Properties of Gas-phase Ions

J. Chem. SOC.,Faraday Trans. 2, 1987, 83, 69-87

Properties of Gas-phase Ions Information to be obtained from Photoelectron Spectroscopy of Unstable Molecules

John M. Dyke Department of Chemistry, The University, Southampton SO9 5NH The information to be obtained from the study of unstable molecules in the vapour phase by photoelectron spectroscopy is reviewed, using examples drawn from high-temperature pyrolysis experiments and rapid atommolecule reaction. An example is given of a chemi-ionization reaction studied by electron spectroscopy, and the information to be obtained from such studies is outlined.

Vacuum ultraviolet photoelectron spectroscopy (p.e.s.) is a technique which may be used to investigate ionic states obtained by removal of one electron from a neutral molecule. Because of the energy of the radiation used (typically ca. 20 eV) only valence electrons can be removed and the resolution of the technique (typically ca. 200 cm-') is such that for small molecules only electronic and vibrational information on ions is obtained. Rotational structure is not normally resolved. As a result, p.e.s. is a relatively low-resolution spectroscopic method. Despite this, however, p.e.s. measurements on short-lived gas-phase molecules have provided a wealth of useful information on molecular ions, some of which has not previously been obtained by any other method. In order to study short-lived molecules in the vapour phase a number of preparative methods have been used. These include: ( a ) microwave discharge of a flowing gas mixture, ( b ) use of a suitable gas-phase reaction, typically a rapid atom-molecule reaction and ( c) high-temperature pyrolysis. Of these methods, only high-temperature pyrolysis will be described, as the first two methods are well established techniques which have been extensively utilized in other areas such as gas kinetics. When the vapour pressure above a solid sample is low at room temperature, a study of the vapour phase by p.e.s. will require an increase in temperature of the solid. A range of heating methods are possible in principle, but for a number of technical reasons radio-frequency (r.f.) induction heating has been chosen by the Southampton p.e.s. group as being particularly applicable to photoelectron spectroscopy. 1,2 After experimenting with a number of designs and geometries, the heating arrangement shown in fig. 1 was chosen. In this diagram, the r.f. induction heating assembly is mounted inside the ionization chamber of a photoelectron spectrometer, which in turn is mounted on a large liquidnitrogen-trapped diffusion pump. As shown in fig. 1, an induction coil is wound coaxially around the furnace which contains a solid sample. When an r.f. current is passed along these coils eddy currents are induced in the surface of the furnace and this gives rise to a heating effect. In practice, a pulsed form of the radio frequency is used so that interference effects with the detection electronics can be eliminated by using a suitable gating unit and photoelectron signals are recorded during the 'off' pulse. The solid sample is vaporized downwards into the photon beam of the spectrometer and the photoelectrons produced are sampled at 90"to the photon beam by the slits of the spectrometer and the energy of the electrons is analysed by a 150" hemispherical analyser. With a 16 kW induction heater, for a carbon furnace of wall thickness 1 mm, a maximum furnace temperature of 2800 K can be achieved, whereas with a tungsten 69

70

Photoelectron Spectroscopy of Unstable Molecules

5

0

cm

Fig. 1. Diagram of a radio-frequencyinductively heated furnace assemblyused in high-temperature p.e.s. experiments.',' A, aluminium alloy; B, brass; C, graphite; D, ceramic; E, insulator; F, tungsten; 0, induction coil; 0, water cooling.

furnace of similar dimensions a maximum furnace temperature of 2600K can be obtained. Further details of this heating method as used in photoelectron spectroscopy have been described elsewhere.lY2 A photoelectron spectroscopic study of a molecule produced by high-temperature pyrolysis or a rapid atom-molecule reaction often yields spectroscopic information on a molecular ion which has not previously been obtained by any other method. In order to assess the reliability of the spectroscopic parameters obtained it is useful to consider photoelectron spectra of small molecules, where the ionic states observed have previously been studied by other methods, such as optical emission spectroscopy. If the vibrational frequency in the ground electronic state of the neutral molecule is large with respect to kT, the only vibrational level populated in this state is the lowest vibrational level and vibrational structure is present in the photoelectron band, as shown in fig. 2. For the photoelectron band obtained, the separation of the vibrational components can be measured and values of the spectroscopic constants 6,and 0,x, can be derived, provided that the vibrational energy levels in the ion are assumed to fit an anharmonic oscillator expression. A very simple example where this method has been applied occurs in the He1 photoelectron spectrum of N2. This spectrum shows three bands, which all show clearly resolved vibrational structure, and by measuring the vibrational separations in each band values for the ionic constants, 0,and G,x,, can be obtained. Table 1 shows a comparison of the values of 0,obtained for each ionic state with the corresponding values obtained previously3by the higher resolution method of optical emission spectroscopy. As well as providing separations of vibrational levels in a given state of an ion, the photoelectron spectrum contains further information in that the relative vibrational intensities in a band can be measured and this can be used to calculate the change in equilibrium bond length between the molecule and ion. Hence the equilibrium bond

J. M. Dyke

Ei

71

k-

I-----

/M+

-

Fig. 2. A diagram showing the ionization of M to M+ and the formation of a typical photoelectron band. Table 1. N l spectroscopic constants obtained from photoelectron spectrum

parameter

NT (X22i)

NT (A211,)

N l (B' X:)

re (p.e.s.) re (spectroscopic)"

ionic equilibrium bond length/A 1.115 1.177 1.116 1.175

1.078 1.074

6,(p.e.s.) 6, (spectroscopic)"

vibrational constant/cm-' 2220 1900 2207 1904

2415 2420

a

Ref. (3)

length in the ion can be obtained if the equilibrium bond length in the molecule is known. The method which involves the calculation of Franck-Condon factors for a range of trial ionic equilibrium bond lengths can be summarized in the following way? For a diatomic molecule the intensity of individual components, Iut,ufl, in a photoelectron band is given by:

where

72

Photoelectron Spectroscopy of Unstable Molecules

and

In these equations, qUlruIf is the so-called Franck-Condon factor, defined as the squared overlap of the vibrational wavefunctions belonging to the ion (t,bUf) and the neutral molecule ( t,hutr), integrated over the internuclear distance, r. R+.(r ) is the electronic transition moment and is the expectation value of the electronic dipole moment operator, Me, with respect to the electronic wavefunctions of the ionic (+;) and neutral (+;) states, integrated over the electron coordinates qe. If &(r) is assumed to be a slowly varying function of r and of the kinetic energy of the ejected electron, eqn (1) can be approximated by: in which R, is evaluated at the r centroid, Fut,ugo,a weighted mean of the internuclear separation. In the determination of ionic equilibrium bond lengths from photoelectron band envelopes it is usual to assume that the first term in eqn (4) is a constant ( i e . that the electronic transition moment does not vary over a photoelectron band) and hence the relative intensities of vibrational components in a photoelectron band are determined by the Franck-Condon factors, qur,u8t. In order to calculate values of qur,uet at trial values of the ionic equilibrium bond length the following procedure has been adopted by the Southampton p.e.s. group. (1) A Morse potential is assumed for the molecular and ionic state. This is determined by the parameters G,,Gexeand re in each state. For the neutral molecule, these values are usually already available from other spectroscopic studies, whereas Ge and 6,xe in the ion are obtained from measurement of the vibrational component separations in the photoelectron band. The equilibrium bond length in the ion is set at its trial value. (2) Vibrational energies and wavefunctions are then calculated as numerical solutions of the radial Schrodinger equation. The required values qof,vff can then be obtained from the computed wavefunctions in the initial and final states. (3) The procedure is repeated for different values of re (ion) until good agreement is obtained with the experimental envelope. In fact, a least-squares procedure is used to determine the value of re (ion) which gives the closest agreement with the experimental vibrational component intensities. The ionic equilibrium bond lengths obtained from the three bands observed in the He I photoelectron spectrum of N2 are shown in table 1. As can be seen from this table, the agreement between the values of 6,and re derived from the photoelectron spectrum with the corresponding values obtained from optical emission spectroscopy is good. Similar comparisons can also be made for ionic states observed in the He I photoelectron spectra of NO, CO, O2 and CS, where the ionic states have also been investigated by independent spectroscopic experiments. For the ionic states where clearly resolved, unoverlapped vibrational structure is observed in the photoelectron spectrum, the general result is that when the photoelectron data are used to derive ionic spectroscopic parameters, re values are obtained to within k0.005 A of the spectroscopic value and 6,is obtained to within *30 cm-'. This result indicates that the observed vibrational component intensities are determined almost completely by the quf,urr values and that the variation of the electronic transition moment over the band is small. This assumption can be tested by plotting the observed vibrational component intensities in a band, corrected for the analyser transmission function, divided by the qur,u.tvalue [computed with the spectroscopically determined re (ion) value] against the vibrational numbering in the upper state. This has been done for the second band of N2 (see fig. 3 ) and, as expected, this plot deviates only slightly from a horizontal straight line.

73

J. M. Dyke

0

2

1

v

3

Fig. 3. An investigation of the variation of the electronic transition moment over a photoelectron band for the N l ( A 211,)6 N2(X *Zg)ionization recorded with He I, radiation.

This general result has been obtained for all diatomics that have been investigated by p.e.s. where ionic spectroscopic constants are available from other studies for comparison, with the notable exception of H2, where the variation of the electronic transition moment with internuclear distance has to be taken into account to reproduce the experimental photoelectron relative inten~ities.~’~ As well as the assumption of a constant electronic transition moment over a photoelectron band, two other assumptions are made. First, it is assumed that the electronic states involved can be satisfactorily described by Morse potentials and secondly it is assumed that the photoelectron band is not distorted by autoionization. This latter assumption can be investigated experimentally by recording a photoelectron band with several different photon sources. The application of this method to a spectrum of a short-lived molecule is illustrated by considering the photoelectron spectrum of S2 obtained by heating solid sulphur to 600 K.* The He I photoelectron spectrum of S2 shows nine bands, five of which clearly exhibit resolved vibrational structure (fig. 4). As an example, an expanded scan of the first band of S2 is shown in fig.5. This band, which corresponds to the ionization S, + X 211s + S2 X ’Xi, shows vibrational structure as well as evidence of spin-orbit splitting in the X 211gstate. The ionic parameters obtained from analysis of the S2 photoelectron spectrum are shown in table 2. These values are, of course, subject to an error of k0.005 A in re and *30 cm-’ in G e . The relative positions of the ionic states of S2 observed in the photoelectron spectrum and the ionic spectroscopic constants derived from the photoelectron spectrum allow ionic emission envelopes to be computed. For example, S,f(A21X,)-+ (x21X,) is an optically allowed transition. From the photoelectron data, the energy of transitions from the lowest nine vibrational levels in the A 21Xu state to the lowest nine vibrational

Photoelectron Spectroscopy of Unstable Molecules

74

h

II

h

~

18.0

17.0

16.0

15.0

14.0

ionization potential/eV

Fig. 4. ( a ) The He I photoelectron spectrum of S2(X ’Xi). ( b ) -3 V acceleration. 3000

4

‘m U m

a

s

9.70

9.60

9.50

9.40

9.30

ionization potential I eV

Fig. 5. The band assigned to the Sl(X211,) t S 2 ( X 3 H ; ) ionization ndenotes spin-orbit splitting.

levels in the lower X 211g state can be calculated and the Franck-Condon factors for these transitions can be computed. The advantage of this procedure is that although the position of a ( d - u ” ) transition may be in error by up to 60 cm-’, the relative intensities of the components for a transition from a given vibrational level ( u ’ ) in the upper state is reasonably accurately predicted. This allows the vibrational numbering (d, u ” ) of the bands in the emission spectrum to be established and as a result, improved spectroscopic constants of the S l states can be obtained (see fig. 6). The ST spectroscopic constants

75

28

26

24

22

20

18

16

lo3 energylcm-'

Fig. 6. Comparison of the emission spectrum computed from p.e.s. data' and observed experimentally' for the SZ(A ' n u ) - ( X211g) transition. (-) Optical data, (- - -) p.e.s. data.

Table 2. Spectroscopic constants obtained for S l from p.e.s.8 S,' state 2% 4nu

2Kd "z,

"z,

re/ A a

1.824 (1.823) 2.058 2.047 (2.044) 1.936 1.983

(3Jcm-l

a

790 (807 f 3) 620 547 (551 *3) 58 1 546

Values in brackets are those derived subsequently from optical emission s t u d i e ~ . ~ ~ ~ ~

a

obtained from the emission approximately five years after the initial photoelectron study have been included in table 2. This demonstrates the point that photoelectron measurements are often important precursors to higher-resolution spectroscopic studies. The application of these methods to a molecule produced by a rapid atom-molecule reaction can be illustrated by considering the N F radical produced from the F + N 3 reaction.' '-12 This reaction is sufficiently exothermic to produce both the ground electronic state of N F (the X 3C- state) and the first excited state (the N F a 'A state). Removal of one electron from both of these states produces the NF+ ( X 'n) state and the measurement of the difference in the experimental adiabatic ionization energies (see fig. 7) allowed the separation of the zeroth vibrational levels of the X '2- and a 'A NF) states to be obtained as 1.42*0.01 eV, in agreement with a value of 1.418 eV obtained by observing the a 'A-X 'C- e m i ~ s i o n . ' ~Also, measurement of the vibrational separations in each band allowed 6,to be obtained in the NF+ ( X 'n) state as 1520* 40 cm-' and use of the vibrational component intensities in each band allowed the equilibrium bond length in this state to be derived as 1.180f 0.006 At present this appears to

76

Photoelectron Spectroscopy of Unstable Molecules A r ( H e [ @1

n

NF?X Zfl)--NF(X3E)

m NF+(X 'n)-NF(o'A)

m

14

13

12

1.1

10

9

Ei/ eV

Fig. 7. The first bands observed in the He I photoelectron spectra of NF(X 3X)and NF(a 'A).

be the only experimental determination of these quantities for the NF+ ( X 'II) state, although values of 0, = 1499 cm-' and re = 1.182 A were subsequently computed from ab initio calculations which include the effects of extensive configuration intera~tion.'~ Fig. 8 illustrates the least-squares fitting method used to obtain the equilibrium bond length of a diatomic ion from vibrational component intensities in a photoelectron band. As discussed earlier, Franck-Condon factors are calculated at various trial re values for the NF+ ( X 'II) state and the least-squares error, E u I [Iur(calc) - I,,!(expt)]', is calculated for each value of the ionic re. The least-squares error can then be plotted against re for both the NF' ( X 'n) + N F ( X 'E-) and NF+ ( X 21-1) + NF ( a 'A) band envelopes to yield the plots shown in fig. 8. Ideally the minima of both these curves should occur at the same value of the equilibrium bond length in the ion. As can be seen from fig. 8, however, a small difference was observed and the equilibrium length for NF+ (X 'II) was derived as 1.178k0.004 A and 1.182f0.004 A from the bands associated with the N F ( X 'Z-) and NF ( a 'A) states, respectively. Combining these results gave a value for re of 1.180k 0.006 A for NF+ ( X 'I).' A typical spectrum of a molecule produced by high-temperature vaporization is shown in fig. 9. This shows the spectrum, recorded in the 7.0-8.5 eV ionization energy region, of the VO molecule obtained by heating stoichiometric VO(s) to 1980 30 K. The first band shown in fig.9 corresponds essentially to a metal (4s)-' ionization, whereas the second band is much stronger and corresponds essentially to a metal (3d)-' ionization. These relative intensities are consistent with those seen in the photoelectron spectrum of the metal15316 and the experimental band envelopes are consistent with computed band envelopes obtained from a6 initio molecular-orbital calculations and Hartree-Fock-Slater ~alculations.'~Only the first band in fig. 9 shows vibrational structure and measurement of the vibrational component separations allows 0,to be obtained in the ground state of the ion (the X 3Zcstate) as 1060*40 cm-' and the experimental component intensities can be used to obtain re in the VO+ ( X 'Z-) state as lS4* 0.01 A. The error in re quoted here is larger than that normally quoted because of the possibility of weak 'hot band' contributions to the band shown in fig. 9. This is thought likely as it has been found in previous studies that for the high-temperature

*

77

J. M. Dyke 0.10 0

0.08 N

n n c,

2 0.06

a

v %a

I

-cf

h

ri

0.04

v %a U

w

-9

0.02

0

Y/1,170 '

1

1.1 90

1.180

Fig. 8. Results of the least-squares fit performed at various trial re values for the NF+ ( X 'n) state. The experimental vibrational component intensities are from fig. 7 for (a) the NF+(X 'II) t NF(X 3E-) ionization and ( b ) the NF+(X 'n) t NF( a 'A) ionization.

8.5

7.5

8.0

7.0

Ei /eV

Fig. 9. The first two bands seen in the photoelectron spectrum of VO. ( a ) VO+(A 'A) + VO(X 'E-), ( b ) VO+(X 'E-) t VO(X 'X-).

Photoelectron Spectroscopy of Unstable Molecules

78

- lo01

(a

I Ili v) I

c 0

11.0

10.0

9.0

EJeV

Fig. 10. The first band seen in the He I photoelectron spectrum of PF2, produced as a secondary product of the F+ PH3 reaction.

spectrometer used, evaporation of a metal oxide at a furnace temperature of ca. 2000 K leads to molecules in the photoionization region with a vibrational temperature of ca. 800 K. This difference arises because of collisional deactivation between the furnace and the photoionization region.17 In contrast, similar p.e.s. studies on metals'6-18show that if a metal is evaporated at ca. 2000 K, the electronic excitation temperature of the metal in the photoionization region may be ca. 1500 K, indicating that, as expected, much more inefficient deactivation of electronic excitation on collision occurs than vibrational deactivation. In the study of triatomic molecules by p.e.s., it is not generally possible to determine the equilibrium geometry of the ion from the experimental spectrum as usually at best only one vibrational series is resolved. However, measurement of the adiabatic ionization energy allows the heat of formation of the ion to be determined, provided the heat of formation of the neutral molecule is known. An example of a triatomic band where vibrational structure is resolved and the adiabatic component can be readily identified occurs in the first band of the PF2 As shown in fig. 10, the first band of PF2 shows regular vibrational structure and the adiabatic and vertical ionization potentials were measured as 8.84*0.01 and 9.09*0.01 eV, respectively. On the basis of the bonding character of the molecular orbital from which ionization occurs and the known frequencies of the normal modes in the neutral molecule, PF2 ( X 2B1),20,21 the observed structure in fig. 10 is assigned to excitation of vl ,the P-F stretching mode, in the ionic state, PF,f ( X 'Al). Measurements of the vibrational separations gave (3, = 980 30 cm-' for v l in the PF,f ( X 'Al) state. As observed in the first band of the photoelectron spectrum of NF2,22this value is higher than vl in the ground state of the neutral molecule because ionization occurs from the outermost half-filled level which is antibonding in the P-F direction and bonding the F-F direction. In the case of a triatomic molecule where a large equilibrium geometry change occurs on ionization, identification of the adiabatic ionization energy may be very difficult. Such a case occurs in the first band of the formyl radical where the ionization process corresponds to a transition from a neutral molecule with a bent C, equilibrium structure to an ion with a C,, equilibium geometry. In the first photoelectron study of the HCO radical with a single detector ~ p e c t r o m e t e ra~long ~ vibrational series was seen in the HCO deformation mode, as anticipated. Unfortunately, however, it was not possible to observe the adiabatic ionization energy. A study of this band with a multidetector spectrometer allows two extra vibrational components to be observed in the low ionization energy region of the HCO photoelectron band (fig. 11) with the lowest component

*

J. M. Dyke

9.2

8.8

79

8.4

8.0

8.4

8.0

EJeV

6000

2000

w

~

~.

9.2

8.8

Ei/ eV

Fig. 11. The low ionization energy region of the first band of HCO recorded with ( a ) He1 and ( b ) Ne I radiation.

*

being seen at 8.35 0.01 eV. It is clear, however, that the adiabatic component is not directly observed in fig. 11, as the HCO adiabatic ionization energy has recently been determined from a series of photoionization mass-spectrometric measurements on a number of organic compounds as 8.10k0.05 eV.24 In order to establish the ionic vibrational numbering in the series seen in fig. 11 it is necessary to utilize two pieces of information derived from independent studies; the adiabatic ionization energy of 8.10f 0.05 eV determined by photoionization mass s p e ~ t r o m e t r yand ~ ~ the IJ = 0-1 ( v2) band origin for HCO+ measured by infrared diode laser spectroscopy as 829.72 ~ m - ' . ~ 'This added information shows that two vibrational components are unobserved in fig. 11. Having established the ionic state vibrational numbering, the separation of the observed vibrational components in the photoelectron spectrum can be plotted against (u'+ l ) , where v' is the upper state numbering, to yield (3, = 850 f 25 cm-' and O,x, = 9 f 10 cm-'. The HCO adiabatic ionization energy is then derived as 8.14f0.04 eV. The adiabatic ionization energy of DCO can then be calculated from the HCO value, by making appropriate zero-point energy corrections, and used to determine the ionic state vibrational numbering in the DCO photoelectron spectrum. Plotting the separation of the observed vibrational components in the DCO case against ( v ' + l ) gives (3,= 655 f25 cm-', O,x, = 4 * 10 cm-' and a IJ = 0-1 (v,) band centre of 647 f 25 cm-'. This value is important as u2 in DCO+ has not been measured by high-resolution spectroscopy and it is hoped that the photoelectron work will assist the search for this fundamental band by higher-resolution methods.26

80

Photoelectron Spectroscopy of Unstable Molecules A I*O*

I

15

14

13

12 Ei/eV

11

10

9

8

co+ \

O+

Ar+

70

60

50

40

30

20

10

amu

Fig. 12. ( a ) The He I photoelectron spectrum of A120 and ( b )The electron impact mass spectrum recorded at the same time as ( a ) using 60 eV electron energy.

Some triatomic molecules show no resolved vibrational structure in their photoelectron spectra. Even in these cases, however, it usually proves possible to derive some structural information. An example of this type occurs in the photoelectron spectrum of A1202' obtained by heating an Al-A1203 mixture to 1600K. The photoelectron spectrum obtained [fig. 12(a ) ] was confirmed as being attributable to A120 by recording the mass spectrum of the high-temperature vapour [fig. 12(6)] using electron im act at 60 eV electron kinetic energy simultaneously with the photoelectron spectrum.27 P A120 is an interesting molecule as, although it seems likely from previous spectroscopic studies that the valence isoelectronic molecules Ga20, In20 and T120 have C2" equilibrium geometries, the equilibrium geometry of A120is not well established. If A120has a Dmhequilibrium geometry, it has the following valence electronic configuration:

'

4ai

2 ~ : 6a2, 5a;

and in practice the photoelectron spectrum can be assigned on this basis because four bands were seen. Experimentally, the first two bands were sharp with the same experimental half-width and are assigned to the (5uU)-'and (6a,)-' ionizations. HartreeFock-Slater calculations show that these two ionizations correspond essentially to removal of electron density from the A l 3 s atomic orbitals with a small reduction in

J. M. Dyke

81

Al3p electron density. In contrast, the second two bands correspond to ionization from the 27ru and 4uu molecular orbitals, which are composed mainly of Al3p and 0 2p

contribution^.^^ If the equilibrium symmetry of A120 is lowered from Dmh to C2, then the valence electronic configuration of A120 changes in the following way: 4a2,

I

6a2, 502,

I

I

bl a1 bl. This indicates that five bands would be expected in the photoelectron spectrum. Also, it might be expected that the 2Al determinants obtained by the two (al)-' ionizations would be close in energy and would interact causing the lower 2A1state to acquire some character of the upper 2Al state and vice versa. This effect would be seen in the experimental spectrum by a broadening of the second band compared to the first. However, as already stated, only four bands were seen experimentally and the observed half-widths of the first and second bands were the same. Hence the experimental photoelectron spectrum is consistent with a Dcohequilibrium geometry for the ground state of A120.This result has been confirmed by a6 initio, minimum-energy geometry calculations for this molecule using a double-zeta quality gaussian basis set with two added d-polarization functions on each centre and incorporating the effects of electron correlation via fourth-order many-body perturbation theory.27 After the photoelectron investigation of A12027was complete, a Dmhequilibrium geometry was independently derived from an investigation of the Raman spectrum of A120trapped in an argon matrix28and further a6 initio molecular-orbital calculation^.^^ The type of information to be obtained from the spectra of tetratomic and polyatomic molecules can be illustrated by considering the photoelectron spectra of the hydroxymethyl (CH20H) and methoxy (CH30) radicals.30931These can both be conveniently produced by the reaction of fluorine atoms with methanol, i.e. F+CH,OH

--*

CH20H+HF

F+CH,OH

--*

CH,O+HF

AH?& = -163 kJ mol-' = -141

kJ mo1-l.

(5)

(6)

Hydroxymethyl is more stable than the methoxy radical and, at a reagent mixing distance of 1.0cm above the photon beam, the first band of C H 2 0 H can be clearly identified (see fig. 13). As can be seen from fig. 13, the band assigned to CH20H shows one vibrational series with evidence of a second series. The main series could be analysed to give 6,= 1650+ 30 cm-' for the vibration excited, whereas tFe second series had an average separation of 1370*30 cm-' (see table 3). In the ground electronic state of CH20H, the half-filled molecular orbital can be described in terms of a C 2p,-O 2p, antibonding atomic orbital combination with C-H bonding and H-H bonding contributions in the CH2group. On this basis, the 1650 cm-' structure is assigned to excitation of a C-0 vibrational mode increased from the neutral molecule C-0 value of 1183 cm-', whereas the 1370 cm-' structure is assigned to a CH2 deformation mode decreased from the value of 1459 cm-' in CH20H ( X 2 A ) . In CD,OH, two series were again observed (see fig. 14). The main series now increases slightly and analysis of the component separations yields 6,= 18 10 f 30 cm-', whereas the smaller series decreases to yield an average separation of 1100 f30 cm-'. The decrease in the average vibrational separation of the second series from 1370 to 1100 cm-' on going from CH,OH+ to CD20H+is consistent with assignment of this structure to excitation of a CH2 deformation mode. However, the slight increase in the C-0 mode on deuteration is more difficult to explain. It is thought, however, that this is due to a 'C-D' stretchkg mode lying below the 'C-0' mode in (CD,OH)+. These modes interact strongly causing the 'C-0' mode to be increased slightly. Some support

82

Photoelectron Spectroscopy of Unstable Molecules

1000

I

9.5

9.0

8.5

8.0

I

7.5

Ei/ eV

Fig. 13. The first band of the CH,OH radical produced from the F+CH,OH reaction.

Table 3. Summary of the data obtained far the first p.e. bands of the CH20H and C H 3 0 radical^^'*^'

ionization energy/ eV radical

vertical

adiabatic

(3,/cm-'

c/cm-'

CH20H CH20D CDZOH CD2OD CH30

8.14*0.01 8.14*0.01 8.13*0.01 8.14*0.01 8.13*0.02

7.56*0.01 7.55*0.01 7.55*0.01 7.56*0.01 7.37*0.03"

1650*30 1610*30 1810*30 1770*30 1530*40

1370 30 1390* 30 ll00*30 1130*30

*

-

Obtained from analysis of main vibrational series; see for example fig. 13 and 14. Average separation of secondary series; see fig. 13 and 14. " Band onset. Average separation of vibrational structure; see fig. 15. (I

for this suggestion has been provided by force-field calculations which have been performed on CH20H+ ( X ' A ) and CD20H+ ( X ' A ) . Also, the suggested assignments are consistent with the vibrational structure observed in the photoelectron spectra of other deuterated CHzOH species (see table 3). The main results to be derived from this study are an improved value for the first adiabatic ionization potential of CH20H and measurement of two of the vibrational frequencies in CH20H+ ( X ' A ) . Also, since it appears from the literature that the heat of formation AH&,8 of the ion, as calculated from the proton affinity of f ~ r m a l d e h y d e , ~ ~ is better determined than the heat of formation of the neutral molecule, use of the p.e.s. value for the adiabatic ionization energy of CH20H of 7.56k0.01 eV with AHgg8(CH20H+)of 7.30 f0.12 eV32leads to a redetermination of AH&8(CH20H) of -0.25 f0.13 eV. It should also be noted that some weak structure was seen in the F + C H 3 0 H spectra in the 8.5 eV ionization energy region (see fig. 13 and 14). This structure was not seen

83

J. M. Dyke 3500

e 'v)

U v)

1

8

0 8.5

9.0

8.0

7.5

Ei/ eV

Fig. 14. The first band of the CD20H radical produced from the F+CD,OH reaction.

CH,O ( H e I )

1

9.0

I

L

8.0

1

7.0

EJeV

Fig. 15. The first band of the methoxy radical obtained from pyrolysis of dimethyl peroxide.

in spectra obtained from the Cl+CH,OH reaction, which is less exothermic than the F+CH,OH reaction and produces CH20H as the only detectable radical product. It was thought that this structure at ca. 8.5 eV ionization energy may be due to the methoxy radical and, as a result, attempts were made to make C H 3 0 in the absence of CH20H. A suitable way of achieving this is via pyrolysis of dimethyl peroxide and a spectrum obtained from this route is shown in fig. 15. As can be seen a broad band was observed centred at 8.13 f 0.02 eV ionization energy and this has been assigned to the CH30+ (X 3A2)+CH,O(X 2 E ) ionization. The vibrational series seen in fig. 15 had an average separation of 1530 40 cm-'. Assignment of this structure can be achieved in an analogous way to assignment of the

*

84

Photoelectron Spectroscopy of Unstable Molecules

Fig. 16. A schematic representation of chemi-ionization in the 0 + CH reaction.

main series in the CH20H first band, as the molecular orbital from which ionization occurs is antibonding in the C-0 direction. This structure is therefore assigned to excitation of the C-0 stretching mode in the ion, with a vibrational frequency increased over the corresponding value (1053 cm-l) for the neutral molecule. Having described some existing examples of photoelectron spectroscopy of unstable molecules, it is useful to describe one relatively new area of electron spectroscopy where information on small molecular ions can also be obtained. This involves the use of electron spectroscopy to study chemi-ionization. A chemi-ionization reaction can be defined as a reaction in which new chemical bonds are formed and the number of charge carriers increases. Reactions of this type in which electrons are produced have been termed chemi-electron reactions.33 The only chemi-ionization reaction to be investigated previously by electron spectroscopy is the reaction of oxygen atoms with the CH radical, i. e. O+CH

+

HCO++e-.

(7)

Jonathan et al.33measured the electron energy distribution obtained from the reaction and obtained a sharp, unstructured band centred at 0.23h0.01 eV. As the heats of formation of 0, CH and HCO+ are well estab!ished, the exothermicity of reaction (7) can be calculated readily as 0.23eV, in good agreement with the maximum in the experimental electron distribution for the 0 CH reaction. With the aid of potentialenergy surfaces computed for HCO and HCO+ by MacGregor and Berry,34the 0 + CH reaction can be envisaged as occurring as shown in fig. 16. As illustrated in this diagram, the 0 and CH reactants can combine to give either HCO in its ground state or HCO in an excited state. For certain reaction coordinates, the HCO excited state will lie above HCO+ in its ground state and, as a result, electron emission will occur. The maximum in the electron kinetic energy of the ejected electrons will correspond to the line marked (A) on fig. 16. Unfortunately, the observed electron energy d i ~ t r i b u t i o n ~ ~

+

J. M. Dyke Ce+O,

85

photon source off on

(He I )

I-

’

O:(b4E,)

‘ m

2000

I I

I I I I I I ,-

I

‘v) v) c ,

I

1

I

0

I

I

I I

I I I

0 0

1

2

kinetic energy/eV

Fig. 17. The electron energy spectrum obtained from the C e + 0 2 reaction.

showed no vibrational structure and hence it was not possible to obtain any additional information on HCO+ over that which can be obtained from direct ionization of HCO. At the time that the first chemi-electron study was made, extension of this method to the study of other reactions proved difficult. This arose because most known chemiionization reactions which yield electrons involve reactions of metals, produced in the vapour phase by heating the solid metal, with a suitable oxidant. Unfortunately, the high-temperature capability in electron spectroscopy had not at that time been developed. However, with the development of high-temperature pyrolysis methods in p.e.s., these reactions can now be studied and should provide an alternative way of probing the ground state of metal oxide ions other than direct ionization from the neutral molecule. An example of a spectrum obtained from a reaction of this type is shown in fig. 17, which was recorded for the Ce+0, reaction. The chemi-electron band observed has a peak maximum of 0.90 f 0.04 eV and showed clear vibrational structure with an average separation of 790* 30 cm-’. The way in which this spectrum is interpreted is shown in fig. 18. As shown in this diagram, transitions take place from a CeO, excited state to CeOl, with the maximum observed electron energy corresponding to a transition to the lowest vibrational level of CeOl. Measurement of this quantity should allow the heat of formation of CeOl to be obtained and hence the adiabatic ionization energy of CeO, can be determined if the heat of formation of the neutral molecule is known. Also, if, as seems llkely,35336 the electronically excited state of CeO, shown in fig. 18 and the ground state of Ce0; have C2vequilibrium

Photoelectron Spectroscopy of Unstable Molecules

86

Fig. 18. A schematic representation of chemi-ionization in the Ce + O2 reaction.

structures, then the observed vibrational structure can be assigned to excitation of the v1 mode in the ion. Other metal plus oxidant reactions are currently being studied by chemi-electron spectroscopy and it is anticipated that the information obtained on metal-oxide ions will complement that obtained by direct ionization. It is a pleasure to acknowledge that the results used in this report have been obtained by members of the Southampton p.e.s. group, both past and present. The current research group consists of Dr Alan Morris, Vivienne Butcher, Andrew M. Ellis, Miklos Feher, Steven Harris, Julian Stevens and Hadi Zamanpour. Dr Alan Morris is acknowledged for skillfully designing and building the apparatus used in these studies and Prof. Neville Jonathan is thanked for numerous stimulating discussions. This research has been financed by grants from the S.E.R.C. and C.E.G.B. and was also supported in part by the Air Force Office of Scientific Research (grant no. AFOSR-83-0283) through the European Office of Aerospace Research (EOARD), United States Air Force.

References 1 D. K. Bulgin, J. M. Dyke, N. Jonathan, E. Lee and A. Morris, J. Electron. Spectrosc. Relat. Phenom., 1977, 12, 67. 2 A. Moms, J. M. Dyke, M. P. Hastings, G. D. Josland and P. D. Francis, High Temperature Science, 1986, accepted for publication. 3 (a)A. E. Douglas, Can. J. Phys., 1952,30,302; ( b ) E. A. Colbourn and A. E. Douglas, J. MoZ. Spectrosc., 1977, 65, 332; (c) K. P. Huber and G. Herzberg, Molecular Spectra and Molecular Structure IV (Van Nostrand, New York, 1979). 4 J. M. Dyke, N. Jonathan and A. Moms, Int. Rev. Phys. Chem., 1982, 2, 3. 5 J. M. Dyke, N. Jonathan and A. Moms, Electron Spectroscopy (Academic Press, London, 1979), vol. 3, p. 189. 6 J. Berkowitz and R. Spohr, J. Electon Spectrosc. Relat. Phenom., 1973, 2, 143.

J. M. Dyke 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36

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Y. Itikawa, J. Electron Spectrosc. Relat. Phenom., 1973, 2 , 125. J. M. Dyke, L. Golob, N. Jonathan and A. Moms, J. Chem. SOC.,Faraday Trans. 2, 1975,71, 1026. M. Tsuji, I. Murakami and Y. Nishimura, Chem. Phys. Lett., 1980, 75, 536. A. J. Capel, J. H. D. Eland and R. F. Barrow, Chem. Phys. Lett., 1981,82,496. J. M. Dyke, A. E. Lewis, N. Jonathan and A. Morris, Mol. Phys., 1982,47, 1231. J. M. Dyke, A. E. Lewis, N. Jonathan and A. Moms, J. Chem, SOC.,Faraday Trans. 2, 1982,78, 1445. W. E. Jones, Can. J. Phys., 1967, 45, 21. M. BettendorfT and S. D. Peyerimhoff, Chem. Phys., 1985,99, 55. J. M. Dyke, B. Gravenor, M. P. Hastings and A. Morris, J. Phys. Chem., 1985,89, 4613. J. M. Dyke, 8. Gravenor, M. P. Hastings, G. D. Josland and A. Moms, J. Electron Spectrosc. Relat. Phenom., 1985,35,65. J. M. Dyke, B. Gravenor, R. A. Lewis, G. D. Josland and A. Moms, Mol. Phys., 1984,53,465. J. M. Dyke, B. Gravenor, R. A. Lewis and A. Moms, J. Phys. B, 1982, 15,4523. J. M. Dyke, A. Moms and A. M. A. Ridha, unpublished results. J. K. Burdett, L. Hodges, V. Dunning and J. H. Current, J. Phys. Chem., 1970,74, 4053. D. Solan, Ph. D. Thesis (Brooklyn College, City University of New York, 1965). ( a ) A. B. Cornford, D. C. Frost, F. G. Herring and C. A, McDowell, J. Chem. Phys., 1971, 54, 1872; ( b ) A. B. Cornford, D. C. Frost, F. G. Hemng and C. A. McDowell, Faraday Discuss. Chem. SOC., 1972, 54, 56. J. M. Dyke, A. Moms, N. Jonathan and M. J. Winter, Mol. Phys., 1980, 39, 629. J. C. Traeger, Znt. J. Mass Spectrom. Ion Processes, 1985, 66, 271. P. B. Davies and W. J. Rothwell, J. Chem. Phys., 1984, 81, 5239. K. Kawaguchi, A. R. W. McKeIlar and E. Hirota, J. Chem. Phys., 1986, 84, 1146. J. M. Dyke, M. Feher, M. P. Hastings, A. Moms and A. J. Paul, Mol. Phys., 1986, 58, 161. I. V. Ovchinnikov, L. V. Serebrennikov and A. A. Maltsev, Russ. J. Phys. Chem., 1985, 59,923. V. G. Solomonik and I. G. Sazonova, Rum. J. Znorg. Chem., 1985, 30, 1100. J. M. Dyke, A. R. Ellis, N. Jonathan, N. Keddar and A. Morris, Chem. Phys. Lett., 1984, 111, 207. J. M. Dyke, A. R. Ellis, N. Jonathan and A. Moms, unpublished results. K. Tanaka, G. I. MacKay and D. K. Bohme, Can. J. Chem., 1978, 56, 193. N. Jonathan, A. Moms, M. Okuda and D. J. Smith, J. Chem. Phys., 1971,55,3046. M. MacGregor and R. S. Berry, J. Phys. B, 1973, 6, 181. S. D. Gabelnick, G. T. Reedy and M. G. Chasanov, J. Chem. Phys., 1974,60, 1167. R. L. DeKock and W. Weltner, J. Phys. Chem., 1971, 75, 514. Paper 6/1202; Received 13th June, 1986