 and CH requires short wavelength photolysis and these processes can often be non-specific resulting in the generation of several radicals in the photolysis pulse. If one is only interested in the removal rate of the radicals then the production of multiple radical species is not usually an issue unless the target radicals are generated from any secondary chemistry of the photolysis co-products. An additional problem can be the generation of species in electronically or vibrationally excited states; for example both the singlet ground state and first excited triplet state of C2 are formed in the multiphoton dissociation of halogenated ethenes.95,96 The addition of molecular oxygen rapidly quenches the excited state preventing any interference when monitoring the kinetics of ground state radicals. Flow methods provide alternative methods of radical generation. Micro-wave discharge dissociation of triatomic and polyatomic molecules generally produces a wide range of radicals. Many hydrocarbon species, for example CH3 and C2H3 are be generated by the reaction of F atoms (produced from a micro-wave discharge of F2, a clean process) with methane or ethene.109 However production of highly endothermic species such as CH requires use of metal atom stripping (Na or K)110,111 and careful control of conditions to ensure clean radical production. K + CHBr3- KBr + CHBr2

(R8)

K + CHBr2 - KBr + CHBr

(R9)

K + CHBr - KBr + CH

(R10)

A schematic of a metal atom source is shown in Fig. 12.110 Such sources are also relevant for the direct study of the chemistry of metal atoms112 produced via meteoric ablation in the upper atmospheres of the planets (see Section 3.5).113

Table 4 Radical CH 3 CH2 1 CH2 NH2

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Fig. 12 Metal atom source for the production of CH radicals.110 Reprinted with permission from ref. 110. Copyright 1998 American Chemical Society.

3.3 Temperature control The importance of measuring rate coefficients at the appropriate temperatures for atmospheric models has been discussed by Hebrard et al.114 Although reactions may have been studied in detail at room temperature, lack of knowledge of the temperature dependence results in major uncertainties at the relevant temperatures and a list of key reactions for study at temperatures relevant to Titan was produced. Flash photolysis with laser induced fluorescence probing has significant advantages for temperature dependent measurements as the temperature only needs to be defined and determined at the point at which the photolysis and probe lasers overlap. Smith and co-workers have used this technique to measure reactions close to 80 K using liquid nitrogen.115 In later experiments on relaxation of CH(v = 1) by N2 and CO results using the cryogenically cooled system were in good agreement with that obtained using the CRESU Laval system.116 Cooled nitrogen gas was used by Berman and Lin to obtain 160 K.91 A major limitation to such measurements is the vapour pressure of the radical precursor and molecular species. Even at higher temperatures (say 195 K obtained using acetone–dry ice), partial condensation of the species onto the walls of the reactor can be a problem, so that the concentration of the molecular species is less than would be calculated from the measured input flow rates. Ideally the concentration of the molecular species should be measured using an in situ technique. Of course such issues are not limited to low temperatures; molecular species with low vapour pressures, such as amines, can be lost at surfaces at ambient temperatures.117,118 Concentration limitations related to vapour pressures are much less severe in the Laval process described in the following section. High temperature studies are relevant for the Venusian atmosphere and the ever increasing number of ‘hot Jupiter’ exoplanets (see Section 1) that have been discovered, suggest that there will be an increased need for high temperature

Selected examples of photolysis sources for radicals relevant to planetary atmospheres Precursor (wavelength/nm) 91

92–94

CHBr3 (266), (248) H2CCO (351)98,99 H2CCO (308)102 NH3 (193)106

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Radical

Precursor (wavelength/nm)

C2 C2H CN CH3

C2Br4 (266)95 C2Cl4 (193)96 (266)97 C2H2 (193)100,101 ICN (248)103 (266)104 BrCN, (CN)2105 (CH)2CO (193)107 (248)108

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measurements to model these environments. High temperature measurements with flash photolysis are relatively straight forward up to B600 K with a Pyrex/quartz cell and to 1000 K with metal cells. Above 1000 K, more care is required with cell construction (materials, cooling for window flanges), temperature measurement and high flow rates are required to prevent significant thermal decomposition of molecular species. The work of Fontijn provides good examples of high temperature studies.119 When absorption spectroscopy is used to probe the reaction more care needs to be taken in ensuring temperature uniformity. Absorption methods are typically less sensitive than fluorescence and to monitor low concentrations reagents and/or products need to be monitored over a long pathlength and the temperature needs to be uniform over this region. Taatjes and co-workers120 developed an elegant method for combining multipass IR absorption methods with laser flash photolysis for high temperature studies and the method has been adapted by other groups to study reactions at sub-ambient conditions.121,122 The central portions of a pair of Herriot cell mirrors are drilled out to allow photolysis light to pass through the Herriot cell. IR probe light is introduced through a second hole close to the edge of one mirror and exits through the second hole of the other mirror. As can be seen in Fig. 13, in a Herriot cell the probe light traces out a pattern of spots around the edge of the mirrors, crossing the photolysis region only in central portion of the reaction cell. Therefore it is only this central portion of the cell that needs to be maintained at a constant temperature and the end flanges, holding the Herriot mirrors and cell windows can be kept close to ambient temperature. Flow tube measurements, whether radicals are generated by continuous methods (m-wave discharge, metal atom stripping)

or by photolysis (coupling flowtubes and pulsed laser photolysis has combined the advantages of flash photolysis and mass spectrometric detection), pose greater challenges for temperature dependent measurements as uniform temperatures have to be maintained along the reactive length of the flow tube. Jacketed flow tubes can be used for sub-ambient measurements and for temperatures up to B350 K and with efficient circulation temperature gradients along the tube are minimal, but it is difficult to avoid temperature gradients at the interface of the flow tube with the detection system. Higher temperatures can be achieved via resistive heaters wrapped around the flow tube, however, this needs careful design and implementation to ensure uniform temperatures. 3.4 Radical–radical reactions Radical–radical reactions are important in the growth of higher molecular weight hydrocarbons in the outer planets, removing radicals in the termination steps of chain reactions. Radical–radical reactions such as: CH3 + H + M - CH4 + M

(R11)

CH3 + CH3 + M - C2H6 + M

(R12)

have received extensive study as prototypical recombination reactions.123–126 The Infrared Space Observatory recorded much lower column abundances of CH3 than predicted by modelling; an under estimation of k12 in the models at the appropriate temperature has been suggested as one explanation.127,128 Fig. 14 shows a schematic of a flow tube apparatus set up to monitor reaction (R12) with mass spectrometric detection of methyl radicals.

Fig. 13 Schematic of how pulsed photolysis light (pump beam in the figure) and IR probe light interact in the apparatus developed by Taatjes and co-workers.120 Note that the interaction region is in the central portion of the reaction cell, where temperatures can be easily controlled. Reproduced with permission from ref. 120.

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Fig. 14 Schematic of a discharge flow apparatus used by Stief and co-workers to study radical reactions. In this diagram reproduced from Cody et al.,127 the apparatus is configured to study CH3 reactions, with methyl radicals being generated from the reaction of F + CH4.

Recombination reactions such as methyl recombination are relatively straightforward to study as only a single radical needs to be generated. Of course the radical decay follows second-order kinetics and therefore absolute concentrations are required to extract the rate coefficient. For atom radical studies two radicals need to be generated, but by having the atomic species in excess (recombination of atomic species is usually slow at low and moderate pressures and therefore the concentration remains approximately constant), the molecular reagent reacts under pseudo-first-order conditions. Reactive species such as Cl and F atoms can be used to generate both species, e.g. F + D2 - DF + D

(R13)

F + CH4 - HF + CH3

(R14)

Analysis becomes more complex when looking at the reactions of two molecular radicals as both species need to be monitored; mass spectrometry is the ideal technique here and Stief and co-workers have studied a range of radical–radical reactions including Cl + CH3,129 H + C2H5,130 CH3 + CH3,127,131 CH3 + C2H3,109 C2H3 + C2H3,132 H + C2H3,133 N + C2H5,134 N + C2H3135 and N + CH3136 using discharge flow techniques coupled with mass spectrometric detection. The reaction of C2H3 with CH3 illustrates many of the issues.109 C2H3 + CH3 - C3H6

(R15a)

- CH4 + C2H2

(R15b)

- C3H5 + H

(R15c)

Reagent radicals were generated by the reaction of F atoms produced upstream in a microwave discharge with ethene and methane. The fast titration reaction of F with Cl2, monitoring the reduction in the Cl2 was used to determine absolute concentrations. Concentrations of the stable species were 6328

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Fig. 15 Plot of [C2H3] vs. reaction time at 298 K and 1 Torr (He). The solid line is the result of the numerical fit with k15 set at 1.1 1010 cm3 molecule1 s1. Taken from Thorn et al.109 (Copyright 2000, with permission John Wiley and Sons).

arranged such at CH3 was in excess, but the decay of the vinyl radical is non exponential as illustrated in the curved log-plot in Fig. 15. Numerical modelling was required in order to account for variations in the methyl radical concentration and secondary reactions of the vinyl species and allow extraction of the bimolecular rate coefficient. A range of products can be formed depending whether the reaction occurs via recombination (R15a), disproportionation (R15b) or recombination/fragmentation (R15c). The stable products propene and acetylene were observed confirming that reactions (15a) and (15b) take place. Allyl, C3H5, radicals were also observed, but could originate from propene fragmentation in the mass spectrometer, however, the observation of a signal at m/z equivalent to C4H8, suggests that recombination/fragmentation to allyl + H (R15c) is significant, with C4H8 being formed in the secondary reaction of allyl with the excess methyl radicals. This journal is

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A range of radical–radical126,137–139 reactions have also been studied by Gutman, Slagle and Knazyev at Catholic University, Washington DC. The focus of their work has tended to be more on combustion and pyrolysis applications, however, such results will be directly relevant for hot-Jupiter environments. In contrast to the work of Stief, radicals are generated via laser flash photolysis as opposed to discharge. Complementary methods of radical generation are important in identifying potential systematic errors associated with the method of radical generation. Once again mass spectrometry is used, but with photoionization giving some selectivity and less fragmentation at the expense of less flexibility (different ionization lamps are required to observe different species) compared to electron impact ionization. Radical–radical reactions also play a role in the catalytic recombination of CO and O as illustrated in Fig. 2 (Section 1). The reaction of OH with CO regenerates CO2 (R16) and the OH is generated from a number of routes including HO2 recombination, followed by photolysis of hydrogen peroxide (R17, P8) or the reaction of O atoms with HO2 (R18).140 Reaction (18) is also relevant for the generation of OH planetary airglow.141,142 OH + CO - H + CO2

(R16)

HO2 + HO2 + M - H2O2 + O2 + M (R17) H2O2 + hn - 2OH HO2 + O - OH + O2

(P8) (R18)

Studies of HO2 chemistry and particular recombination have been facilitated by the development of absorption techniques in the near infrared, monitoring HO2 either by cavity ring down spectroscopy143 or conventional multipass absorption spectroscopy.122 An important issue in studies of reaction (17) is the role that other species such as water and methanol (often the source of HO2) can play in facilitating HO2 recombination.122 Reaction (17) can occur via two mechanisms, a third body dependent, initial recombination of HO2 or a second order direct production. For application at the very low pressures appropriate to the Martian atmosphere it is important that the relative contributions of the two mechanisms to the experimental data can be quantified. However, obtaining experimental data at low pressures can be challenging; for example photolytically generated radicals may not be fully thermalized. Master equation models, discussed in Section 7.1 are useful tools to extrapolate pressure dependent laboratory Table 5 Metal Na, K Mg+ Ca Ca+ Mn a

data to conditions of temperature and pressure relevant for planetary models. 3.5 Metal atom chemistry Metal atom and ion chemistry plays important roles in the upper atmospheres of all the planets. Ablation of meteoritic materials introduces large quantities of iron, calcium, magnesium and sodium into the upper atmospheres. Laboratory studies of metal atom, ion and metal compounds can be carried out with essentially the same apparatus that is used in studies of non-metallic chemistry. A major issue is the generation of the reagent species. Table 5 summarizes some of the methods that have been used in kinetic studies. Sodium and potassium can be generated by simple heating for flow tube studies. More refractory metals or metals for flash photolysis studies need to be generated by more complex means. This can either involve laser ablation from solid samples (the sample is generally rotated to ensure a fresh sample is available for each pulse) or the photolysis of metal containing molecules, such as metal halides or organometallic species, that have been introduced into the gas phase. Metal atom (ion) concentrations can be followed by laser induced fluorescence in both flow tube and flash photolysis or by mass spectrometry in flow systems. Examples of such techniques can be found in a recent publication by Whalley and Plane on the longevity of magnesium ion layers in the upper Martian atmosphere.148 Magnesium ions formed in the ablation of meteoric material at B80 km react with CO2 to form the carbonate ion: Mg+ + CO2 + M - Mg+CO2 + M

(R19)

Mg+CO2, as opposed to Mg+, can react rapidly with electrons through a dissociative recombination process with loss of ionic species, however the observation of long-lived sporadic ion layers suggests that an alternative process must be occurring which rapidly recycles Mg+CO2 back to Mg+. Reaction (19) was studied by laser flash photolysis with Mg+ being generated from the 248 nm laser flash photolysis of MgAcAc (Mg(C5H7O5)2, magnesium acetyl acetonate) and Mg+ removal monitored by laser induced fluorescence in the presence of excess CO2 and helium bath gas. At high concentrations of CO2, the greater efficiency of CO2 over helium means that the bimolecular plots are no longer linear, but such plots potentially provide useful information as CO2 will be the primary third body in the Martian atmosphere.

Examples of metal atom or ion sources for kinetic studies Method a 144,145

Photolysis of metal halide 193 nm photolysis of Mg(AcAc)2.147 Laser ablation (532 nm) of MgO148 Thermal heating of Ca pellets.112 248 nm laser photolysis of Ca(FOD)2150 Laser ablation (532 nm) of calcite.152 Laser photolysis of Ca(TMHD)2 at 248 nm153 Laser ablation (1064 nm) of Mn or MnO target155

Metal

Method

Cu Fe+

Thermal heating of Cu146 Laser ablation of iron target149

Cr

Laser photolysis (248 nm) of chromium carbonyl151 Microwave discharge of trimethylaluminium154

Al V

Laser photolysis (248 nm) of cyclopendadienylvanadium-tetracarbonyl151

Photolysis with UV flashlamp through suprasil window (l > 165 nm).

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Due to difficulties in absolute radical measurements many branching ratios have been studied using a calibration reaction, i.e. reaction that relates reagents and products in a known, usually 1 : 1, relationship. Good examples are found in the study of CH radical reactions. The high heat of formation of the CH radical (DfH = 595 kJ mol1) means that for many reagent species several product channels are open, however, the reaction with methane has only one exothermic channel, the production of H and C2H4 and therefore CH and H can be linked. CH + CH4 - C2H4 + H Fig. 16 Schematic of Mg and Mg+ chemistry. The rapid conversion of molecular ions to MgO+ and Mg+ competes with dissociative electron recombination accounting for the significant lifetime of ionic species. Reproduced from ref. 147 with permission from the Royal Society of Chemistry.

The reaction of Mg+CO2 with oxygen was studied using a fast flow tube. Mg+CO2 + O2 - MgO2+ + CO2

(R20)

The study illustrates the flexibility of flow tube systems to generate a range of reagents via relatively complex, but well defined chemistry. Mg+ was generated from laser (loosely focused 532 nm light from a Nd:YAG laser) ablation of an MgO sample, CO2 is introduced down-stream and following the titration of some Mg+ into Mg+CO2, molecular oxygen is introduced at a second inlet port. The reaction is followed by observing, via mass spectrometry, the reduction in the Mg+ CO2 signal as more oxygen is added. Continued generation of Mg+CO2 complicates the analysis and k20 is extracted from numerical analysis. MgO2+ reacts with atomic oxygen forming MgO+ and then Mg+ as shown in Fig. 16. The rapid reaction of MgO2+ with O2 and O prevents dissociative recombination and thus sustains the ion layer consistent with observations.

4. Product detection and reaction branching ratios 4.1 Competition between different chemical pathways; CH, CN and C2H reactions Many radical–radical or radical–molecule reactions are highly exothermic with several available product channels.156 As with the Earth’s atmosphere such reactions can be important branching points in reaction mechanisms and in these cases it is as important to know the branching ratios between various product channels as the rate coefficient for the overall reaction. For most studies determination of branching ratios requires measuring products and then relating the measured product signal to the initial radical concentration. Determining absolute radical concentrations, whether reagents or products, is challenging; mass spectrometric or fluorescence methods are sensitive and in many cases selective, but require calibration to determine absolute concentrations. Absorption measurements can provide absolute measurements but only if the absorption cross section and the path length over which radicals are generated are known. 6330

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(R21)

This calibration reaction has been used by two groups to determine H atom product yields in the reactions of CH with several hydrocarbons. McKee et al.157 used the photolysis of bromoform to generate CH radicals and compared the H atom signal, determined by vuvLIF from reference reaction to that from the reaction of CH with C2H2, C2H4 and C2H6. For the unsaturated species the H atom yield was B100% (C2H2 = (105 9)%, C2H4 = (109 14)%). Such measurements do not directly identify the nature of the co-product, e.g. allene, propyne or cyclopropene in the reaction of CH with C2H4, but Kaiser and co-workers have shown how deuteration can identify the co-product as being dominated by allene formation consistent with calculated barriers on the C3H5 surface (Section 6.2).158 For the reaction with ethane, the H atom yield was significantly reduced ((14 6)%), but was consistent with the expected decomposition of a 1-propyl intermediate with fragmentation of the weaker C–C bond dominating: 1-C3H7 - CH3 + C2H4

(R22a)

1-C3H7 - H + C3H6

(R22b)

159

Loison and Bergeat used the metal atom stripping technique (Section 3.2) in a flowtube to generate CH, also monitoring H atom product via vuvLIF. This technique avoids the complications associated with excited CH species, but uncertainties in the time–distance at which CH is generated means that CH self reaction can be an additional source of H atoms which needs to be determined. The two contrasting methods of study give broadly similar H atom yields (C2H2 = (90 8)%, C2H4 = (94 8)%) suggesting that the potential complications of both studies have been overcome. Note that reaction (R21) is important in its own right in Titan’s atmosphere. Dobrijevic has shown that photochemical models of Titan’s atmosphere are extremely sensitive to reaction (R21) and the competition for CH between CH4 and H which could potentially lead to bistability for some compounds in the upper reaches of Titan’s atmosphere.160 Developments in the vuvLIF detection system (gating the photomultipliers and the use of a second photomultiplier to normalize the signal with respect to probe light intensity and absorption) have allowed the Leeds group to improve the sensitivity and time resolution of the apparatus. It is now possible to monitor the temporal growth of the H atom product confirming that the H atom is produced with the same kinetics as the removal of the reagent radical in the target reaction. Fig. 17 shows the CN removal and H atom from the This journal is

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Fig. 17 Time profiles of CN removal and H atom production from reaction (23). Figure adapted from Choi et al.161 (Copyright 2004, with permission Elsevier).

reaction of CN with H2, the reaction used to calibrate CN and H (highlighted in R23). CN + H2 - HCN + H

(R23)

The reaction of CN with ethene is of interest for forming complex species in Titan’s atmosphere and can potentially take place via two channels. CN + C2H4 - (CN)HCQCH2 + H

(R24a)

CN + C2H4 - HCN + C2H3

(R24b)

The addition–elimination mechanism produces (CN)HCQCH2 + H efficiently coupling carbon, hydrogen and nitrogen in a molecule with a double bond site for subsequent chemistry, whilst the direct abstraction produces HCN + C2H3. Product yields show that the addition–elimination reaction (R24a) has essentially unity yield and therefore that reactions of CN radicals with ethene will lead to molecular growth. Trevitt et al.162 have used laser flash photolysis with product detection via photoionization mass spectrometry to probe reactions of CN with higher alkenes. The tuneability of the synchrotron radiation source (the Berkeley Advanced Light Source, ALS) allows differentiation of isomeric species and enhances the selectivity of the detection system. Following the addition of CN to the double bond, either H or CH3 can be eliminated and the H : CH3 is in good agreement with earlier studies.103 The advantages of the synchrotron based detection

system come in the observation of the molecular co-products. H atom elimination leads to the formation of cyanopropene (Scheme 1). The particular isomer formed depends on the position of the initial CN addition and subsequently which H atom is eliminated. Addition at the terminal carbon forms the more stable radical and an H atom can be eliminated from the 1 or 3 position. Addition at the central carbon leads to the 2-cyanopropene. Earlier studies163 suggested that the 3-cyanopropene was a minor product, but these later studies162 with improved detection of the ionization potential (see Fig. 18) and the use of selectively deuterated propene (CH2QCHCD3) demonstrates that production of 3-cyanopropene is significant. C2H radical reactions are also important in planetary atmospheres (and combustion).164 Generating C2H from acetylene photolysis (P9) provides an internal standard for H atom yields of C2H reactions as C2H and H are produced with a 1 : 1 correspondence at low photolysis intensities. Fig. 19 demonstrates the principle. The H atom trace for the production of C2H from acetylene photolysis and subsequent reaction with acetylene shows that the photolytic and reactive yields are identical within error. Monitoring the growth of the H atom signal produces a bimolecular rate coefficient for reaction (25) that is in excellent agreement with previous literature. C2H2 + hn - C2H + H C2H + C2H2 - C4H2 + H

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(R25) 100

The technique has been used by Kovacs et al. to study the reaction of C2H with C2H4 which is isoelectronic to reaction (24) above. As might be expected the addition–elimination reaction again dominates (the yield of H is 0.94 0.06) making the reaction an efficient route to the formation of long chain unsaturated species. Fig. 20 emphasises the importance of C2H chemistry for generating >C2 hydrocarbons in the lower atmosphere of Titan.30 In the upper atmosphere the flux of Lyman-a radiation is relatively high and methane photolysis (see Section 2.1) is relatively efficient, however, lower in the atmosphere only longer wavelength radiation is available and methane photolysis doesn’t occur below 400 km. However, acetylene, C2H2, can still be photolysed providing a catalytic route to methane destruction and via methyl radical recombination, ethane production. C2H2 + hn - C2H + H

(P9)

C2H + CH4 - C2H2 + CH3

(R26)

CH3 + CH3 + M - C2H6 + M

(R12)

Net:

Scheme 1

(P9)

2CH4 - C2H6 + 2H

The Leone group have performed a comprehensive investigation of C2H kinetics as a function of temperature, including reactions of C2H with H2,165 C2H2,166 CH4,167 C2H4,165 C2H6,165 C3H4 isomers,168 C3H8,169 and O2.170 More recently the Leone group have extended their work on C2H chemistry to lower temperatures via pulsed Laval experiments described in Section 5.2. A majority of kinetic studies have been undertaken using transient IR absorption spectroscopy to follow the Chem. Soc. Rev., 2012, 41, 6318–6347

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Fig. 18 (a) Experimentally determined photoionization cross sections of the different cyanopropene isomers. Earlier studies163 relied on calculated cross sections. Reprinted with permission from ref. 162. Copyright 2011 American Chemical Society. (b) Measured photoionization cross section of m/z = 67. The red line is the fit based on 1-cyanopropene = 0.50 0.12, 2-cyanopropene = 0, 3-cyanopropene = 0.50 0.24. Reprinted with permission from ref. 162. Copyright 2011 American Chemical Society.

removal of C2H,166,171 the technique is relatively insensitive and long path lengths are required to achieve appropriate sensitivity with associated issues of temperature uniformity (Section 3.3).

Van Look and Peeters172 developed an alternative technique where the reaction is studied in the presence of oxygen which intercepts a fraction of the C2H converting into the first excited state of CH and A - X emission at 431.9 nm is used to sensitively follow the course of the reaction. 4.2 Competition between chemical reaction and electronic relaxation; 1CH2 chemistry The first excited state of methylene, CH2 (a 1A1) is one of the important products of methane photolysis (see Section 2.1) and is involved in the formation of higher hydrocarbons in the atmospheres of Titan and the giant planets. For example, at low pressures reaction with acetylene (R27) leads to the formation of the propargyl radical and recombination of propargyl radicals137 is one suggested route to benzene, which has been observed in Titan’s atmosphere.27 At higher pressures the C3H4 intermediate can be stabilized – see Section 7.1. CH2 + C2H2 - C3H3 + H

(R27a)

CH2 + C2H2 + M - C3H4 + M

(R27b)

C3H3 + C3H3 - C6H6

(R28)

1

Fig. 19 H atom time profile showing prompt and reactive production in a 1 : 1 ratio.100 Adapted with permission from ref. 100. Copyright 2010 American Chemical Society.

1

However the chemical reaction of excited methylene with acetylene is in competition with relaxation to form the triplet ground state, (X 3B1) 3CH2. 1

Fig. 20 Profile of CH4 removal and C2H6 production as a function of altitude in Titan’s atmosphere. Methane photolysis (Section 2.1) dominates removal in the upper region; catalytic removal of methane via acetylene photolysis is primarily responsible for the bulk of methane removal and ethane production in the lower region.30 Reprinted with permission from ref. 30. Copyright 2009 American Chemical Society.

6332

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CH2 + C2H2 - 3CH2 + C2H2

(R27c)

The branching ratio between reaction and relaxation has been measured for several reactions at room temperature by observing the production of 3CH2 by laser magnetic resonance with relaxation accounting for approximately 20% of the reaction.173 The reaction of 1CH2 with inert gases, where only relaxation can occur, has been shown to have a positive temperature dependence.58,174–176 Some models have applied the same temperature dependence to relaxation in reactions where there is a competition between reaction and relaxation, enhancing reactive loss at temperatures relevant for the stratospheres of the outer planets. Recently we have measured the competition between reaction and relaxation by measuring the H atom yield relative to the reference reaction of 1CH2 with molecular hydrogen.102,177 In contrast to reaction with inert gases, relaxation, in reactions where there is competition between electronic relaxation and chemical reaction, decreases This journal is

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Fig. 22 Schematic reproduced with permission from Smith181 of the CRESU technique used for studying reactions of C atoms generated from the photolysis of C3O2 and probed via laser induced fluorescence. Copyright r 2006 WILEY-VCH Verlag GmbH. Fig. 21 Schematic of stationary points and the minimum energy crossing point on the 1CH2 + C2H2 surface.102 Energies in kJ mol1. Reproduced from ref. 102 with permission from the Royal Society of Chemistry.

with temperature and formation of 3CH2 dominates at the low temperatures relevant for Titan and the outer planets. The chemical reaction occurs via an attractive potential energy surface forming a C3H4 intermediate; the singlet and triplet surfaces therefore cross at energies below the entrance channel as shown in Fig. 21. Theoretical studies have qualitatively reproduced the observed negative temperature dependence.102

5. Low temperature kinetics measurements using Laval expansions 5.1

The Laval expansion technique

Reducing the uncertainties in photochemical models of the outer planetary atmospheres or the interstellar medium (ISM) requires the generation of input kinetic data at temperatures relevant to the model.114 This is not always possible but one wants to minimise the temperature range over which any extrapolation is made, and to base the extrapolation on theory which has been benchmarked against kinetic data obtained over an extensive range of temperatures. Condensation ultimately limits the temperature range of conventional techniques such as flash photolysis, but the CRESU technique, developed originally by Rowe and co-workers178 for ion– molecule reactions, and first adapted by Sims et al.179,180 to study radical molecule reactions, has no such limitations. CRESU (cintique de reaction en ecoulement supersonique uniforme) is based on the formation of uniform beam of gas in a vacuum chamber; the beam of gas, which can be characterised by a temperature, i.e. collisions within the beam (unlike that of a molecular beam (see Section 6)) is an essentially wall-less flow tube. A low temperature uniform jet is achieved by the isoentropic supersonic expansion of a high pressure of gas, using a convergent–divergent Laval nozzle into a preset low downstream pressure, where the density of cold gas in the jet, 1016–2 1017 molecule cm3, is high This journal is

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enough to promote thermalization through many collisions. There have been several reviews of the technique,181–184 so only a limited description of the technique is presented and the focus is on the impact that the CRESU/Laval nozzle technique has played in the understanding of outer planetary chemistry. The original CRESU setup in Rennes, France178 and the second setup in Birmingham, UK used a continuous flow of gas, which meant the apparatus required vast pumping capacity – a room full of pumps – and an equally extensive supply of gas, limiting the reagent and buffer gases to relatively low cost materials. Fig. 22 shows a schematic of the CRESU apparatus set up to monitor the reactions of C(3P) atoms. C(3P) was generated from the pulsed photolysis of carbonsuboxide at 193 nm and probed at B166 nm by laser induced fluorescence, with the probe light being generated by four-wave mixing in xenon. More recently, pulsed Laval systems185–188 have been developed which lead to, typically, an order of magnitude reduction in the pumping and gas requirements. In an additional development, a miniature continuous CRESU apparatus has been constructed,189 which has pumping/gas requirement somewhere in between the continuous and pulsed Laval setups. Construction of the miniature version has been made possible through engineering improvements, which allows the manufacture of miniature Laval nozzles while still maintaining the high precision design requirements. In all these setups, a different Laval nozzle is required for each temperature, where a lower temperature is achieved by narrowing the throat of the nozzle to increase the Mach number. In principle, each nozzle is defined by one gas jet density but in practice a range of pressures can be obtained by varying the pressure behind the nozzle.190 In general, the continuous systems can obtain lower temperatures, down to 7 K191 compared to the >50 K for the pulsed systems. However, with improvements in design and machining tolerances it is expected that the lowest temperature achieved by pulsed systems will improve. There are limitations to kinetic studies in the Laval jet due to the length of stable flow of the jet and its stability as the gas Chem. Soc. Rev., 2012, 41, 6318–6347

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composition is varied. In general, the jet is only stable, at most, over a few 10’s of cm, which translates to a reaction time of o500 microseconds, and stability requires no significant change in the heat capacity of the carrier gas reagents, so reagents must be present at no more than 1–2%. Overall, studies of kinetics are limited to reactions with rate coefficients above 1012 cm3 molecule1 s1. In contrast, in conventional flash photolysis studies there is generally no limitation on the amount of substrate gas which can be added to make the pseudo-first-order loss rate for the target dominate over other radical loss processes. For studies of reactions relevant to outer planetary atmospheres, generally only reactions with high rate coefficients (>1011 cm3 molecule1 s1) at low temperatures contribute significantly to the chemistry, so limitations in rate coefficients are not significant; important exceptions are the reactions of radicals with gases present in very high concentrations such as methane on Titan (k298 (C2H + CH4) = 2.3 1012 cm3 molecule1 s1 167) or H2 for the giant planets ((k298 (C2H + H2) E 5 1013 cm3 molecule1 s1).100,165 If such reactions have positive temperature dependencies, then they may not be applicable for study in Laval expansion systems (or only over a limited temperature range). Within the gas jet, kinetic measurements are made by generating and monitoring the species of interest. The original setup focused on ion–molecule kinetics,178 where an electron beam close to the Laval nozzle was used to make the ions, and a mass spectrometer sampled the ions in the jet as the reagent concentration was varied. The first experiments to determine radical kinetics in the jet179,180 used the laser flash photolysis/ laser induced fluorescence technique, where the pulsed excimer laser was directed through the throat of the nozzle, irradiating all the gas in the jet to ensure the same chemistry at all parts of the jet (Fig. 22). The radicals in the jet can be probed by pulsed laser induced fluorescence – a very sensitive technique – achieved by probing the jet at right-angle and at a fixed point from the nozzle. As radicals have been created in the jet, the probe samples only these radicals and by varying the time delay between the excimer and LIF probe, the time profile of the radical is built-up. The fact that the excimer laser evenly irradiates the gas jet means that even though the probe has sampled difference sections of the gas jet it is still sampling the same chemistry. 5.2

Low temperature studies using the Laval expansion method

The CRESU method originally focused on determining the kinetics of ion–molecule reactions at low temperature relevant to ISM.178 It was later extended to fast neutral chemistry involving highly reactive radicals such as CH/CN/C that have been identified as being important in cold temperature ISM chemistry.182 Some of these reactions are directly relevant to the chemistry of the outer planets, and because of the higher temperatures in these atmospheres no extrapolation of the Laval measurements is required. More recently, especially following the results from the Cassini–Huygens spacecraft voyage to Titan, there has been a focus by Laval experiments on important reactions of Titan’s atmosphere. Table 6 collates some of the measurements relevant to Titan and the outer planets. 6334

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CH4 photolysis plays a significant role in the atmospheres of the giant planets and Titan, as discussed in Section 2.1, and the additional catalytic destruction of CH4 by C2H was introduced in Section 4.1. The importance of minimising the extrapolations from laboratory to atmospheric temperatures has already been mentioned and the CRESU/Laval kinetic studies on the C2H radical have significantly reduced the uncertainty in our understanding of the chemistry in these systems. The detection of the C2H relies on the chemiluminescence tracer reaction as introduced in Section 4.1: C2H + O2 - CH(A2P) + CO2

(R29)

where the blue emission from the excited CH(A2P) is monitored. In general, C2H reactions with unsaturated hydrocarbons are characterised by rate coefficients that are close to gas kinetic limit, see Table 6, and exhibit little temperature dependence. This does mean that rate coefficients can be accurately estimated for other unsaturated compounds which have not been measured. However, as mentioned below, a present drawback of the CRESU is the lack of information on the products of reaction, which is a significant uncertainty in these C2H addition reactions with the larger unsaturated hydrocarbons, where a variety of atoms or radicals can be eliminated following the initial addition of C2H to the double bond (Section 4.1). While the above table gives the impression that CRESU experiments simply measure fast rate coefficients that vary little with temperature or have negative temperature dependence, the reaction of C2H + nitriles do show normal Arrhenius behaviour, such that the reaction with methyl nitrile, CH3CN, is too slow to be measured in the Laval apparatus below 165 K;198 C2H reactions with H2 and CH4 also fall into this category. However, the larger alkanes and nitriles have correspondingly smaller activation energies such that rate coefficients measurements in the Laval system can be extended down to 104 K. A summary of the temperature dependencies of the rate coefficients for C2H with some saturated species is shown in Fig. 23. For these reactions that exhibit a significant temperature dependence, the CRESU experiments are playing a vital role in deciding the importance of a given reaction in the system. From Table 6, C2H is observed to react with NH3 with a significantly increasing rate coefficient as the temperature is reduced consistent with barrierless formation of products. However, the kinetic isotope effect (KIE) where k(C2H + NH3) : k(C2H + ND3) is E2 over the measured temperature range 104–296 K is qualitatively similar to reactions proceeding with an activation barrier.197 Clearly a conventional interpretation of a KIE in terms of zero-point energy differences between reagent and transition state is not applicable. The KIE can in principle give detailed information on the nature of the transition state as has been discussed by Taatjes and Klippenstein.207 There are three major possibilities via which the reaction can proceed: formation of C2H2 + NH2 (either by direct abstraction or via initial complex formation), an addition–elimination reaction producing HC2NH2 + H or an addition reaction followed by rearrangement before elimination giving potentially more stable products such as This journal is

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Table 6 Collation of a sample of kinetic studies undertaken using Laval expansion techniques relevant to the study of Titan and the outer planets Radical C2 H

Substrate

Temperature

C2H2192 HCCCH2CH3193 H2CCCH2

103 K 74–295 K 103 K192 63–295 K145 103 K192 63–295 K194 103–296 K


CH3CCH C2H4 CH3CHCH2 1-Butene195 C2H6 C3H8 n-C4H10196 NH3 ND3197 Isobutene 1-Butene isobutylene 1,3-Butadiene198

96–296 K 104–296 K 104–296 K

CH3CN C2H5CN C3H7CN198 Benzene199

104–296 K

C2H2 C2H4 C2H6200 Benzene Toluene201 NH3180 CH3CCH194 CH4178 CO H2 CO, N2, CO2 NH3, SO2, H2S178 NH3202 CH4, C2H2, C2H4, C2H6, but-1-ene203

25–296 K

104–296 K

CN

N+ H3+ CH

>1010 >1010 >1010

25–295 K 25–296 K 8K 30 K

>1010

104–296 K

26–295 K 23–295 K

CH4, C3H6 H2CCCH2 CH3CCH204

77–295 K

C2H4, C3H6, 1-C4H8, H2CCCH2, 1,3-Butadiene, C2H2, CH3CCH, HCCCH2CH3205

39–300 K

CH4 C2H6 C3H8, C4H10206

39–300 K

>1010 >1010 >1010 >1010 >1010 >1010 >1010 >1010 >1010 >1010 >1010 >1010 0.7 (200 K)–2.1 1011 1.7–0.4 1010 >1010

CH2 NH2

Radical detection method/ comment on study Chemiluminescence, C2H + O2 - CH* Products detected via PIMS at 295 K193

>1010 >1010 >1010 >1010 >1010 3–4 1011 4.7–7.7 1011 5.9–9.7 1011 12.0–5.1 1011 5.8–2.4 1011 >1010 >1010 >1010 >1010 0.2–1.5 1012 4.1–4.8 1012 1.3–1.8 1011 >1010 >1010 >1010 12.2–2.5 1011 >1010 >1010 46.8–2.8 1011 >1010 >1010

C4 H

LIF detection

Products detected via MS at 295 K

LIF in the blue

LIF in the blue, 408 nm

LIF in the visible LIF

methylnitrile + H. Theoretical calculations by Nguyen et al.208 suggest that the initial channel dominates. This reaction channel may be important in the Jovian upper troposphere as a source of NH2. NH2 has been suggested as a possible precursor to HCN formation by Kaye and Strobel.209 The reaction between C2H with acetylene is known to form diacetylene, C4H2, which has long been recognised as a key species in the atmosphere of Titan. C2H + C2H2 - C4H2 + H This journal is

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(R25)

Its red shifted absorption spectrum with respect to acetylene means that it can readily be photolysed to C4H at lower altitudes, potentially further enhancing the catalytic breakdown of methane as discussed in Section 4.1. C4H2 + hn - C4H + H

(P10)

C4H has recently been studied using the CRESU technique, where it was probed using LIF in the blue at 408 nm. All of the reactions of C4H with unsaturates205 were observed to be fast, Chem. Soc. Rev., 2012, 41, 6318–6347

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Included in the above Table 6 are CRESU measurement on reactions with N+, and H3+. The electrostatic potential between an ion and the reagent – Langevin potential – occurs at a much larger distance compared to neutral reactions, and as such if reaction is possible it is characterised by 109 rate coefficients, with significant negative temperature dependences. However, the electrostatic potential can be accurately calculated, and from which theory is able to predict ion–molecule rate coefficients with good accuracy.182 For this reason the above table has not included Laval studies on ion reactions. 5.3 Product detection and future directions

Fig. 23 Temperature dependence of the reactions of C2H with hydrogen and saturated alkenes. Note the strong positive dependence of the rate coefficients for the reactions of C2H with H2 and CH4 in contrast to the weak temperature dependence (or indeed temperature independence) of the reactions with higher alkanes. Data below 160 K have been obtained using a pulsed Laval expansion technique,196 data above by conventional laser flash photolysis. For details of the references see Murphy et al.196 Reproduced from Murphy et al. (Copyright 2003, with permission Elsevier).

and a similar result was observed for the alkanes,206 except CH4 which reacted slowly and showed normal a positive temperature behaviour such that no Laval measurements were possible below 200 K. The rate coefficient at 200 K for the reaction of C4H with methane (B7.7 1013 cm3 molecule1 s1) is only 20% below that of C2H with methane, so C4H may also contribute to the catalytic removal of methane. Coupling of the N2 and CH4 chemistry leads to the formation of the CN radical, which plays an important role in the chemistry of Titan, even though the initial impetus to study CN kinetics in the Laval stems from its prevalence in the ISM. Its kinetics show much similarity to the isoelectronic C2H radical in that many of its reactions are fast and only approach the limit for capture as the temperature is lowered, except CH4 and H2 which show normal Arrhenius behaviour and are consequently too slow for Laval measurement. However, Nizamov and Leone have pointed out that there may be subtle differences in some mechanisms as the dipoles of the two radicals are different.197 An interesting feature of the reactions of C2H–C4H and CN, all of which play an important role in the upper atmosphere of Titan, is that they ultimately lead to chain lengthening of unsaturated molecules. A consequence of these types of reaction is aerosol formation, which ultimately leads to absorption of sunlight and the thermal inversion observed in Titan’s atmosphere. A unique feature of Titan is its cold N2 atmosphere. This means the existence of N+ in its upper atmosphere, which gives rise to chemistry that can be coupled to the CH4 present. 6336

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The majority of the work on the C2H radical has be done in the group of Stephen Leone, and from Table 6 it can be seen that their work is progressing towards the identification of the products of reaction. In the reaction C2H + 1-butyne, HCCC2H5,210 the products of the reaction were characterised in complementary 295 K experiments, using energy resolved photo-ionization mass spectrometry, which is capable of identifying isomers. Most recently, this complimentary technique, schematically shown in Fig. 24, was used to directly probe the products of C2H + C2H2 (R25) in a pulsed Laval experiment.211 In the future, it is expected that the range of species monitored in the Laval jet will increase, both for reagents and products. However, the low gas density of the jet means that the reacting species must be probed by ultrasensitive techniques over r500 ms time-window, which is a major constraint of the technique. This is where theory should play

Fig. 24 Schematic diagrams of methods to interface mass spectrometric techniques to pulsed Laval systems. Reproduced with permission from Soorkia et al.211

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a more significant role, especially in the area of product assignment, which is the major uncertainty in many of reactions, e.g. most the reactions in Table 6 are so exothermic that they produce multiple products (Section 7.1). Included in Table 6 are two species that have not been studied in the Laval, (a 1A1) 1CH2 and NH2 but are potentially important for planetary atmospheres. 1CH2 is highly reactive, but as it is an excited state, it can also be deactivated to the less reactive triplet, (X 3B1) 3CH2, as discussed in Section 4.2. Experiments down to 195 K suggest that deactivation is more competitive than reaction, so the chemical product yields at the temperatures of the outer planets are probably low.59,102 NH2 radicals are generally unreactive towards closed shell molecules212 – 3 CH2 is also – so are likely to react with other radicals, e.g. itself or H, which requires third-body stabilization. In general, radical– radical reactions in a Laval are problematic due to the need to make significant concentrations in a low density medium, but it has been done for bimolecular reactions OH + O213 and N + OH.214 For an association radical–radical reaction the low total pressure may render the effective bimolecular rate coefficient too slow for the reaction to be measured in the r500 microsecond time-window of the Laval expansion. Typically the temperature region from 170 K to 230 K is difficult to cover for many reactions, condensation prevents the use of conventional laser flash photolysis or discharge flow and the upper temperature of Laval expansion methods is about 170 K. This temperature region is particularly relevant for the Earth’s upper troposphere/lower stratosphere (UTLS) and for other regions of the outer planets. For many reactions, the temperature variation is relatively gradual and estimations of rate coefficients in this region from parameterization of conventional kinetic data and lower temperature Laval expansion measurements can be made with confidence (Fig. 23). However, it has been shown that several reactions which appear to show a conventional positive temperature dependence down to B250 K, become much more efficient at the temperatures measured in a Laval expansion system.215 Measurements in this important temperature region of 170–250 K

would provide more insights into the change in reaction mechanism as well as providing useful kinetic data relevant for photochemical models of the UTLS.

6. Reaction dynamics 6.1 Crossed molecular beam apparatus Our earlier discussion has looked at measurements of the overall rate coefficients of reactions and on the increasing focus on the product distributions of multichannel reactions under thermal conditions. Crossed molecular beam (CMB) studies are ideal techniques for product studies; as they involve single collision events, interference from secondary chemistry, which may influence ‘bulb’ experiments, is absent. Recently collaborative papers have been published bringing together temperature dependent studies on rate coefficients with complimentary CMB studies on products, sometimes in combination with theory.216,217 Several review articles are available on CMB studies.218,219 Whilst there have been relatively few CMB studies of reactions relevant to terrestrial atmospheric chemistry, the complexity of the upper atmosphere of the outer planets (including Titan) including the involvement of a wide range of exotic radicals (C2, CH, CH2, CN, C2H) means that CMB studies are still a major tool for the studies of such environments. Fig. 25 illustrates the basic layout of a CMB system. Two molecular beams are generated and directed into the reaction chamber. The relative angle of the beams can often be adjusted to alter the collision energy. Products are detected as a function of angle generally via mass spectroscopy, but optical techniques can also be used. Many aspects of the technique can be illustrated by the study of Geppert et al.220 on the reaction: C(3PJ) + C2H4 - [C3H4]* - C3H3 + H DH00 = 189 kJ mol1

(R30a)

- C3H2 + H2 DH00 = 214 kJ mol1

(R30b)

- c-C3H2 + H2

(R30c)

DH00 = 284 kJ mol1

Fig. 25 Schematic of the Crossed Molecular Beam Apparatus of Casavecchia and co-workers.218 The beams can be rotated relative to each other altering the collision energies. Reproduced from ref. 218 with permission from the PCCP Owner Societies.

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where the above represent the three most exothermic of many product channels. Two techniques were used; firstly cross sections integrated over all angles were measured with a CMB system at Bordeaux with detection of the H atom product by laser induced fluorescence and secondly by the CMB system in Perugia where soft electron impact ionization, time of flight (TOF) mass spectrometry was used to study the molecular products. Laboratory angular and TOF distributions of m/z 38 (C3H2) and 39 (C3H3) were identical, strongly suggesting that m/z 38 arises solely from fragmentation of C3H3 and not from the reaction. Although the results show that C3H3 is the sole molecular product, there are four possible C3H3 isomers; the angular distributions show that at low collision energies propargyl is the dominant product, but as collision energies are increased other isomers may be formed. The observations of alternative product channels at high collision energies were supported by analysis of the Doppler profiles of the H atom fluorescence signals. The domination of an H atom product channels confirms an earlier room temperature thermal study by Bergeat and Loison, where the H atom yield was measured as 0.92 0.04.221 Atom–molecule reactions have been studied since the earliest CMB experiments as many atomic species can readily be produced either by heating (e.g. sodium) or by microwave discharge, but radical–molecule reactions have received less focus due to difficulties in generating sufficient concentrations. Recent developments have allowed for the development of a range of radical sources pertinent to reactions occurring in the upper atmosphere of the outer planets. In the apparatus of Kaiser and co-workers, a radical precursor is mixed with carrier gas and then expanded through a pulsed valve/nozzle system. Pulsed laser photolysis then produces the required radical and a chopper wheel selects a part of the radical beam with a defined peak velocity. Laser photolysis can generate a range of products especially for multiphoton processes such as the generation of CH from bromoform, and with significant internal excitation. Unlike the kinetic experiments described above, there is no way to temporally link reagents and products, and whilst there are other ways to distinguishing between products formed from different reagents, characterisation of the products of photolysis is very important. In a study on CH + C2H4, Zhang et al.158 used laser induced fluorescence to measure a rotational temperature of 14 1 K and o6% vibrationally excited CH. CH may also be formed in higher electronic states, but the natural lifetimes of the A and B states are short compared to the transit time to reach the reactive region. The metastable quartet a state may however cause complications. Pulsed laser excitation can also be used to selectively excite internal modes of the reagents. Riedel et al.222 describe an elegant multipass reflector design which significantly enhances IR absorption of the CH2D2 molecule allowing a CMB study of the reaction of Cl + CH2D2(v) to investigate site selective chemistry. Casavecchia and co-workers have taken an alternative approach and use a high power, high pressure radio frequency discharge which excites the rare gas atoms forming metastable species which can collide with precursor molecules premixed into the carrier gas flow.223 Such reactions can be 6338

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indiscriminate in nature and a range of species with high internal energy can be produced in certain cases, however, the source is continuous and hence the duty cycle of the system is enhanced. Expansion through the nozzle will produce significant cooling, but internal relaxation may not be complete and characterisation of the radicals is desirable. Leonori et al. describe in detail the production of C2 and CN radicals and measurement of internal state distributions by LIF.223 6.2 Alternative ionization methods Soft electron impact reduces the fragmentation of ions, but generally doesn’t eliminate it altogether. Photoionization can provide yet softer ionization; as mentioned earlier, the ideal light source is a synchrotron, providing tuneable ionizing radiation. Such radiation removes complications from fragmentation and allows for the identification of particular isomers from their ionization potentials. A CMB apparatus has been interfaced with synchrotron ionization radiation at the Taiwan National Synchrotron Radiation Research Centre.224 A recent publication from this group on the reaction of N + C2H4 illustrates the benefit of synchrotron radiation.225 Possible products include: N + C2H4 - C2H3N + H

(R31a)

N + C2H4 - C2H2N + 2H

(R31b)

Reaction (R31) is of relevance to the atmosphere of Titan where both N atoms and hydrocarbons are present in the extensive stratosphere. An earlier study Balucani et al.226 had observed products at m/z = 40 (C2H2N) and 41 (C2H3N). The similarity of the angular distributions suggested that C2H2N was a daughter fragment of C2H3N and that C2H3N was the dominant reaction product. With high energy photoionization Lee et al. observed similar angular distributions of both products, however as the ionization energy was reduced, it became clear that both C2H2N and C2H3N are products of the reaction; Fig. 26 shows the different angular distributions of the two m/z at low ionization energy. The tuneability of the synchrotron radiation allowed identification of the structure of products; Fig. 27 shows a photoionization spectrum of the C2H3N clearly identifying contributions from CH2NCH and c-CH2(N)CH isomers. The experimental observations were supported by both ab initio and RRKM calculations and Lee et al. suggest that the mechanism involves the addition of N to the p-bond to form a cyclic C2H4N complex which eliminates an H atom. A majority of C2H3N go on to eject a further H atom forming CH2CN (Fig. 27b). As discussed above, partial deuteration can often be used to elucidate chemical mechanisms and a good example which is very relevant to outer planetary atmospheres is the reaction of CH with ethene. The reaction, which occurs on a C3H5 surface (and hence links to studies on allyl photochemistry227,228) leads predominantly to the formation of C3H4 + H products157,159 but the nature of the C3H4 product was not known. By studying the combinations of CH–C2H4, CH–C2D4 and CD–C2H4 Zhang et al. were able to show This journal is

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7. Comparisons with terrestrial atmospheric studies Studies of gas phase chemistry mechanisms relevant to the Earth’s atmosphere can be broken down into the study of elementary reactions by experiment and theory, simulations of mechanisms in atmospheric chambers and the production of chemical models at varying levels of complexity ranging from explicit models looking at detailed chemistry, to highly parameterized chemical models which can be including in global circulation models (GCMs). Additionally there is a major focus in experiment, theory, simulation and modelling in how gas phase chemistry leads to the production or growth of aerosols and reviews on comparative studies for other atmospheres, particularly tholin production on Titan, can be found in Lebonnois et al.,26 Imanaka et al.229 Lavvas et al.230,231 and Rannou et al.232 The main focus of this review has been on experimental studies of elementary gas phase radical reactions relevant to planetary atmospheres, but in this section we briefly consider the roles of theory, simulation and modelling as applied to non-terrestrial atmospheres highlighting differences from approaches used for terrestrial studies. 7.1 Theoretical studies

Fig. 26 Newton Diagrams and product velocity contour plots for the reaction of N(2D) with C2H4 leading to reaction with the loss of one or two H atoms.225 The dashed lines denote the laboratory detection angles. Reproduced from ref. 225 with permission from the PCCP Owner Societies.

that the reaction proceeded via addition across the double bond to form a cyclopropyl intermediate (as opposed to insertion into C–H bond) followed by ring opening to form the allyl radical. The angular distribution of the products suggests that the intermediates are long lived and that H atom elimination occurs from the central carbon to almost exclusively form allene.

Ab initio studies are increasingly being used to complement and interpret experimental kinetic data.233 This is certainly the case in studies of reactions relevant to non-terrestrial atmospheres and several examples have already been discussed above. Developments in ab initio calculations have been applied to range of kinetic problems and predictions from theoretical calculations are becoming increasingly useful as the uncertainly in calculations continue to decrease. Ab initio structure theory is now capably of calculating the potential energy surface (PES) of a given reaction to B1 kcal mol1 for systems containing 10 or less heavy atoms. This is still not sufficient to calculate accurate rate constants for radical– neutral as the PES, as the reactants initially approach, needs to be known even more accurately – for ion–molecule reactions the PES is sufficiently known that accurate rate constants can be calculated.182 Ion–molecule studies using the Laval or other methods are relatively straightforward allowing extensive comparison of experiment and theory for such reactions.

Fig. 27 (a) Low-resolution photoionization spectrum at m/z = 41. The black circles and line are the experimental data with increments in the experimental trace showing the contribution of the two isomers. Reproduced from ref. 225 with permission from the PCCP Owner Societies. (b) Photoionization spectrum at m/z = 40 confirming that the product is CH2CN and not CH2NC. Reproduced from ref. 225 with permission from the PCCP Owner Societies.

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Fig. 28 Zero point energy corrected stationary points on the 1CH2 + C2H2 potential energy surface calculated using the B3LYP functional and 6-31G(2df,p) basis set.177 Energies in kJ mol1. Reprinted with permission from ref. 177. Copyright 2010 American Chemical Society.

Many important reactions relevant to planetary atmospheres involve reactions of highly reactive radicals and therefore there are frequently many open product channels resulting in complex PES with many interconnecting wells such as that shown in Fig. 28. PES that have multiple products normally require large adjustments in order to switch product channels.103 Therefore calculations can often make reliable predictions on the products of reactions, exactly where Laval experiments are most difficult. Product yields are temperature and pressure dependent with higher pressure resulting in increased stabilization into the wells. Determination of product yields on such surfaces requires the use of master equation calculations234 to determine the competition between the transfer between wells and to products versus collisional stabilization into the wells. Internal states in the wells are divided up into energy grains and differential equations, coupling the transfer of material between grains in the same well (by collisions) and between wells are solved. Several packages such as MULTIWELL235 and MESMER236 are available to model such complex systems. The low temperatures relevant for the outer planets pose significant challenges for master equation calculations. In the study of the 1CH2 + C2H2 reaction the PES of which is shown in Fig. 28 (see also Section 4.2), experiments were limited to a total pressure of 1 Torr (otherwise relaxation to ground state 3 CH2 dominates 1CH2 removal), but data are required at a range of temperatures and pressures for combustion and planetary modelling.177 The deep wells and the low value of kBT under outer planetary conditions, means that if grains are established for the whole of the well, then there can be problems with the calculations. Such problems can be overcome by the use of ‘reservoir states’, a single state spanning from the bottom of the well to a few kBT below the entrance energy. The probability of a given isomer being activated from below a certain threshold is negligible and therefore energies 6340

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Fig. 29 Pressure dependence of the H atom yield following the formation of C3H4 adducts calculated via the master equation. The lines represent the reservoir state (rs) calculations at the various temperatures (e.g. rs_300), the open symbols are the exact calculations at 400 (J) and 600 K (n) using the full master equation. Note that these calculations do not include the fraction of 1CH2 relaxed to 3CH2. Adapted with permission from ref. 177. Copyright 2010 American Chemical Society.

below this threshold can be considered as a single state. Fig. 29 shows the pressure dependence of the H atom yield from reaction (27a) as a function of pressure, i.e. competition between decomposition of the C3H4 intermediate versus collisional stabilization. CH2 + C2H2 - C3H3 + H

(R27a)

CH2 + C2H2 + M - C3H4 + M

(R27b)

1 1

The symbols represent the full calculations, whilst the lines use the reservoir approximation. The agreement between the two methods is excellent. This journal is

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Whilst significant strides have been made in the study of kinetics at the low temperatures relevant to most planetary atmospheres, the complexity of low temperature apparatus such as pulsed Laval systems, means that, for many reactions, it is almost always more straightforward to carry out experiments out at temperatures close to ambient. Theory can then be matched to the experimental data with reasonable adjustment of the PES and other parameters and the resultant model used to extrapolate outside this experimentally easily accessible temperature range. It is likely that this approach, coupled with Laval studies for particularly important species, will provide the improved kinetic data required to further improve the reliability of photochemical models. 7.2

Simulation chambers

Simulation chambers play important roles in the study of the Earth’s atmospheric chemistry. Many radical species are hard to generate and/or study in isolated studies and chambers provide a way to study the reactions of such species with a degree of control over the chemistry and conditions. The larger sizes of simulation chambers and the longer timescales of the chemistry (typically minutes to hours rather than micro to milliseconds for direct radical studies) mean that a wider variety of experimental techniques can be deployed. Increasingly simulation chambers are equipped with high sensitivity radical detection methods, so that reactions can be simulated under concentrations that are representative of the real atmosphere. Simulation chambers therefore provide suitable testbeds for atmospheric mechanisms.237 Two major issues have limited the application of chambers to the complex hydrocarbon chemistry of the outer planets. Firstly, as we have discussed, the chemistry in such environments has been driven by very short wavelength radiation, e.g. Lyman-a at 121.6 nm, and there are few convenient sources of such radiation that can be coupled to chambers. Secondly, at the low temperatures of the outer planets most species would condense onto the walls of the chamber; studies are therefore limited to higher temperatures reducing the applicability of the simulation. Nevertheless, simulations, particularly focusing on the chemistry of Titan’s atmosphere, are performed, but they are often quite different in nature from their terrestrial counterparts. For example Gautier et al.25 simulated the gas phase formation of nitriles by a plasma dissociation rather than by photolysis and Thissen et al. have used synchrontron source.238 A review of simulations relevant to the formation of tholins can be found in the recent publication of Cable et al.239 Two examples of work with stronger links to conventional atmospheric chambers are presented below. Romanzin et al.64 have used the apparatus shown in Fig. 30a to study the stable products of methane photolysis at either 121.6 nm (Lyman a) or 248 nm. Sources of Lyman a, such as microwave driven resonance lamps, are relatively weak. Romanzin et al. wanted to explore the potential of two photon absorption at 248 nm as an alternative method of methane photolysis. End product analysis of the stable photoproducts was obtained in each case by in situ FTIR analysis. Fig. 30a shows the experimental setup for 121.6 nm photolysis. For 248 nm photolysis the resonance lamp was replaced by a This journal is

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Fig. 30 (a) FTIR end-product analysis of 121.6 nm methane photolysis. Reproduced from Romanzin et al.64 (Copyright 2010, with permission Elsevier). (b) Species profiles of methane and methane photolysis products with model predictions. Reproduced from Romanzin et al.64 (Copyright 2010, with permission Elsevier).

pulsed excimer laser whose output was focussed into the centre of the chamber. Fig. 30b shows an example of the evolution of the stable products acetylene, ethene and ethane and the data were interpreted on the basis of a chemical model in a similar to the analysis of most chamber data. The product distribution from the 248 nm photolysis of methane was quite different with virtually no ethane being formed. Romanzin et al. interpreted this observation as being due to photolysis occurring via a three, rather than two, photon process with the generation of CH4+ ions and hence a very different subsequent chemistry. Vuitton, Doussin and co-workers240 have used a large Pyrex reactor (surface–volume ratio of 9 m1) to study aspects of the CH4–C2H2 chemistry discussed in Section 4.1. The studies were carried out at room temperature and pressures slightly above ambient (to minimize ingress of O2) with photolysis at 185 and 254 nm from low pressure mercury lamps, monitoring the production/loss of hydrocarbons by long-path FTIR absorption spectroscopy. It is important to note that Vuitton et al. do not see their experiments as realistic simulations of Titan’s atmosphere, but rather as a method of assessing chemical models under controlled conditions. Of course the assessment is not ideal as even if there is good agreement at room temperature, the model will only be valid if the temperature dependence of all the relevant reactions are correct, but one has to start somewhere! The study was partially successful; earlier models were not able to reproduce the observations which could be traced to issues around the quantum yield of acetylene photolysis and neglect of reactions of heavier reactive species such as C4H4. Vuitton et al. were able to obtain good agreement with a more comprehensive model with kinetic parameters fixed at literature values. It was not possible avoid some contamination with oxygen, allowing Vuitton et al. to suggest roles for oxygenated chemistry in the formation of ethene in Titan’s atmosphere. An alternative approach, with closer links to combustion studies, has been adopted by Ferris and co-workers.241,242 They have used flow reactors in which, instead of passing through a heated region before end-product analysis, the gas mixture passes through an illuminated (185 and 254 nm) Chem. Soc. Rev., 2012, 41, 6318–6347

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region before analysis of stable products by FTIR, UV spectroscopy or chromatography (GCMS or DNPH cartridges for carbonyl analysis). Solid material is collected for FTIR or XPS analysis. Tran et al. have used such methodology to investigate the modification of the methane photo-products in the presence of HCN and CO.242


7.3

Modelling

Many chemical models exist for the atmospheres of the planets including: Jupiter,16 Saturn,17 Titan,243 Neptune19 and exoplanets.45,244–246 For experimentalists, modelling studies, where sensitivity analysis has been carried out to identify crucial steps in the mechanism, are particularly useful. An example was presented above with regard to methane photolysis (Fig. 5, Section 2.1).51 More recently Dobrijevic et al.19 have identified 7 ‘main key’ and 26 key reactions from a sensitivity based analysis of a comprehensive (75 compounds, 381 reactions, 50 photolysis processes) photochemical mechanism of Neptune’s atmosphere. The reactions are of differing significance in different regions of Neptune’s atmosphere as shown in Table 7, reproduced from Dobrijevic et al.19 Many of the tabulated reactions have been discussed above. As with the Earth’s atmosphere, comprehensive chemical models are complex and cannot be run in combination with a detailed consideration of atmospheric physics and dynamics; condensed models are required for such applications and Dobrijevic et al.247 has developed his sensitivity analysis approach to produce a condensed mechanism of 25 compounds and 46 reactions (again including many discussed in this review) from an initial photochemical model with 90 compounds and 608 reactions. Dobrijevic et al. use MonteCarlo methods to assess the uncertainty of the mole fraction profiles with the uncertainties of the rate coefficients and quantum yields in the chemical schemes. The reduced model distribution was obtained from 500 individual runs. Fig. 31 shows the comparison between the reduced chemical scheme and the full model for the profiles of ethene and ethane. In comparison to terrestrial models there are a lot less experimental data (rate coefficients, branching ratios, photolysis rates and quantum yields) to input and much greater

Fig. 31 Red line: abundance profiles of C2H4 and C2H6 with the reduced chemical scheme. Blue line: initial profile obtained with the initial chemical scheme. Black dotted line: median profile. Black dasheddotted lines: 5th and 15th 20-quantiles of the distribution. Black longdashed lines: 1st and 19th 20-quantiles of the distribution.247 Reproduced from Dobrijevic et al.247 (Copyright 2011, with permission Elsevier).

uncertainties in observational data with which to both constrain input parameters and to compare species concentrations and profiles. Data evaluations are key tools in constructing terrestrial atmospheric248–252 or combustion chemistry253 models. Such evaluations do not exist for planetary atmospheres and

Table 7 ‘Main key’ reactions and photolysis processes in Neptune’s stratosphere. For each pressure the compounds which show high sensitivity to the given process are shown. Adapted from Dobrijevic et al.19 Reaction/pressure (mbar)

10

1

101

102

103

104

CH4 + hn - CH3 + H CH4 + hn - 1CH2 + H2 H + CH - C + H2

C2H6

C2H6

C2H6 C2H6

C2H6 C2H6 CH3C2H, C4H2

CH3, C2H6 CH3, C2H2, C2H6 C2H2, C2H4, CH3C2H, C4H2, C6H6 C2H4, CH3C2H, C4H2, C6H6 C2H4, CH3C2H, C4H2, C6H6 CH3, C2H2, C2H6 C6H6

CH3, C2H6 CH3, C2H2, C2H6 C2H2, C2H4, CH3C2H, C4H2, C6H6 CH3C2H, C4H2, C6H6 CH3C2H, C4H2, C6H6 C2H6

C2H2, C2H4

C2H2

CH3

CH3, C2H6

CH + H2 - 3CH2 + H 3

CH2 + H2 - CH3 + H

C2H2

H + CH3 (+M) - CH4 CH3, C2H6, CH3, C2H6 H + C2H2 (+M) - C2H3 C2H4, CH3C2H, CH3, C2H2, C6H6 C2H4, C2H6, CH3C2H, C6H6 H + C2H4 (+M) - C2H5 C2H4 C2H2, C2H4, C6H6 CH3 + CH3 (+M) - C2H6 CH3 CH3, C2H6

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CH3, C2H6 CH3, C2H4, CH3C2H, C6H6

C2H2, CH3C2H, C4H2, C6H6 CH3, C2H2, CH3C2H, C4H2 CH3, C2H6 CH3, C2H4, CH3C2H, C6H6

C2H2, C2H4, C4H2, C6H6 CH3, C2H6

CH3, C2H2, C2H4, C4H2 CH3, C2H6

C2H2

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reviews are more limited,254 but the Kinetic Database for Astrochemistry (KIDA, http://kida.obs.u-bordeaux1.fr) does contain evaluations of reactions relevant to planetary atmospheres as many reactions are relevant for both interstellar chemistry and planetary atmospheres. A review of gas phase astrochemistry has recently been compiled by Smith.255


8. Summary Approaching an understanding of terrestrial atmospheric chemistry based on a combination of observation, simulation, laboratory studies and modelling is replicated in the study of extraterrestrial atmospheres, albeit with a significant reduction in the detail of the observations. Many of the issues around the laboratory studies of photochemistry, kinetics and reaction dynamics of radical reactions are similar for both applications, but particularly for studies relevant to the hydrocarbon dominated, reducing atmospheres of the giant planets, there are additional challenges posed by the importance of short wavelength photolysis, low temperatures and pressures and highly reactive radical species. Absorption cross sections, as well as rate coefficients, can be strongly temperature dependent and temperature dependent studies should be made wherever possible. Simulations of such environments are particularly challenging. Advances in experimental techniques, such as the increased use of wavelength selective photoionization mass spectroscopy and similar approaches, is increasing the number of reactions for which product distributions are known and increasing the quality of data obtained (e.g. isomer specific product ratios or competition between reaction and relaxation in excited state reactions). The Laval expansion technique provides an excellent method for studying a range of reactions at appropriate temperatures, developments in the Laval methodology will increasingly allow for product detection and may be able to operate at temperatures relevant for the Earth’s UTLS region. Interactions between experiment and theory for all applications of kinetics and dynamics continues to develop.233 Thousands of reactions are needed for comprehensive models of complex environments and experimental determination of all reactions will never be realistic even if appropriate conditions can be achieved. Collaboration between theory and experiment to benchmark and tune calculations is essential to allow for extrapolation of results to appropriate conditions or for accurate ab initio calculations of rates and product yields. Both bulk and molecular beam studies are important. Finally, the discovery of increasing numbers of exoplanets with a wide range of conditions opens up new areas for laboratory study, where high temperature chemistry may be of more relevance and where, in the absence of detailed observations, laboratory studies and modelling will be crucial in developing our understanding of these exciting new (to us!) environments.

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