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Threshold electron impact ionization of carbon tetrafluoride, trifluoromethane, methane and propane

This content has been downloaded from IOPscience. Please scroll down to see the full text. 2000 J. Phys. B: At. Mol. Opt. Phys. 33 4263 (http://iopscience.iop.org/0953-4075/33/20/306) View the table of contents for this issue, or go to the journal homepage for more

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J. Phys. B: At. Mol. Opt. Phys. 33 (2000) 4263–4283. Printed in the UK

PII: S0953-4075(00)50715-9

Threshold electron impact ionization of carbon tetrafluoride, trifluoromethane, methane and propane T Fiegele, G Hanel, I Torres, M Lezius† and T D M¨ark‡ Institut f¨ur Ionenphysik, Universit¨at Innsbruck, Technikerstr 25, A-6020 Innsbruck, Austria Received 8 May 2000, in final form 13 July 2000 Abstract. We have studied the ionization behaviour close to threshold for the fragment ion production from CF4 , CHF3 , CH4 and C3 H8 . In particular, we use a reliable method to extrapolate weak ion signals towards the appearance energy. Our evaluation is based on an extension of Wannier’s law for the ionization of atoms towards the case of polyatomic molecules. In the experiment we use a hemispherical electron monochromator with an energy resolution of ≈135 meV. The vibrationally cold target molecules are prepared in a supersonic expansion and the resulting ions are mass analysed using a quadrupole mass spectrometer. We observe several disagreements with recommended ionization energy values, reaching up to 5 eV in the CHF3 case. This is in contrast to a number of recent reliability tests that we have performed on Ne, Ar, Kr, Xe, H2 , D2 , N2 , O2 and N2 O and that have reproduced the tabulated literature data within the resolution of the monochromator.

1. Introduction The determination of reliable ionization and appearance energies (AEs) is of crucial importance to the understanding of high-energy chemical processes. The presence of energetic electrons and photons leads to the production of very reactive fragment ions and radicals, which are then able to trigger a series of consecutive physico-chemical processes. Such phenomena are for example present in the upper layers of the Earth’s atmosphere or in technical applications such as plasma processing in the semiconductor industry, gas discharges, pulse power switching, gaseous dielectrics etc. Today, in many of these applications the family of fluorinated hydrocarbons plays an important role. In particular, carbon tetrafluoride (CF4 ) is currently used in manufacturing (see [1] and references therein). The gas is technically useful as an important source for very reactive species such as neutral F atoms, and for this reason it is used in various semiconductor etching processes [1]. However, CF4 is a severe greenhouse gas and sometimes it would be desirable to substitute it by trifluoromethane (CHF3 ), which has a 200 times shorter lifetime in the Earth’s atmosphere [2]. Nevertheless, in order to replace CF4 by CHF3 , the latter has to be well characterized with respect to high-energy chemical behaviour. Today, the number of data available on CHF3 is much less than what we know about CF4 [2]. While most fluorinated hydrocarbons are recent man-made compounds that are increasingly contaminating important layers of the atmosphere, methane (CH4 ) and propane (C3 H8 ) are natural gases which are emitted from areas such as rice cultures, forests and farms and also from industrial areas and gas pipelines. Despite their natural origin, today † Also adjunct Professor at the Comenius University, Bratislava, Slovak Republic. ‡ To whom correspondence should be addressed. 0953-4075/00/204263+21$30.00

© 2000 IOP Publishing Ltd

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a growing number of sources for these compounds are man-made. Thus, such hydrocarbons are increasingly present in the atmosphere, but they usually react away very rapidly with OH radicals and their influence on the Earth’s climate is much less severe than that of fluorinated hydrocarbons. On the other hand, electron collisions with hydrocarbon molecules are important in many other fields of science and technology, ranging from astrophysics [3] to semiconductor processing [4] to fusion plasmas [5]. For instance, almost all currently operating fusion devices contain carbon as one of the plasma-facing materials. Chemical erosion surface processes caused by plasma–wall interactions are an abundant source of hydrocarbon molecules contaminating the hydrogenic plasma. The composition of hydrocarbon fluxes entering the plasma covers a wide spectrum of molecules from CH4 to C3 H8 [6]. The emission of more complex hydrocarbons such as C2 H6 and C3 H6 becomes increasingly significant as the impact energy of plasma ions impinging on the surface decreases [7]. Despite this we present our data on methane and propane here as a consistency check for the experiments we have performed on the fluorinated hydrocarbons. The appearance energies of methane and propane for parent and fragment ion formation can be compared to a number of elaborated works in the literature, most of which are listed in [8]. Nevertheless, with our data evaluation technique we are able to give new insight into the ionization of such hydrocarbons, especially regarding the approximate form of their partial ionization cross sections very close to the threshold. The accurate determination of ionization and fragmentation energies is an important basis for the understanding of many electron impact phenomena (for recent reviews on electron–molecule collisions see e.g. [9, 10]). In particular, most of the high-energy chemical processes are initiated by the interaction of medium energy electrons (5–50 eV) with matter. Consequently, the accuracy of our knowledge of electron–molecule interactions in this energy range is most relevant. In this respect it is important to note that electron impact should generally lead to a vertical instead of an adiabatic excitation. The reason is that the incoming electron travels through the molecular system in an extremely short period. More illustratively, a classical electron with an energy of 1 eV passes a molecule with a diameter of 10 Å in approximately 1.7 fs. Thus, for energies sufficiently large to induce electronic excitation and/or ionization (>6 eV), the collision time easily reaches the attosecond time regime. These timescales are about two orders of magnitude faster than most vibrational periods of molecules. Nevertheless, in this context it should be mentioned that it was recently reported by Hildenbrand [11] that electron impact with a number of molecules unexpectedly seemed to agree with the adiabatic rather than with the vertical ionization energies. Within the framework of the involved collision timescales this energetics cannot be understood, at least if we assume that the adiabatic and vertical energies are different for the cases observed in [11]. Instead, more complicated intermediate complex formation and internal conversion processes are probably responsible for such a result. After many years of research the accurate determination and interpretation of threshold energies at which a molecule is ionized after electron impact remains difficult [11–13]. Reasons for this are a number of technical obstacles, and, additionally, the complicated physical situation of a quantum mechanical many-body system. Regarding the technical obstacles first, it is a challenge to prepare electrons with a precisely defined, constant energy resolution that is of sufficient quality and current intensity to produce enough ion signal close to the onset of ionization. Even if it is possible to achieve acceptable experimental conditions, there are numerous possibilities that the situation changes and/or deteriorates during the experiment. ‘Sticky’ gases tend to contaminate the sensitive electron optics with insulating dielectric layers. Non-constant electromagnetic fields or local space charge can deteriorate the quality of the electron beam. Power supplies, vacuum conditions and temperatures have to be kept constant to a very strong degree. The use of spectroscopy, or photoionization experiments, often appears to

Threshold electron impact ionization of CF4 , CHF3 , CH4 and C3 H8

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be technically less difficult [12]. Nevertheless, it is important to note that the threshold energies derived from photoionization experiments are not directly comparable to those obtained by electron impact. Both processes are intrinsically different, because the transition complex is not the same. In a photoionization event one electron is kicked into the vacuum from the platform of a neutral molecule. Instead, the reactive complex during electron impact ionization is a highly excited anion state that is subsequently decaying (presumably much faster than any vibrational period) under the ejection of two electrons. 2. Theoretical background The nature of the underlying many-body process close to ionization threshold was successfully described nearly 50 years ago by Wannier [14] for the case of electron impact ionization of hydrogen. Originally, Wannier divided the problem into three different radial zones: the reaction zone (between 0 and 1 Bohr), where quantum mechanics has to be applied, the Coulomb zone (up to 1 µm), where the escape of the electrons is influenced by Coulomb forces, and the outermost free moving zone (up to ∞), where all particles essentially move freely. Wannier’s elaborated treatment of the phase space situation lead to several important postulates. First, he finds that ionization proceeds via an asymptotic double escape at an angle of π between the two leaving electrons. This basically means that the final angular momentum of the system is L = 0. Second, both electrons seem to share their energy equally. In other words, they leave the reaction zone (on opposite sides) at approximately the same time and with nearly identical energies. Finally, for the hydrogen atom, Wannier was able to predict that the ionization cross section should follow an exponential law (Wannier law) in the threshold region: σ (, µ) ∝  µ/2−1/4 ≡  p µ(Z) = 21 [(100Z − 9)/(4Z − 1)]1/2 .

(1)

Here,  is the energy above the ionization energy and Ze is the charge of the ion after ionization. It should be noted that for Z = 1 the coefficient µ = 2.75 ≈ e, and that the Wannier exponent p ≈ 1.127. Twenty years later, Rau [15] and Peterkop [16] were able to extend the quantum mechanical treatment of the problem into the Coulomb region, but they essentially confirmed Wannier’s law. Moreover, Roth [17] suggested that in principle Wannier’s formalism could be extended to systems with a final angular momentum L = 1. In 1973, Temkin [18, 19] started a different approach, in which he included non-classical coupling of the electrons outside the reaction zone. He concluded [20] that the cross section should follow a modulated quasi-linear law which is extendable to L > 1:
  σ () ∝ (ln )−2 1 + CL sin(αL ln  + µL ) (2) L −1/2

with αL = [RL − L(L + 1) − CL = RL , µL ≈ 0 and RL is the lower limit of the Coulomb–dipole approximation (7 < RL < 160) [20]. Compared to Wannier, Temkin’s treatment was essentially based on the Coulomb–dipole interaction, a case in which one electron is close to the nucleus, and the other one is comparably far away. Therefore, his approach leads to an unequal energy partitioning between the two departing electrons. It should be noted that, in contrast to Temkin’s postulates, during the 1980s three high-resolution experiments essentially confirmed Wannier’s most important predictions, the exponential law (1) and the asymptotic double escape [21–23]. On the other hand, an accurate numerical treatment of the problem became possible after the rapid growth of computational speed in the previous decade. In 1993 Bray and 1 1/2 ] , 4

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Stelbovics [24] were able to introduce the convergent close-coupling (CCC) method. Their algorithm performed a full quantum mechanical treatment of the three-body H + e system. As a result, cross sections up to 500 eV initial electron energy could be calculated with an astonishing 5% accuracy. In the same period also several other authors made important theoretical contributions to the problem [25–29], and some of them have even proposed new, or modified threshold laws. For example, one of the more recent studies based on CCC by Watanabe et al [30] suggested an exponential law of the form σ ∝ exp[−γ  −λ + G(E)]

(3)

where λ ≈ and G(E) is a function that solely depends on the higher excited hydrogen states. Up to today, nearly all of the theoretical treatments remain restricted to the fundamental threebody reaction H + e → H+ + e + e. For this system, at the current refinement of the theoretical treatment possible differences between the threshold laws and the available experimental data become nearly indistinguishable due to today’s state-of-the-art electron monochromator technology. Regarding the description of more complex systems, Klar [31] has elaborated an extension of Wannier’s law towards the more general reaction type 1 6

−

A± + BC → A± + BC∗ → A± + B+ + C .

(4)

Klar [31] suggested modifying the coefficient µ and including information about the mass and charge of the leaving fragments (B : (Z, m) and C : (Z3 , m3 )):  (100Z3 − Z)m3 + 128Z3 m 1 µ= . (5) σ () ∝  αµ−1/4 2 (4Z3 − Z)m3 The coefficient α should be 21 for symmetric and 23 for antisymmetric wavefunctions. However, Klar’s approach remains limited to three-body systems and cannot in principle be applied to polyatomic molecules that have numerous accessible internal degrees of freedom. Nevertheless, an important result of Klar’s work is that the modified Wannier exponent is strongly dependent on the collision system and the final re-partitioning of energy and mass. In [31] exponents p = αµ − 41 are listed for (5) that vary between 1.126 89 (e + infinite nuclear mass) and 961.71 (7 Li3+ + 7 Li2+ → 2 7 Li3+ + e). As a first step towards a deeper understanding of how Wannier’s description can possibly be extended to more complex many-body collisions, we present here an experimental approach in which we demonstrate the possibility to determine close to the appearance energy the exponent p for a number of exemplary cases. Here, we investigate in particular the ionization and fragmentation behaviour of polyatomic molecules. As an additional outcome we are able to provide improved data regarding the appearance energies in a number of cases where we find that the previous tabulated values are of insufficient accuracy. Nevertheless, it is important to understand that our present approach starts from a rather phenomenological point of view. We are aware that the results for the exponents p perhaps do not have a simple physical basis. An important reason is that additional ionization channels close to the threshold lead to a superposition of several individual Wannier functions:  σ (E) = Ai exp{pi ln0 (E − Ei )} i

pi = αi µi −

1 4

ln0 (E − Ei ) =



E
Ei : E > Ei :

−∞

(6)

ln(E − Ei ).

As an example, the situation is sketched for the case of three states close to the threshold region in figure 1. It can be seen that the individual states lead to an additional increase of the

σ[arb.units]

Threshold electron impact ionization of CF4 , CHF3 , CH4 and C3 H8 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 -1

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MLA of Model: A1*exp(p1*ln(E-E1)) A1 1.499 –0.07386 p1 1.690 –0.03585 11.013 –0.05843 E1

10

11

12

13

14

15

Electron Energy [eV] Figure 1. Simulation of a sum cross section σ (E) produced via three different states starting at thresholds Ei = 11.0, · · · · · ·, 12.0, - - - - and 13.0 eV, —— with individual Wannier coefficients pi = 1.13, 1.26 and 1.35, respectively. A small random scatter was added to the points ( ). The thick solid curve was obtained using a Marquart–Levenberg algorithm (MLA) fitting technique (see text) with the model function σ (E) = A exp{p ln(E − E1 )}. It can be seen that the fit reproduces the smallest threshold within 25 meV and the simulated data points within an energy interval of 3 eV above the threshold E1 .

◦

cross section above their respective threshold energies Ei . The probability of each individual contribution is given by the coefficients Ai , which are in principle functions of the transition matrix elements for each individual reaction pathway i. In general, Ai and pi will be unknown for an arbitrary system under observation. Even with ‘infinitely good’ resolution of the kinetic electron energy E only in a few cases will the change of the slope of the cross section enable the experimentalist to identify the energies Ei unambiguously, and to extract accurate values for pi and Ai from the experimental data (a possible example would be the excellent work of Williams and Hamill [32] on a series of hydrocarbons). Instead, from the dataset a global p can be derived. In a good approximation p should model the overall shape of the cross section in the threshold region in close correspondence to the lowest threshold energy level E1  Ai exp{pi ln0 (E − Ei )}. (7) A¯ exp{p ln0 (E − E1 )} ≈ i

Using this approximation, the result of E1 should be closer to the threshold compared to conventional linear extrapolation techniques. Moreover, for an energy interval that is chosen smaller than |Ei>1 − E1 |, the value for the coefficient p is maintained and will correspond to p ≈ p1 . Thus, in principle it will be possible to obtain p1 by successively choosing smaller energy intervals above E1 from the same dataset until p approaches a somewhat constant value. Unfortunately, with this approach the error on p also increases dramatically due to statistical effects (fewer and fewer data-points are used). Therefore, such a convergence method can only be applied if the dataset has an extremely small scatter and Ei>1 is sufficiently larger than E1 .

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Figure 2. Sketch of the experimental set-up used in the present experiment. A neutral molecular beam is produced via adiabatic expansion in the left-hand vacuum chamber. The beam enters the centre of the apparatus passing through a molecular beam skimmer. It is then crossed by a quasimonoenergetic (135 meV) electron beam obtained from a hemispheric monochromator. The ions produced by electron impact are subsequently analysed with a quadrupole mass spectrometer.

3. Experiment 3.1. Apparatus The experimental apparatus has been described in detail elsewhere [13]. In short, we have combined a molecular beam source with a home-built hemispherical electron monochromator and a commercial quadrupole mass analyser (Balzers QMG 421C). The experimental arrangement is depicted in figure 2. Typical FWHM electron monochromator energy spreads at large enough electron currents to measure small cross sections in the threshold region are approximately 135 meV. Note that better resolution (best values reaching 35 meV [33]) is possible but leads to significantly lower electron currents. The performance of the monochromator experiment was reached after careful attention to a number of technical details. The complete monochromator plus the sample inlet system has been constructed from identical material (stainless steel) to improve the uniformity of surface potentials. Such potentials are produced from contamination with vacuum pump oils or test gases and can lead to significant drawbacks in the day-to-day quality of operation of sensitive high-resolution electron monochromators. With frequent bake-outs it was possible to improve the situation significantly. Additionally, we have introduced differential pumping between the sample inlet and the analyser chamber. In this way the background pressure in the monochromator region can be kept below 10−5 Torr, which also reduces the effects of surface contamination. Finally, it was very important to control the magnetic fields in the environment of the monochromator. Instead of µ-metal shielding we have installed an active compensation technique with instrument size solenoids. The sample is prepared as a molecular beam by supersonic expansion in a separately pumped chamber, a technique that is well described by Scoles in [34]. Using molecular beams intrinsically leads to a large forward momentum of the molecules which are passing through the ionizing region. Our quadrupole mass analyser is allocated in line with the molecular beam after the crossing with the electron beam. In this way the ions are collected in the forward direction with respect to the molecular beam and the ion extraction fields can be kept

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minimal. Such a geometry is important because it minimizes shifts of the absolute energy and deterioration of the energy distribution in the ionization chamber. 4. Data analysis In the case of complex many-body systems, especially regarding the polyatomic molecules we are investigating here, the Wannier exponent p ≈ 1.127 represents a lower limit due to the multi-dimensional phase space situation. More rigorously, a significant number of electronic, vibrational and rotational levels have the possibility to contribute to the ionization efficiency in the threshold region. As discussed in the theoretical part, this leads to a stronger power dependence as compared to the atomic case. Unfortunately, for arbitrary molecular cases the energy partitioning and thus the exponent p is usually unknown. However, it is possible to treat p as a variable and to extract it from the experimental data. In such an approach one should restrict p to between 1.127 and 4.0, because higher exponents are not common in the case of single ionization. The simplest possible way to obtain p, already proposed by Wannier [14], would be to raise the data-points close to threshold to the power of 1/p, and to vary p until a straight slope up to about 2 eV above threshold is obtained. The line can be fitted to the rescaled data-points and the intersection with the constant background signal will define the AE. However, this approach leads to a slight systematic overestimate of p and E1 if the dataset contains a large amount of background signal (see [35]). In the present study we have elaborated a more sophisticated approach. We used a weighted nonlinear least-squares fitting procedure on our raw data, applying a Marquart–Levenberg algorithm (MLA) [36, 37]. The fitting function (8) is chosen in close relation to the Wannier law (1) and its approximated extension to superimposed states (7), respectively:  b E < E1 : (8) f (E) = E  E1 : b + c(E − E1 )p . The parameter b describes the underlying constant background. In the fitting algorithm the parameters b, c, E1 and p are varied until an optimum overall agreement of the trial function with the data is reached. With this approach, both the poly-atomic Wannier exponent p and the threshold energy E1 can be extracted from the experiment. As an advantage, the MLA allows us to weight the data-points with different importance. In our case we have chosen a weighting function 1/(n + 1) where n is the total number of counts per energy bin above the threshold E1 . Thus, the weak ion signals close to threshold are not suppressed and an overestimation of E1 can be avoided. More recently, our data fitting technique has been improved by the determination and deconvolution of the electron energy distribution. For this, we assume that the electron energy distribution is approximately a Gaussian distribution centred around E1 with a full width at half maximum (FWHM) of E. Then the fit function (8) can be numerically convoluted with the electron energy distribution  ∞ (E−E1 )2 e− 2E (b + c(E − E1 )p ) dE. (9) P (E) = −∞

Now, we calibrate the experimental data using xenon in two ways. First, we correct our result for the Xe+ AE with the tabulated value of 12.129 87 eV [38], thus obtaining an absolute energy scale. Xenon is advantageous, because all higher excited ionic states are more than 1.3 eV above the AE for Xe+ . Therefore such states contribute in a negligible way to the cross section at the threshold and Wannier’s law with a coefficient p = 1.127 should be an applicable model. Second, a comparison between the theoretical and the experimental xenon

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Ion signal (arb. units)

1.0 0.8 0.6

0.05 0.4 0.2

0.00

0.0

12.0 11

12

12.1

12.2

13

Electron energy (eV) Figure 3. Evaluation of the experimental data on xenon for electron impact ionization near threshold with the theoretical cross section calculated using Wannier’s power law (1) and an electron energy distribution of 135 meV (——, left-hand side). In the magnified region (right-hand side) it can be seen that the theoretical cross section (——, right-hand side), after convolution with the electron distribution of 135 meV ( , right-hand side), corresponds well with the experimentally obtained data-points ( , right-hand side).

◦

Table 1. Experimentally determined appearance energies and Wannier exponents p for electron impact ionization of Xe, Ar, Kr, H2 , O2 , N2 and N2 O in comparison with earlier electron impact studies (EIS) and data recommended by the US National Institute of Standards (NIST) [8]. Molecule

Present

p

Earlier EIS

Recommended value [8]

Xe Ar Kr H2 O2 N2 N2 O

12.13 15.7 ± 0.05 13.99 ± 0.02 [40] 15.42 ± 0.05 12.07 ± 0.02 [40] 15.52 ± 0.05 12.86 ± 0.01 [40]

1.13 1.35 1.22 1.19 1.24 1.27 1.28

12.13 [38] 15.78 ± 0.03 [67] 13.99 [38] 15.44 ± 0.01 [54] 12.1 ± 0.1 [61] 15.58 ± 0.01 [62] 12.91 ± 0.03 [62]

12.129 87 15.759 ± 0.001 13.999 61 ± 0.000 01 15.425 93 ± 0.000 05 12.069 7 ± 0.000 2 15.581 ± 0.008 12.889 ± 0.004

intensity yields information on the experimental FWHM of the electron energy resolution. In the present study we determine a resolution of approximately 135 meV (see figure 3). It should be mentioned that a comparable electron energy resolution has been observed under similar experimental conditions for the Cl− ion yield from s-wave attachment to CCl4 and CF2 Cl2 at around zero electron energy [33, 39]. Applying the MLA technique to our experimental data for xenon, argon, N2 and H2 yields results that are shown in table 1 and in figure 4. It can be seen that in the case of xenon we reach excellent agreement with Wannier’s power law (p = 1.127). We absolutely calibrate our energy scale with the tabulated xenon ionization energy (12.13 eV). In the case of argon the AE is slightly lower (15.7 ± 0.05 eV), as compared to [8] (15.759 ± 0.001 eV), and the exponent is slightly higher (p = 1.35). This may be due to some close-lying excited states that contribute to the ion formation in the threshold region. It can be seen that for N2 and H2 the appearance energies correspond well to the values tabulated in [8]. To consolidate our approach even further we have obtained appearance energies for a number of other systems, i.e. Kr, Ne, O2 , D2 and N2 O [13, 40]. Our results are compared to the literature data in table 1. In general, we reach an agreement within 50 meV with the tabulated values [8], if we limit the MLA to less than 3 eV above threshold.

Threshold electron impact ionization of CF4 , CHF3 , CH4 and C3 H8 25000 20000 15000

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+

Xe 12.13 eV

10000 5000 0 60000

Ion signal (cps)

50000 40000 30000 20000 10000

6

+

8

10

12

14

16

14

16

18

20

Ar 15.70 eV p = 1.35

0 2000 1500 1000 500

10

12 +

N2 15.52 eV p = 1.27

0 25000

10

20000

+

15000 10000 5000

12

14

16

18

H2 15.42 eV p = 1.19

0 10

12

14

16

18

20

E lectron energy (eV ) Figure 4. Ion signals close to the threshold ionization energy of xenon, argon, nitrogen and hydrogen. The ionization energy (indicated with an arrow) was obtained after a fit (——) of the experimental data ( ) using the function (8). Absolute calibration of the energy scale used the tabulated ionization energy of xenon (12.13 eV). From the fit it was also possible to obtain the modified exponents p for Wannier’s power law (1). As can be expected, in general the values for p exceed Wannier’s original value for hydrogen ionization (p = 1.127, see text).

◦

In some cases we observe that the background contamination does not remain constant with the electron energy. This is an effect which is caused by background molecules such as pump oils and H2 O, if the fragment ion under investigation has a low abundance and similar molecular mass (within 0.1 u). In such cases we are able to improve our evaluation by the subtraction of a linear rising background. More precisely, if Eb is the AE of the impurity, the fit function is modified to  b   E
Eb : b + c(E − Eb ) (10) f () = Eb
E
E1 :   p E > E1 : b + c(E − Eb ) + d(E − E1 ) . It is important to mention that fitting with equation (10) is only reliable if the influence of

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Ion signal (arb. units)

+

C /C F 4 constant background (C B) linear rising background (LR B)

150

34.77 eV (LR B )

100

30.10 eV (C B ) 50

0 29

30

31

32

33

34

35

36

37

38

Figure 5. Typical example for an impurity correction shown for the production of C+ from CF4 using electron impact. The measured data-points are indicated with . Between 30 and 34 eV the signal is contaminated by a linear rising background produced from pump-oils that contain hydrocarbons. Assuming only a constant background the AE would be 30.10 eV (- - - -), whereas with the impurity correction for a linear rising background a value of 34.77 eV is obtained (——).

◦

the impurity ion remains very small compared to the ion under observation. As an example, figure 5 demonstrates the C+ ionization efficiency curve from CF4 fitted with and without the impurity correction technique, demonstrating the importance of this correction procedure. 5. Results and discussion 5.1. Electron impact on carbon tetrafluoride In our experiment electron impact on carbon tetrafluoride leads to the observation of the fragment ions C+ , CF+ , CF+2 and CF+3 . The experimentally determined ionization behaviour of these ions in the threshold region is shown in figure 6, together with the fit determined from the application of the MLA to functions (8) and (10), where necessary. The corresponding appearance energies are compared with earlier electron impact results and photoionization studies in table 2. The parent ion CF+4 itself has a tabulated electron impact ionization energy of 16.2±0.1 eV [8], which exceeds the AE determined by photoionization for CF+3 (14.9±0.1) by more than one electron volt. CF+4 is unstable, with a lifetime of less than 10 µs, thus in mass spectra only CF+3 is observed. The first transition energies for electronic excitations (12.6 eV for 1t1 → 3s and 13.6 eV for 1t1 → 3p [41, 42]) are rather high and lead directly to a dissociation of the molecule (CF3 + F). The decay of the parent ion can be mass spectrometrically detected from metastable ion peaks [43]. In the case of the production of CF+3 a pronounced difference exists between the spectroscopically determined AE for CF+3 formation (14.9 ± 0.1) [44] and the present electron impact value of 15.69 ± 0.05 eV. The AE value that we obtain here is slightly lower than the value which has been suggested previously, 15.9 ± 0.5 [1]. However, the two values agree within the error bars that have been stated.

Threshold electron impact ionization of CF4 , CHF3 , CH4 and C3 H8

600

CF

4273

+ 3

1 5 .6 9 e V 300

p = 2 .0 4

0 10

Io n s ig n a l (c p s )

150 100 50

12

CF

14

16

18

20

+ 2

2 1 .4 7 e V p = 1 .9 8

0 18 20

20

CF

22

24

26

28

+

2 9 .1 4 e V 10

p = 1 .7 8

0 40 30

C

+

26

28

30

32

34

36

34

36

38

40

3 4 .7 7 e V 20

p = 1 .9 8

10 0 30

32

E le c tro n e n e rg y (e V ) Figure 6. Fragment ion signals close to threshold for electron impact ionization of carbon tetrafluoride (CF4 ). The appearance energies have been obtained using a fit (——) of the experimental data ( ) with MLA using the function (8), (see text). The evaluated threshold energies and the exponents p for the power law dependence (1) are indicated.

◦

In contrast, for the production of CF+2 we observe a threshold value of 21.47±0.1 eV, which is about 0.8 eV lower than the tabulated one (22 ± 0.5 eV) [1]. The first electron impact data published on this decay channel are by Dibeler and Reese [45] (20.3 ± 0.5 eV) and deviate by nearly 2 eV. We find that the comparably large error bars stated in the literature have probably

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T Fiegele et al Table 2. Appearance energies and Wannier exponents p for fragment ions from carbon tetrafluoride. Fragment

Present

p

Earlier EIS

Photoionization

CF3 CF2

15.69 ± 0.05 21.47 ± 0.1

2.04 ± 0.05 1.98 ± 0.05

14.9 ± 0.1 [44] —

CF

29.14 ± 0.2

1.78 ± 0.2

C

34.77 ± 0.2

1.98 ± 0.2

15.9 ± 0.5 [1] 22.0 ± 0.5 [1] 20.3 ± 0.5 [45] 27.0 ± 0.5 [1] 22.6 ± 0.5 [45] 34.5 ± 0.5 [1] 31.5 ± 0.5 [45]

— —

to be extended to match our result that has been obtained with higher resolution. It should be mentioned that a major difficulty in obtaining accurate values for the tetrafluoromethane molecule is due to the high excess kinetic energy (1–15 eV) of its fragment ions [1, 43]. As a result ion signals are sometimes discriminated, especially if weak extraction fields (in our case ≈100 mV cm−1 ) are applied to the ionization region. On the other hand, strong extraction conditions would certainly deteriorate the energy resolution. In the case of the CF+ ion formation the deviations from earlier experimental data are even more obvious. In their recent review Christophorou and coworkers [1] have suggested a value of 27 ± 0.5 for this decay channel, which is significantly different from the AE value of Dibeler and Reese of 22.6 ± 0.5 eV [45]. Here, we find that the formation of CF+ requires a high primary electron energy of 29.14 ± 0.2. Such large deviations of more than 2 eV from the recommended value, and of approximately 7 eV in comparison to earlier electron impact data, appear curious, and we are currently unable to provide a consistent explanation for this large discrepancy. As shown in figure 6, our data have a small statistical scatter and the fit that we use here follows the experimental data very nicely. Usually we would expect that our threshold value should be below earlier reported values, because we use a nonlinear fit. Compared to the CF+2 and CF+ cases it is especially astonishing that for the much smaller signals in the case of C+ our value (34.77 ± 0.2) is in good agreement with earlier results (34.5 ± 0.5) [1]. On the other hand this excludes any systematic errors that might arise from a nonlinear electron energy scaling or from effects in the data fitting technique. It should be noted that from our present observations it can be concluded that the binding energy per fluorine atom in CF4 appears to be approximately independent of the total number of fluorine atoms that are extracted from the system (≈6.5 eV). Unfortunately, due to its very low cross section the direct production of F+ from CF4 was not observable in our experiment. Thus, we are currently unable to provide reliable values for this dissociating channel. Regarding the Wannier coefficient p we find that for all threshold curves of carbon tetrafluoride the exponent is very close to 2. Such a value is significantly larger than the theoretical and experimental exponent for atoms, but well within our expectations for the polyatomic case. 5.2. Electron impact on trifluoromethane A review on electron interactions with CHF3 was recently published by Christophorou et al [2]. These authors have explicitly stated that for a more substantial characterization some basic data on this molecule are needed with regard to all elastic and inelastic scattering processes, cross sections and electron impact ionization and dissociation. With the present work we believe that we are able to satisfy some of these demands. CHF3 itself is very similar to CF4 and can be obtained after the substitution of one fluorine atom by hydrogen. This changes the point symmetry group and leads to a more polar molecule. As a consequence, the response

Threshold electron impact ionization of CF4 , CHF3 , CH4 and C3 H8 4000

3000

+

CF 3 14.41 eV p = 1.29

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+

1000

CHF 2 15.26 eV p = 1.86

2000 500 1000

Ion signal (cps)

0

0

10 12 14 16 18 20 100

+

CF 2 15.23 eV p = 1.78

50

10 12 14 16 18 20 +

500

CF 17.80 eV p = 2.82

250

0 0

12 14 16 18 20 22 40 30

+

CH 28.29 eV p = 3.77

20

14 16 18 20 22 24 +

200

C 27.52 eV

100

10 0

0

22 24 26 28 30 32 34

26 28 30 32 34 36 38

E lectron energy (eV ) Figure 7. Fragment ion signals close to threshold for electron impact ionization of trifluoromethane (CHF3 ). The appearance energies for these ions have been obtained using an MLA fit (——) of the experimental data ( ) with function (8) (see text). The evaluated threshold energies and the exponents p for the power law dependence (1) are indicated.

◦

to changing electric fields is enhanced and the molecule is slightly destabilized. We find that such a general behaviour is also reflected in the appearance energies. In our experiment we are able to observe CF+3 , CF+2 , CF+ , CHF+2 and CHF+ . Quite similarly to the CF4 case, CF3 H fragments easily under electron impact ionization and the lifetime of the parent ion itself is too short to be observed in the millisecond time regime. The threshold behaviour of all observable ions is shown in figure 7 and we have summarized and compared our results in table 3. The heaviest fragment CF+3 can be detected at the lowest initial electron energy.

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T Fiegele et al Table 3. Appearance energies and Wannier exponents p for fragment ions from trifluoromethane. Fragment

Present

p

CF3

14.41 ± 0.05

1.29 ± 0.05

CF2

15.23 ± 0.3

CF CHF2

17.8 ± 0.1 15.26 ± 0.05

CH C

28.29 ± 0.3 27.52 ± 0.2

Earlier EIS

14.42 [50] 14.03 ± 0.06 [46] ≈14.5 [48–50] 15.2 [47] 1.78 ± 0.1 17.5 ± 0.3 [48, 50] 14.7 ± 0.4 [52] 2.82 ± 0.1 ≈20.6 [47, 48, 50] 1.86 ± 0.05 15.75 [50] 16.4 ± 0.3 [48] 3.77 ± 0.3 33.5 ± 0.5 [47] 3.15 ± 0.3 —

Photoionization 14.19 ± 0.02 [51]

— — — — —

Surprisingly, from our experiments most appearance energies for the CHF3 molecule and its fragments did not correspond to tabulated electron impact AE values within the suggested error bars [8]. We find the closest agreement for the formation of CF+3 , which we observe at 14.41 ± 0.05 eV. For comparison, a rather precise electron impact value mentioned in [8] has been reported by Martin et al [46] with 14.03 ± 0.06 eV. However, recent findings by Goto et al [47], using threshold ionization mass spectrometry, have suggested an ≈1 eV higher AE of 15.2 eV. A possible reason why we obtain a higher AE compared to [46] may be the additional vibrational cooling effect during the adiabatic expansion in our molecular beam source. Our threshold energies for CF+3 are in much better correspondence with electron impact studies that have been reported by Hobrock and Kieser [48] (14.67 eV), Farmer et al [49] (14.53 eV) and Lifshitz and Long [50] (14.42 eV). In addition, it should be noted that our result is 0.22 eV higher but very close to the photoionization (adiabatic) value of 14.19 ± 0.02 eV [51]. We find the best fit for the threshold law using an exponent that is slightly higher than in the atomic case (p = 1.29). This points towards the situation that higher excited states contribute only weakly to the CF+3 ion formation. Approximately 1 eV above the onset for the formation of CF+3 the molecule starts to eject fluorine instead of hydrogen. More precisely, we determine a value of 15.26 ± 0.05 eV for the production of CHF+2 . This result is more than 1 eV below the tabulated value of Hobrock and Kieser [48] of 16.4 ± 0.3 eV, using electron impact. In better correspondence is an EI value of 15.75 eV reported by Lifshitz and Long [50]. As in the CF+3 case it seems that the data of Goto et al [47] (16.8 eV) are too high when compared to our result and to older literature data. Figure 7 demonstrates that our data for this channel have very good statistics and can be fitted with an exponent of p = 1.86. Approximately in the same energy region we additionally observe the onset for the production of CF+2 (15.23 ± 0.3 eV). From earlier data, using electron impact, the production of CF+2 [52] should be possible above 14.7 ± 0.4 eV. Consequently, these data [52] suggest that one additional hydrogen would destabilize the molecular ion CHF+2 by more than 1 eV. Instead, we find here that the additional hydrogen stabilizes the CF+2 with approximately 100 meV binding energy. It should be mentioned that for the formation of CF+2 Lifshitz and Long [50] as well as Hobrock and Kieser [48] have reported an AE of approximately 17.5 eV, which appears too high compared to the present dataset. Moreover, we observe that the production of CF+ from trifluoromethane is possible with electron energies exceeding 17.8 ± 0.1 eV. This is about 2.5 eV less energetic than has been reported in the literature up to now [47, 48, 50]. It should be noted that we find a comparably large exponent (p = 2.82) for this dissociation channel, pointing towards a superposition of

Threshold electron impact ionization of CF4 , CHF3 , CH4 and C3 H8

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several fragmentation channels via highly excited ionic states. The comparably large exponent causes a strong bending of the curve close to the ionization threshold and may be responsible for a failure of the linear extrapolation technique that has been applied in earlier experiments. Moreover, it is interesting to compare the energy necessary for the formation of CH+ with that for CF+ . An AE of 28.29 ± 0.3 eV indicates that more than 10 eV additional energy is necessary to excite CHF3 sufficiently highly to eject the last fluorine atom instead of the hydrogen. It should be noted that the enthalpy of formation for CH is 594.13 kJ mol−1 while for CF it is only about 255.22 kJ mol−1 [8]. Consequently, using the tabulated ionization energies for CH (10.64 eV) and CF (9.11 eV) [8], approximately 16.79 eV are necessary to produce CH+ and only 11.75 eV for CF+ . If one accounts for energy repartitioning in the intermediate excited complex (using for example RRKM), the large difference in the appearance energies becomes clear. We find that our value for the formation of CH+ is about 5 eV lower than has been reported by Goto et al [47], which might be due to the large exponent (p = 3.77) that we obtain from our MLA analysis. As can be seen in figure 7, the ion signals of CH+ and CF+ were slightly contaminated by a linearly rising background. Therefore, we had to obtain our values using equations (10). However, in most cases we observe only weak agreement between our values and those obtained by Goto et al [47], but it should be noted that our work was focused on high resolution and the accurate determination of the appearance energies, while Goto et al [47] were keener to investigate radicals and neutrals from CHF3 dissociation reactions. Therefore, we suggest that our present values should be more reliable. Most interestingly, up to now the formation of C+ from CHF3 by electron impact has not been reported in the literature, either in electron impact or in photoionization experiments. There is certainly no reason why this reaction channel should be forbidden, especially because in the carbon tetrafluoride case the corresponding decay channel is known and appears at around 34.5 eV. We have mentioned above that the extraction of hydrogen from CHF3 is less energetic than the subtraction of one fluorine atom (by ≈1 eV). Similarly, the production of CF+2 from carbon tetrafluoride requires about 6 eV larger initial electron energy than that from trifluoromethane. The same tendency can be observed from a comparison of the CF+ appearance energies. In order to produce CF+ from CF4 about 7–10 eV more initial energy is required in comparison to CHF3 . Therefore, it can be expected that the formation of C+ from trifluoromethane should be possible at electron energies which are about 7–10 eV below the C+ formation from CF4 . Indeed, we find here that the C+ production from CHF3 , possible above an AE of 27.52 ± 0.2 eV, needs about 7 eV less initial electron energy compared to CF4 . It should be noted that less energy is needed to produce C+ than CH+ from trifluoromethane. The reason is that the gas phase production of HF is exoenergetic by about 2.83 eV (−273.3 kJ mol−1 ) [8], while the production of F is endoenergetic by 0.82 eV (79.38 kJ mol−1 ). If we assume fragmentation down to CHF+ prior to the formation of C+ , this leads to the following reaction energetics (all values given in kJ mol−1 and taken from [8]):

CHF+ (IE + H ) → CH+ (IE + H ) + F(H ) (971.04 + 125.52) → (1027.02 + 594.13 + 79.38) → C+ (IE + H ) + HF(IE) → (716.68 + 1086.9 − 273.3).

(11)

Thus, the upper reaction path in (11) requires 1.76 eV more energy, which in principle explains the relatively low AE for the C+ formation observed here.

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T Fiegele et al Table 4. Appearance energies and Wannier exponents p for fragment ions from methane. Fragment

Present

p

CH4

13.08 ± 0.02

1.54 ± 0.05

CH3

14.27 ± 0.02

CH2

15.01 ± 0.05

CH

22.02 ± 0.1

Earlier EIS

12.63 ± 0.02 [64] 12.6 ± 0.4 [53]