- Email: [email protected]
On the adiabatic ionisation energy of the CF3 free radical
2 October 1998
Chemical Physics Letters 295 Ž1998. 145–151
On the adiabatic ionisation energy of the CF3 free radical G.K. Jarvis a
a,1
, R.P. Tuckett
b,)
School of Physics and Astronomy, UniÕersity of Birmingham, Edgbaston, Birmingham B15 2TT, UK b School of Chemistry, UniÕersity of Birmingham, Edgbaston, Birmingham B15 2TT, UK Received 16 July 1998; revised 14 August 1998
Abstract The different methods that have been used to measure the adiabatic ionisation energy of the CF3 free radical in the gas phase are reviewed, with values covering the wide range 8.6–9.1 eV. In a study of vacuum-UV photofragmentation of perfluoropropane, C 3 F8 , using the threshold photoelectron–photoion coincidence technique, our value for the appearance energy of CF3q at 298 K, 13.0 " 0.1 eV, leads to an upper limit for the adiabatic ionisation energy of CF3 of 8.8 " 0.2 eV, in good agreement both with that deduced from a low-temperature photoionisation mass spectrometric ŽPIMS. study of CF3 Br and with values obtained from three ion–molecule bimolecular studies. It is lower, however, than the value obtained indirectly from a PIMS study of C 2 F4 and from ab initio calculations. q 1998 Elsevier Science B.V. All rights reserved.
1. Introduction CF4 is used extensively as a carrier gas in the plasma dry etching of integrated silicon circuits. The CF3 radical and its cation, CF3q, may play important roles as reactive species in such systems Žsee, e.g., Refs. w1,2x.. The CF3 free radical is also an important species in atmospheric chemistry because of its role in the oxidative degradation of both perfluorocarbons and hydrofluorocarbons Žsee, e.g., Ref. w3x.. For both reasons, the thermochemistry of CF3 and CF3q may be important factors in modelling the reaction kinet-
)
Corresponding author. Fax: q44-121-414-4403; e-mail: [email protected] 1 Present address: Chemical Sciences Division, MS 6-2100, Lawrence Berkeley Laboratory, University of California Berkeley, CA 94720, USA.
ics associated with both systems. Whilst the thermochemistry of the CF3 neutral is now fairly clearly established w4x, that of the ion remains less clear. The link between them is the adiabatic ionisation energy ŽIE. of the CF3 radical. The problem with measuring IE ŽCF3 . stems essentially from the large geometry change, especially in the FCF bond angle q, between the neutral radical Žpyramidal, C 3v symmetry, q s 110.78 and R C – F s ˚ w5x. and the cation Žplanar, D 3h symmetry, 1.32 A ˚ w6x.. Thus the q s 1208, R C – F s ca. 1.23 A Franck–Condon factor at threshold, corresponding to ionisation from the lowest vibrational level of the neutral to that of the cation, is essentially zero. Indeed, in the highest-quality ab initio calculation of IE ŽCF3 . performed to date, Horn et al. w6x obtained a value of 8.98 " 0.05 eV, with a Franck–Condon envelope showing a maximum at nq 2 s 20 in the umbrella bending vibrational mode of the cation, ca.
0009-2614r98r$ - see front matter q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 9 8 . 0 0 9 3 7 - 3
G.K. JarÕis, R.P. Tuckettr Chemical Physics Letters 295 (1998) 145–151
146
1% of the maximum at nq 2 s 8, and negligible Franck–Condon intensity for nq 2 - 8. Attempts to measure an accurate value for the IE ŽCF3 ., therefore, via a direct, single-photon photoelectron or photoionisation mass spectrometric ŽPIMS. experiment are full of difficulties. No He I, threshold or zero kinetic energy photoelectron spectrum of CF3 has been reported. The classic PIMS study of Lifshitz and Chupka w7x, in which CF3 CH 2 ONO was thermally decomposed to give CF3 radicals and photoionised by tunable vacuum-UV radiation from a hydrogen discharge lamp, yielded a threshold, or first onset, value of 9.25 " 0.04 eV. It is now realised Žsee, e.g., Ref. w8x., that first onsets measured in this way yield a value that is too low for the IE because thermal effects in the neutral precursor have been ignored. In this instance, the correction would be small, so that even if the photoion yield was analyzed by modern techniques, this experiment would always give an IE value which is too high because of the unfavourable Franck–Condon factor at threshold. Later experiments to measure IE ŽCF3 . have therefore used a variety of indirect techniques, some of them in effect attempting to circumvent the Franck–Condon effect. In all cases, including the threshold photoelectron – photoion coincidence ŽTPEPICO. measurement reported by our group w9x, the value obtained is less than 9.25 eV. There have been three studies of this problem by ion–molecule bimolecular reactions. Tichy et al. w10x studied the reaction HClqq CF4 ™ CF3q q HF q Cl
Ž 1.
in a conventional selected ion flow tube. The fast rate constant of this reaction Ž1 = 10y1 0 cm3 moleculey1 sy1 ., probably recorded at 298 K, is less than that predicted by the Langevin theory of ca. 2 = 10y9 cm3 moleculey1 sy1 . Assuming the deviation from the Langevin value was entirely due to the Žsmall. endothermicity of reaction Ž1., they concluded that D r H8 Ž298. of this reaction has an upper limit of q0.06 eV, from which an appearance energy ŽAE. of CF3q from CF4 at 298 K of less than 14.23 eV was deduced. Fisher and Armentrout w11x studied the forward and reverse reaction Krq 2 P3 r 2 q CF4 ™ CF3q q F q Kr
ž
/
Ž 2.
as a function of centre-of-mass collision energy in a guided ion beam mass spectrometer. They established that reaction Ž2. in the forward direction was also endothermic by 0.24 " 0.07 eV and, combined with the very accurate value for the ionisation energy of the Kr atom w12x, they obtained an almost identical value for AE ŽCF3qrCF4 . of 14.24 " 0.07 eV. The temperature of this experiment is also not defined, but is probably 298 K. Although not mentioned specifically in either paper, we show later that these studies yield an upper limit to IE ŽCF3 . of 8.69 " 0.13 eV. Very recently, the reaction HCNq Ž v s 0 . q CF4 ™ CF3q q HF q CN
Ž 3.
has also been studied at 300 K in a selected ion flow tube w13x. From the measured rate constant at this temperature of 6 = 10y1 2 cm3 moleculey1 sy1 , an upper limit for AE ŽCF3qrCF4 . 300 of 14.28 eV was established. Tichy et al. w10x comment that their value of AE ŽCF3qrCF4 . is nearly 1 eV lower than that obtained by unimolecular photon or electron impact of CF4 Žsee Section 3.. Their explanation is that a characteristic of ion–molecule reactions is to yield adiabatic enthalpies of reaction because of the intimate contact of sufficient duration to allow molecular geometries to relax to adiabatic configurations. In other words, due to strong ion-induced dipole interactions, long-lived complexes readily form in such reactions. Under these circumstances, Franck–Condon factors are distorted w14x, yielding adiabatic reaction enthalpies. Turning to unimolecular reactions, Noutary w15x and Clay et al. w16x performed PIMS studies of CF3 X ŽX s Cl, Br, I. and CF3 Br, respectively. The former study was performed at room temperature, the latter in the environment of a jet-cooled molecular beam. Both experiments measured AE ŽCF3qrCF3 X. T , with X s Br only in the case of Clay et al. In both cases, AEs were determined as the first onset of the CF3q fragment in the ion yield curve, and no attempt was made to allow for thermal effects in the CF3 X precursor. Thus it might be expected that both studies, but especially that of Noutary due to its higher temperature, would produce IE ŽCF3 . values that are too low. From the CF3 I study Noutary concluded that IE ŽCF3 . F 8.62 eV, from their CF3 Br study Clay et al. concluded that D f H08 ŽCF3q. s 362 " 4
G.K. JarÕis, R.P. Tuckettr Chemical Physics Letters 295 (1998) 145–151
kJ moly1 . Combined with the 0 K heat of formation of CF3 w4x, the Clay et al. result leads to an IE ŽCF3 . value of 8.55 " 0.08 eV. Using a different precursor, Asher and Ruscic w4x have remeasured some early PIMS experiments of Walter et al. w17x on C 2 F4 . Both these studies yielded the ion yield curves of CFq and CF3q from C 2 F4 , i.e. C 2 F4 q hn ™ CFqq CF3 q ey or CF3q q CF q ey Ž 4. By careful analysis of both threshold regions, Asher and Ruscic extrapolated appearance energies at 0 K of these ions from C 2 F4 . Assuming that there were no kinetic shifts or reverse activation barriers in either channel, the difference of these two appearance energies was taken to be the difference in ionisation energy of the CF and CF3 radicals. Using the literature value for the CF radical w18x, they obtained a value for IE ŽCF3 . of 9.055 " 0.011 eV. Their paper reviews comprehensively all the different measurements that have been made of IE ŽCF3 . up to 1996, with the strengths and weaknesses of many of the different methods discussed. For reasons discussed later, they believe that their value is likely to be a lower limit, but they make no attempt to explain why the values from the two early ion–molecule studies w10,11x are, in their opinion, incorrect. Their claim that 9.055 eV is the correct value for the IE ŽCF3 . is partially substantiated by the ab initio calculations of Horn et al. w6x mentioned earlier. Our contribution to this problem arose by serendipity. We have measured the photofragmentation channels of a range of saturated and unsaturated perfluorocarbon cations by TPEPICO spectroscopy, using tunable vacuum-UV radiation from a synchrotron as the photoionisation source w9,19x. The CF3q cation was observed at the onset of ionisation for C 3 F8 , providing ca. 50% of the branching ratio at this energy. We have therefore measured AE ŽCF3qrC 3 F8 . 298 , from which an IE ŽCF3 . - 8.8 " 0.2 eV was deduced. This calculation was described only briefly in our paper w9x. The purpose of this Letter is to describe this calculation in more detail, and to place our value for IE ŽCF3 . in the context of other recent measurements for this quantity.
147
2. Calculation of IE (CF3 ) from TPEPICO spectroscopy Our experiment combined a TPEPICO spectrometer with tunable vacuum-UV radiation in the range 10–25 eV from a synchrotron source. The resolution of the vacuum-UV radiation was 0.4 nm, corresponding to 0.05 eV at 13 eV. Full details are given elsewhere w9x. TPEPICO spectra were recorded continuously as a function of photon energy, from which it was possible to determine the fragment ions formed at any particular energy. Breakdown diagrams, yielding the formation probability of the product ions as a function of the internal energy of the excited parent ion, were obtained in the usual manner. Experiments were performed at 298 K. For the three saturated perfluorocarbons studied, C 2 F6 , C 3 F8 and n-C 4 F10 , no parent ion was observed over the range of photon energies corresponding to electron removal from one of the valence molecular orbitals, 12–25 eV. A similar result was found for all three compounds in the PIMS study of Noutary w15x. The most likely reason is that, as with the simplest perfluorocarbon CF4 w20x, the ground state of these parent ions is repulsive in the Franck– Condon region accessible by one-photon excitation. These states then decay by an impulsive decay mechanism, with the energy localised in one bond and the timescale for this bond to break being shorter than that for intramolecular energy redistribution. One obvious criterion for such a process is that all three perfluorocarbons must possess at least one dissociative ionisation channel below the observed onset of ionisation, 13.0 " 0.1 eV in the case of C 3 F8 w9x Žthe stepsize used in these scanning-energy experiments was 0.352 nm or ca. 0.05 eV at 13 eV. This is too large ideally for an experiment with a spectrometer resolution also of 0.05 eV. Being conservative, we define the error in the appearance energy to be "0.1 eV.. With this molecule, two cations are formed at threshold, C 2 F4q and CF3q, each with a branching ratio of ca. 50% Žnote that Noutary w15x observed the first-onset threshold of CF3q at 13.22 eV.. The presence of these ions at the onset of ionisation for C 3 F8 is interesting for different reasons. The energies of the dissociation channels are such that C 2 F4q can only form with CF4 as the sole neutral fragment; channels such as C 2 F4qq
G.K. JarÕis, R.P. Tuckettr Chemical Physics Letters 295 (1998) 145–151
148
CF3 q F q ey are thermodynamically forbidden. Hence Fy migration must occur across a C–C bond before C–C bond cleavage can occur. By contrast, the interest in the observation of CF3q at the onset of ionisation is thermodynamic, not mechanistic. We consider the process C 3 F8 q hn ™ C 3 F8q q ey C 3 F8q ™ CF3q q C 2 F5 , Overall: C 3 F8 ™ CF3q q C 2 F5 q ey
Ž 5.
for which an AE ŽCF3qrC 3 F8 . 298 of 13.0 " 0.1 eV Žor 1254 " 10 kJ moly1 . is measured. We have used the procedure of Traeger and McLoughlin w21x to convert this appearance energy into a heat of reaction for Ž5. at 298 K,
Dr H2988 F AE Ž CF3q rC 3 F8 . 298 q 298
H0
q
c p Ž C 2 F5 . dT y
298
H0 5 2
c p Ž CF3q . dT
RT
Ž 6.
The upper limit for D r H2988 arises because an appearance energy Žof ŽAqrAB. 298 . defines an upper limit to the thermodynamic energy of Aqq B q ey. Note also that, like Traeger and McLoughlin w21x, we use the stationary electron convention that at threshold the electron has zero translational energy, not 1.5RT in this instance evaluated at 298 K. With this convention, 298
H0
c p Ž ey . dT s 0
Dr H2988 s Df H2988 Ž CF3q . q Df H2988 Ž C 2 F5 . y Df H2988 Ž C 3 F8 . F 1276 " 10 kJ moly1
CF3 ™ CF3q q ey
c p Ž Aq or B . dT s H2988 y H08
Ž 8.
For both CF3q and C 2 F5 , this term will contain contributions from translational Ž2.5RT ., rotational Ž1.5RT ., and vibrational Ž NA hnrwexpŽ hnrk B T . y 1x per vibrational mode. motion evaluated at T s 298
Ž 10 .
Using the most accurate value for the heat of formation of neutral CF3 at 298 K, y466" 4 kJ moly1 w3x, D r H2988 for reaction Ž10. has an upper limit of 852 " 25 kJ moly1 . Formally, the adiabatic ionisation energy of the CF3 radical is the enthalpy of reaction Ž10. evaluated at 0 K, i.e. IE Ž CF3 . s Dr H2988 Ž 10 . y 298
H0
y
298
Ž 9.
Using values for the heats of formation of C 2 F5 and C 3 F8 at 298 K of y893 " 4 and y1783" 7 kJ moly1 , respectively w25x, we obtain an upper limit for D f H2988 ŽCF3q. of 386 " 21 kJ moly1 . Consider now the reaction describing photoionisation of the CF3 radical,
Ž 7.
If the last three terms in Eq. Ž6. sum to zero, as is often assumed w22x, a significant error may be introduced in converting the measured appearance energy into an upper limit for the enthalpy of the reaction. The error is greater the larger the size of the fragments Aq and B. Considering the second and third terms in the right-hand side of Eq. Ž6.,
H0
K. Using values for the vibrational frequencies of CF3q and C 2 F5 from elsewhere w23,24x, we estimate that Ž H2988 y H08. is 4.70RT for CF3q and 6.74RT for C 2 F5 . The last three terms in Eq. Ž6. therefore sum to 8.94 RT, or 22 kJ moly1 at 298 K. We therefore determine D r H2988 for reaction Ž5. to have an upper limit of 1276 " 10 kJ moly1 . With the stationary electron convention, we can then write for this reaction that
298
H0
c p Ž ey . dT
c p Ž CF3q . dT q
298
H0
c p Ž CF3 . dT
Ž 11 . The second term on the right hand side of Eq. Ž11. is zero in the stationary electron convention, and the last two terms only differ in their vibrational contribution. For CF3 , using vibrational frequencies in the literature w23x, we calculate such a contribution to be 0.65RT, whereas for CF3q, as shown above, we calculate 0.70 RT. At 298 K the difference is negligible, 0.1 kJ moly1 . We derive, therefore, an upper limit for IE ŽCF3 . of 852 " 25 kJ moly1 or 8.8 3 "0.2 5 eV. The errors have been deliberately conservative, and we round this number down to an upper limit of 8.8 " 0.2 eV.
G.K. JarÕis, R.P. Tuckettr Chemical Physics Letters 295 (1998) 145–151
3. Discussion The first comment to make about our result is that this experiment cannot be used to measure the absolute value of IE ŽCF3 .. It can only reveal its upper limit. At the onset of ionisation the ground electronic state of C 3 F8q is repulsive in the Franck–Condon region, hence impulsive photodissociation occurs via C–C bond cleavage with the production of CF3q and C 2 F5 fragments. In other words, the CF3qq C 2 F5 q ey dissociation channel lies below the onset of ionisation. After allowing for thermal effects in C 3 F8 , the AE ŽCF3qrC 3 F8 . 298 value must therefore transform into an upper limit for both D f H2988 ŽCF3q. and IE ŽCF3 .. Our upper limit for IE ŽCF3 ., 8.8 " 0.2 eV, is in accord with the results from bimolecular ion– molecule reactions w10,11,13x, and from early w15x and later w16x PIMS studies of CF3 X ŽX s Br, I.. However, we are in disagreement with the result of Asher and Ruscic w4x from a PIMS study of C 2 F4 that the true IE ŽCF3 . is as high as 9.055 " 0.011 eV, with the possibility that this might represent a lower limit. The situation in C 3 F8 is very similar to attempts to measure an absolute value of IE ŽCF3 . by photoionisation of CF4 . The ground state of CF4q is also repulsive in the Franck–Condon region, dissociating on a sub-picosecond timescale to CF3qq F by cleavage of one C–F bond with a substantial release of translational kinetic energy w26x. CF4q is therefore not observed on the timescale of a mass spectrometric or coincidence experiment. The AE ŽCF3qrCF4 . value reported by PIMS, 15.35 eV w17x, or the slightly higher values from coincidence spectroscopy w20,26,27x, bears no relation to the energy of the CF3qq F q ey dissociation channel; it only relates to the onset of ionisation of CF4 . Even if the Asher and Ruscic value w4x for IE ŽCF3 ., 9.055 eV, is correct, the energy of this channel, given by D r H08 for CF4 ™ CF3 q F plus IE ŽCF3 ., is as low as 14.67 eV. Thus in the case of CF4 , the difference between AE ŽCF3qrCF4 . and the energy of the CF3qq F q ey dissociation channel has a lower limit of ca. 0.7 eV. In view of other measurements of IE ŽCF3 ., it seems unlikely that the difference between AE ŽCF3qrC 3 F8 . and the energy of the CF3qq C 2 F5 q ey dissociation channel is so great; indeed, it is probably as small as ca. 0.1–0.2 eV.
149
As described earlier, Asher and Ruscic have measured the CFq and CF3q ion yields from C 2 F4 at 298 K. The threshold region of both curves is fit by an appropriate model which allows for a thermal energy distribution of the C 2 F4 reactant at this temperature. By this method they are able to extrapolate AEs for both ions at 0 K. They then make the fundamental assumption that there are no kinetic shifts or reverse activation barriers in either of the two dissociation channels CFqq CF3 q ey or CF q CF3qq ey. In other words, the appearance energy of both ions corresponds exactly to the energy of the appropriate dissociation channel at 0 K. Under these circumstances, AE Ž CFqrC 2 F4 . 0 y AE Ž CF3qrC 2 F4 . 0
Ž s 0.055 " 0.003 eV. ' IE Ž CF. y IE Ž CF3 . Ž 12 . Using the value from Dyke et al. w18x for the adiabatic ionisation energy of the CF radical, Asher and Ruscic obtain IE ŽCF3 . s 9.055 " 0.011 eV, with most of the error arising from the error in IE ŽCF.. They justify that this value could be a lower limit by claiming that, if small kinetic shifts are present, CFq should have a slightly larger shift than that of CF3q. Then the differences in AEs, 0.055 eV, is an upper limit, and the value of IE ŽCF3 ., 9.055 eV, is a lower limit. Our main concern with this analysis arises from the assumption that there are no barriers in either exit channel to dissociation. To form both CF3q or CF3 products from C 2 F4 , a fluorine atom has to migrate across the C5C bond. This could easily lead to an activation energy barrier on the potential energy surface to products. It is true that if such a barrier exists, it is likely to be similar in the two exit channels CFqq CF3 q ey and CF q CF3qq ey. Until full details of the potential energy surfaces for these two processes are known, however, we cannot know whether the observed onsets of these ions correspond to their thermodynamic onsets, or whether there is any difference in the threshold region of the two dissociation channels. We therefore query the assumption made in Eq. Ž12.. On a minor point, whilst we would agree that any kinetic shifts in CFq and CF3q are likely to be small, we would argue that CF3q, and not CFq, should have the larger shift since
150
G.K. JarÕis, R.P. Tuckettr Chemical Physics Letters 295 (1998) 145–151
it is the ion with the larger number of internal degrees of freedom. If this is true, then technically 9.055 eV becomes an upper limit to this value of IE ŽCF3 .. In the Clay et al. study w16x, CF3q is detected from CF3 Br by PIMS using synchrotron radiation. The temperature of the jet-cooled source is not defined, but we estimate an upper limit of ca. 30 K. The CF3q signal is measured at its first onset with an appearance energy of 11.56 " 0.02 eV, and allowance was made for second-order radiation from the synchrotron source. The authors believe that this is an adiabatic value, i.e. this energy corresponds to the CF3qq Br q ey dissociation threshold with no kinetic shift or reverse activation barrier. This threshold then converts into an upper limit for IE ŽCF3 . of 8.55 " 0.08 eV. At the low temperature of their source, it is very difficult to see how ‘thermal’ effects in the CF3 Br precursor could account for a shift as great as 0.5 eV, as the Asher and Ruscic analysis of this result would suggest. Likewise, in the two early ion–molecule studies w10,11x similar values of AE ŽCF3qrCF4 . are reported. The temperature of neither experiment is explicitly defined, but we can safely assume it is ca. 298 K. Using the more accurate value from Fisher and Armentrout w11x of 14.24 " 0.07 eV, the procedure of Traeger and McLoughlin, described in Section 2, can be used to convert this value of AE ŽCF3qrCF4 . 298 into an upper limit for both D f H2988 ŽCF3q. and IE ŽCF3 .. We obtain D f H2988 ŽCF3q. F 373 " 9 kJ moly1 and IE ŽCF3 . F 8.69 " 0.13 eV. Once again, this value is significantly lower than the value obtained by Asher and Ruscic from their unimolecular PIMS study of C 2 F4 .
4. Conclusions We believe that the result of accumulated evidence from three bimolecular ion–molecule studies w10,11,13x, from a low temperature unimolecular PIMS study of CF3 Br w16x, and from our room temperature TPEPICO study of C 3 F8 w9x is that the adiabatic ionisation energy of the CF3 radical is certainly less than 8.8 eV, with its probable value being less than 8.6–8.7 eV. We agree with many of the criticisms made by Asher and Ruscic w4x about
the different methods that have been used to refine the original Lifshitz and Chupka value of 9.25 eV w7x, in particular that many PIMS studies quote first onsets which do not allow for thermal effects in the neutral precursor. However, we believe that their indirect method of photoionisation of C 2 F4 to yield CFq and CF3q cannot constitute a definitive measurement of IE ŽCF3 ., since details of the two potential energy surfaces to these product channels are not yet known. Acknowledgements We thank Dr. B. Ruscic ŽArgonne National Laboratory. for communications, Professor E.E. Ferguson ŽNOAA, Boulder. for informing us about the work of Ref. w12x, and Dr. R.A. Kennedy ŽUniversity of Birmingham. and Professor J.M. Dyke ŽUniversity of Southampton. for helpful discussions. We also thank Mr. K.J. Boyle and Mr. D.P. Seccombe for a critical reading of the manuscript. GKJ thanks NERC for the award of a Research Studentship. References w1x V.M. Donnelly, D.L. Flamm, J. Appl. Phys. 51 Ž1980. 5273. w2x J.W. Coburn, Plasma Chem. Plasma Proc. 2 Ž1982. 1. w3x A.R. Ravishankara, A.A. Turnipseed, N.R. Jensen, S.R. Barone, M. Mills, C.J. Howard, S. Solomon, Science 263 Ž1994. 71. w4x R.L. Asher, B. Ruscic, J. Chem. Phys. 106 Ž1997. 210. w5x C. Yamada, E. Hirota, J. Chem. Phys. 78 Ž1983. 1703. w6x M. Horn, M. Oswald, R. Oswald, P. Botschwina, Ber. Buns. Phys. Chem. 99 Ž1995. 323. w7x C. Lifshitz, W.A. Chupka, J. Chem. Phys. 47 Ž1967. 3439. w8x R.L. Asher, E.H. Appleman, B. Ruscic, J. Chem. Phys. 105 Ž1996. 9781. w9x G.K. Jarvis, K.J. Boyle, C.A. Mayhew, R.P. Tuckett, J. Phys. Chem. A 102 Ž1998. 3219. w10x M. Tichy, G. Javahery, N.D. Twiddy, E.E. Ferguson, Int. J. Mass Spec. Ion Processes 79 Ž1987. 231. w11x E.R. Fisher, P.B. Armentrout, Int. J. Mass Spec. Ion Processes 101 Ž1990. R1. w12x C.E. Moore, NSRDS-Nat. Bur. Stand. 35 Ž1971. Volume II. w13x A. Hansel, Ch. Scheiring, M. Glantschnig, W. Lindinger, E.E. Ferguson, J. Chem. Phys. 109 Ž1998. 1748. w14x M.T. Bowers, D.D. Elleman, Chem. Phys. Lett. 16 Ž1972. 486. w15x C.J. Noutary, J. Res. Nat. Bur. Stand. 72A Ž1968. 479. w16x J.T. Clay, E.A. Walters, J.R. Glover, M.V. Willcox, J. Chem. Phys. 101 Ž1994. 2069.
G.K. JarÕis, R.P. Tuckettr Chemical Physics Letters 295 (1998) 145–151 w17x T.A. Walter, C. Lifshitz, W.A. Chupka, J. Berkowitz, J. Chem. Phys. 51 Ž1969. 3531. w18x J.M. Dyke, A.E. Lewis, A. Morris, J. Chem. Phys. 80 Ž1984. 1382. w19x G.K. Jarvis, K.J. Boyle, C.A. Mayhew, R.P. Tuckett, J. Phys. Chem. A 102 Ž1998. 3230. w20x D.M. Smith, R.P. Tuckett, K.R. Yoxall, K. Codling, P.A. Hatherly, J.F.M. Aarts, M. Stankiewicz, J. Chem. Phys. 101 Ž1994. 10559. w21x J.C. Traeger, R.G. McLoughlin, J. Amer. Chem. Soc. 103 Ž1981. 3647. w22x R.K. Boyd, J.H. Beynon, Int. J. Mass Spec. Ion Phys. 23 Ž1977. 163.
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w23x M.W. Chase, C.A. Davies, J.R. Downey, D.J. Frurip, R.A. McDonald, A.N. Syverud, J. Phys. Chem. Ref. Data 14 Ž1985. Suppl. 1. w24x http:rrwebbook.nist.gov ŽNIST website, updated March 1997. w25x S.G. Lias, J.E. Bartmess, J.F. Liebman, J.L. Holmes, R.D. Levin, W.G. Mallard, J. Phys. Chem. Ref. Data 17 Ž1988. Suppl. 1. w26x J.C. Creasey, H.M. Jones, D.M. Smith, R.P. Tuckett, P.A. Hatherly, K. Codling, I. Powis, Chem. Phys. 174 Ž1993. 441. w27x J.C. Creasey, I.R. Lambert, R.P. Tuckett, K. Codling, L.J. Frasinski, P.A. Hatherly, M. Stankiewicz, D.M.P. Holland, J. Chem. Phys. 93 Ž1990. 3295.
Chemical Physics Letters 295 Ž1998. 145–151
On the adiabatic ionisation energy of the CF3 free radical G.K. Jarvis a
a,1
, R.P. Tuckett
b,)
School of Physics and Astronomy, UniÕersity of Birmingham, Edgbaston, Birmingham B15 2TT, UK b School of Chemistry, UniÕersity of Birmingham, Edgbaston, Birmingham B15 2TT, UK Received 16 July 1998; revised 14 August 1998
Abstract The different methods that have been used to measure the adiabatic ionisation energy of the CF3 free radical in the gas phase are reviewed, with values covering the wide range 8.6–9.1 eV. In a study of vacuum-UV photofragmentation of perfluoropropane, C 3 F8 , using the threshold photoelectron–photoion coincidence technique, our value for the appearance energy of CF3q at 298 K, 13.0 " 0.1 eV, leads to an upper limit for the adiabatic ionisation energy of CF3 of 8.8 " 0.2 eV, in good agreement both with that deduced from a low-temperature photoionisation mass spectrometric ŽPIMS. study of CF3 Br and with values obtained from three ion–molecule bimolecular studies. It is lower, however, than the value obtained indirectly from a PIMS study of C 2 F4 and from ab initio calculations. q 1998 Elsevier Science B.V. All rights reserved.
1. Introduction CF4 is used extensively as a carrier gas in the plasma dry etching of integrated silicon circuits. The CF3 radical and its cation, CF3q, may play important roles as reactive species in such systems Žsee, e.g., Refs. w1,2x.. The CF3 free radical is also an important species in atmospheric chemistry because of its role in the oxidative degradation of both perfluorocarbons and hydrofluorocarbons Žsee, e.g., Ref. w3x.. For both reasons, the thermochemistry of CF3 and CF3q may be important factors in modelling the reaction kinet-
)
Corresponding author. Fax: q44-121-414-4403; e-mail: [email protected] 1 Present address: Chemical Sciences Division, MS 6-2100, Lawrence Berkeley Laboratory, University of California Berkeley, CA 94720, USA.
ics associated with both systems. Whilst the thermochemistry of the CF3 neutral is now fairly clearly established w4x, that of the ion remains less clear. The link between them is the adiabatic ionisation energy ŽIE. of the CF3 radical. The problem with measuring IE ŽCF3 . stems essentially from the large geometry change, especially in the FCF bond angle q, between the neutral radical Žpyramidal, C 3v symmetry, q s 110.78 and R C – F s ˚ w5x. and the cation Žplanar, D 3h symmetry, 1.32 A ˚ w6x.. Thus the q s 1208, R C – F s ca. 1.23 A Franck–Condon factor at threshold, corresponding to ionisation from the lowest vibrational level of the neutral to that of the cation, is essentially zero. Indeed, in the highest-quality ab initio calculation of IE ŽCF3 . performed to date, Horn et al. w6x obtained a value of 8.98 " 0.05 eV, with a Franck–Condon envelope showing a maximum at nq 2 s 20 in the umbrella bending vibrational mode of the cation, ca.
0009-2614r98r$ - see front matter q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 9 8 . 0 0 9 3 7 - 3
G.K. JarÕis, R.P. Tuckettr Chemical Physics Letters 295 (1998) 145–151
146
1% of the maximum at nq 2 s 8, and negligible Franck–Condon intensity for nq 2 - 8. Attempts to measure an accurate value for the IE ŽCF3 ., therefore, via a direct, single-photon photoelectron or photoionisation mass spectrometric ŽPIMS. experiment are full of difficulties. No He I, threshold or zero kinetic energy photoelectron spectrum of CF3 has been reported. The classic PIMS study of Lifshitz and Chupka w7x, in which CF3 CH 2 ONO was thermally decomposed to give CF3 radicals and photoionised by tunable vacuum-UV radiation from a hydrogen discharge lamp, yielded a threshold, or first onset, value of 9.25 " 0.04 eV. It is now realised Žsee, e.g., Ref. w8x., that first onsets measured in this way yield a value that is too low for the IE because thermal effects in the neutral precursor have been ignored. In this instance, the correction would be small, so that even if the photoion yield was analyzed by modern techniques, this experiment would always give an IE value which is too high because of the unfavourable Franck–Condon factor at threshold. Later experiments to measure IE ŽCF3 . have therefore used a variety of indirect techniques, some of them in effect attempting to circumvent the Franck–Condon effect. In all cases, including the threshold photoelectron – photoion coincidence ŽTPEPICO. measurement reported by our group w9x, the value obtained is less than 9.25 eV. There have been three studies of this problem by ion–molecule bimolecular reactions. Tichy et al. w10x studied the reaction HClqq CF4 ™ CF3q q HF q Cl
Ž 1.
in a conventional selected ion flow tube. The fast rate constant of this reaction Ž1 = 10y1 0 cm3 moleculey1 sy1 ., probably recorded at 298 K, is less than that predicted by the Langevin theory of ca. 2 = 10y9 cm3 moleculey1 sy1 . Assuming the deviation from the Langevin value was entirely due to the Žsmall. endothermicity of reaction Ž1., they concluded that D r H8 Ž298. of this reaction has an upper limit of q0.06 eV, from which an appearance energy ŽAE. of CF3q from CF4 at 298 K of less than 14.23 eV was deduced. Fisher and Armentrout w11x studied the forward and reverse reaction Krq 2 P3 r 2 q CF4 ™ CF3q q F q Kr
ž
/
Ž 2.
as a function of centre-of-mass collision energy in a guided ion beam mass spectrometer. They established that reaction Ž2. in the forward direction was also endothermic by 0.24 " 0.07 eV and, combined with the very accurate value for the ionisation energy of the Kr atom w12x, they obtained an almost identical value for AE ŽCF3qrCF4 . of 14.24 " 0.07 eV. The temperature of this experiment is also not defined, but is probably 298 K. Although not mentioned specifically in either paper, we show later that these studies yield an upper limit to IE ŽCF3 . of 8.69 " 0.13 eV. Very recently, the reaction HCNq Ž v s 0 . q CF4 ™ CF3q q HF q CN
Ž 3.
has also been studied at 300 K in a selected ion flow tube w13x. From the measured rate constant at this temperature of 6 = 10y1 2 cm3 moleculey1 sy1 , an upper limit for AE ŽCF3qrCF4 . 300 of 14.28 eV was established. Tichy et al. w10x comment that their value of AE ŽCF3qrCF4 . is nearly 1 eV lower than that obtained by unimolecular photon or electron impact of CF4 Žsee Section 3.. Their explanation is that a characteristic of ion–molecule reactions is to yield adiabatic enthalpies of reaction because of the intimate contact of sufficient duration to allow molecular geometries to relax to adiabatic configurations. In other words, due to strong ion-induced dipole interactions, long-lived complexes readily form in such reactions. Under these circumstances, Franck–Condon factors are distorted w14x, yielding adiabatic reaction enthalpies. Turning to unimolecular reactions, Noutary w15x and Clay et al. w16x performed PIMS studies of CF3 X ŽX s Cl, Br, I. and CF3 Br, respectively. The former study was performed at room temperature, the latter in the environment of a jet-cooled molecular beam. Both experiments measured AE ŽCF3qrCF3 X. T , with X s Br only in the case of Clay et al. In both cases, AEs were determined as the first onset of the CF3q fragment in the ion yield curve, and no attempt was made to allow for thermal effects in the CF3 X precursor. Thus it might be expected that both studies, but especially that of Noutary due to its higher temperature, would produce IE ŽCF3 . values that are too low. From the CF3 I study Noutary concluded that IE ŽCF3 . F 8.62 eV, from their CF3 Br study Clay et al. concluded that D f H08 ŽCF3q. s 362 " 4
G.K. JarÕis, R.P. Tuckettr Chemical Physics Letters 295 (1998) 145–151
kJ moly1 . Combined with the 0 K heat of formation of CF3 w4x, the Clay et al. result leads to an IE ŽCF3 . value of 8.55 " 0.08 eV. Using a different precursor, Asher and Ruscic w4x have remeasured some early PIMS experiments of Walter et al. w17x on C 2 F4 . Both these studies yielded the ion yield curves of CFq and CF3q from C 2 F4 , i.e. C 2 F4 q hn ™ CFqq CF3 q ey or CF3q q CF q ey Ž 4. By careful analysis of both threshold regions, Asher and Ruscic extrapolated appearance energies at 0 K of these ions from C 2 F4 . Assuming that there were no kinetic shifts or reverse activation barriers in either channel, the difference of these two appearance energies was taken to be the difference in ionisation energy of the CF and CF3 radicals. Using the literature value for the CF radical w18x, they obtained a value for IE ŽCF3 . of 9.055 " 0.011 eV. Their paper reviews comprehensively all the different measurements that have been made of IE ŽCF3 . up to 1996, with the strengths and weaknesses of many of the different methods discussed. For reasons discussed later, they believe that their value is likely to be a lower limit, but they make no attempt to explain why the values from the two early ion–molecule studies w10,11x are, in their opinion, incorrect. Their claim that 9.055 eV is the correct value for the IE ŽCF3 . is partially substantiated by the ab initio calculations of Horn et al. w6x mentioned earlier. Our contribution to this problem arose by serendipity. We have measured the photofragmentation channels of a range of saturated and unsaturated perfluorocarbon cations by TPEPICO spectroscopy, using tunable vacuum-UV radiation from a synchrotron as the photoionisation source w9,19x. The CF3q cation was observed at the onset of ionisation for C 3 F8 , providing ca. 50% of the branching ratio at this energy. We have therefore measured AE ŽCF3qrC 3 F8 . 298 , from which an IE ŽCF3 . - 8.8 " 0.2 eV was deduced. This calculation was described only briefly in our paper w9x. The purpose of this Letter is to describe this calculation in more detail, and to place our value for IE ŽCF3 . in the context of other recent measurements for this quantity.
147
2. Calculation of IE (CF3 ) from TPEPICO spectroscopy Our experiment combined a TPEPICO spectrometer with tunable vacuum-UV radiation in the range 10–25 eV from a synchrotron source. The resolution of the vacuum-UV radiation was 0.4 nm, corresponding to 0.05 eV at 13 eV. Full details are given elsewhere w9x. TPEPICO spectra were recorded continuously as a function of photon energy, from which it was possible to determine the fragment ions formed at any particular energy. Breakdown diagrams, yielding the formation probability of the product ions as a function of the internal energy of the excited parent ion, were obtained in the usual manner. Experiments were performed at 298 K. For the three saturated perfluorocarbons studied, C 2 F6 , C 3 F8 and n-C 4 F10 , no parent ion was observed over the range of photon energies corresponding to electron removal from one of the valence molecular orbitals, 12–25 eV. A similar result was found for all three compounds in the PIMS study of Noutary w15x. The most likely reason is that, as with the simplest perfluorocarbon CF4 w20x, the ground state of these parent ions is repulsive in the Franck– Condon region accessible by one-photon excitation. These states then decay by an impulsive decay mechanism, with the energy localised in one bond and the timescale for this bond to break being shorter than that for intramolecular energy redistribution. One obvious criterion for such a process is that all three perfluorocarbons must possess at least one dissociative ionisation channel below the observed onset of ionisation, 13.0 " 0.1 eV in the case of C 3 F8 w9x Žthe stepsize used in these scanning-energy experiments was 0.352 nm or ca. 0.05 eV at 13 eV. This is too large ideally for an experiment with a spectrometer resolution also of 0.05 eV. Being conservative, we define the error in the appearance energy to be "0.1 eV.. With this molecule, two cations are formed at threshold, C 2 F4q and CF3q, each with a branching ratio of ca. 50% Žnote that Noutary w15x observed the first-onset threshold of CF3q at 13.22 eV.. The presence of these ions at the onset of ionisation for C 3 F8 is interesting for different reasons. The energies of the dissociation channels are such that C 2 F4q can only form with CF4 as the sole neutral fragment; channels such as C 2 F4qq
G.K. JarÕis, R.P. Tuckettr Chemical Physics Letters 295 (1998) 145–151
148
CF3 q F q ey are thermodynamically forbidden. Hence Fy migration must occur across a C–C bond before C–C bond cleavage can occur. By contrast, the interest in the observation of CF3q at the onset of ionisation is thermodynamic, not mechanistic. We consider the process C 3 F8 q hn ™ C 3 F8q q ey C 3 F8q ™ CF3q q C 2 F5 , Overall: C 3 F8 ™ CF3q q C 2 F5 q ey
Ž 5.
for which an AE ŽCF3qrC 3 F8 . 298 of 13.0 " 0.1 eV Žor 1254 " 10 kJ moly1 . is measured. We have used the procedure of Traeger and McLoughlin w21x to convert this appearance energy into a heat of reaction for Ž5. at 298 K,
Dr H2988 F AE Ž CF3q rC 3 F8 . 298 q 298
H0
q
c p Ž C 2 F5 . dT y
298
H0 5 2
c p Ž CF3q . dT
RT
Ž 6.
The upper limit for D r H2988 arises because an appearance energy Žof ŽAqrAB. 298 . defines an upper limit to the thermodynamic energy of Aqq B q ey. Note also that, like Traeger and McLoughlin w21x, we use the stationary electron convention that at threshold the electron has zero translational energy, not 1.5RT in this instance evaluated at 298 K. With this convention, 298
H0
c p Ž ey . dT s 0
Dr H2988 s Df H2988 Ž CF3q . q Df H2988 Ž C 2 F5 . y Df H2988 Ž C 3 F8 . F 1276 " 10 kJ moly1
CF3 ™ CF3q q ey
c p Ž Aq or B . dT s H2988 y H08
Ž 8.
For both CF3q and C 2 F5 , this term will contain contributions from translational Ž2.5RT ., rotational Ž1.5RT ., and vibrational Ž NA hnrwexpŽ hnrk B T . y 1x per vibrational mode. motion evaluated at T s 298
Ž 10 .
Using the most accurate value for the heat of formation of neutral CF3 at 298 K, y466" 4 kJ moly1 w3x, D r H2988 for reaction Ž10. has an upper limit of 852 " 25 kJ moly1 . Formally, the adiabatic ionisation energy of the CF3 radical is the enthalpy of reaction Ž10. evaluated at 0 K, i.e. IE Ž CF3 . s Dr H2988 Ž 10 . y 298
H0
y
298
Ž 9.
Using values for the heats of formation of C 2 F5 and C 3 F8 at 298 K of y893 " 4 and y1783" 7 kJ moly1 , respectively w25x, we obtain an upper limit for D f H2988 ŽCF3q. of 386 " 21 kJ moly1 . Consider now the reaction describing photoionisation of the CF3 radical,
Ž 7.
If the last three terms in Eq. Ž6. sum to zero, as is often assumed w22x, a significant error may be introduced in converting the measured appearance energy into an upper limit for the enthalpy of the reaction. The error is greater the larger the size of the fragments Aq and B. Considering the second and third terms in the right-hand side of Eq. Ž6.,
H0
K. Using values for the vibrational frequencies of CF3q and C 2 F5 from elsewhere w23,24x, we estimate that Ž H2988 y H08. is 4.70RT for CF3q and 6.74RT for C 2 F5 . The last three terms in Eq. Ž6. therefore sum to 8.94 RT, or 22 kJ moly1 at 298 K. We therefore determine D r H2988 for reaction Ž5. to have an upper limit of 1276 " 10 kJ moly1 . With the stationary electron convention, we can then write for this reaction that
298
H0
c p Ž ey . dT
c p Ž CF3q . dT q
298
H0
c p Ž CF3 . dT
Ž 11 . The second term on the right hand side of Eq. Ž11. is zero in the stationary electron convention, and the last two terms only differ in their vibrational contribution. For CF3 , using vibrational frequencies in the literature w23x, we calculate such a contribution to be 0.65RT, whereas for CF3q, as shown above, we calculate 0.70 RT. At 298 K the difference is negligible, 0.1 kJ moly1 . We derive, therefore, an upper limit for IE ŽCF3 . of 852 " 25 kJ moly1 or 8.8 3 "0.2 5 eV. The errors have been deliberately conservative, and we round this number down to an upper limit of 8.8 " 0.2 eV.
G.K. JarÕis, R.P. Tuckettr Chemical Physics Letters 295 (1998) 145–151
3. Discussion The first comment to make about our result is that this experiment cannot be used to measure the absolute value of IE ŽCF3 .. It can only reveal its upper limit. At the onset of ionisation the ground electronic state of C 3 F8q is repulsive in the Franck–Condon region, hence impulsive photodissociation occurs via C–C bond cleavage with the production of CF3q and C 2 F5 fragments. In other words, the CF3qq C 2 F5 q ey dissociation channel lies below the onset of ionisation. After allowing for thermal effects in C 3 F8 , the AE ŽCF3qrC 3 F8 . 298 value must therefore transform into an upper limit for both D f H2988 ŽCF3q. and IE ŽCF3 .. Our upper limit for IE ŽCF3 ., 8.8 " 0.2 eV, is in accord with the results from bimolecular ion– molecule reactions w10,11,13x, and from early w15x and later w16x PIMS studies of CF3 X ŽX s Br, I.. However, we are in disagreement with the result of Asher and Ruscic w4x from a PIMS study of C 2 F4 that the true IE ŽCF3 . is as high as 9.055 " 0.011 eV, with the possibility that this might represent a lower limit. The situation in C 3 F8 is very similar to attempts to measure an absolute value of IE ŽCF3 . by photoionisation of CF4 . The ground state of CF4q is also repulsive in the Franck–Condon region, dissociating on a sub-picosecond timescale to CF3qq F by cleavage of one C–F bond with a substantial release of translational kinetic energy w26x. CF4q is therefore not observed on the timescale of a mass spectrometric or coincidence experiment. The AE ŽCF3qrCF4 . value reported by PIMS, 15.35 eV w17x, or the slightly higher values from coincidence spectroscopy w20,26,27x, bears no relation to the energy of the CF3qq F q ey dissociation channel; it only relates to the onset of ionisation of CF4 . Even if the Asher and Ruscic value w4x for IE ŽCF3 ., 9.055 eV, is correct, the energy of this channel, given by D r H08 for CF4 ™ CF3 q F plus IE ŽCF3 ., is as low as 14.67 eV. Thus in the case of CF4 , the difference between AE ŽCF3qrCF4 . and the energy of the CF3qq F q ey dissociation channel has a lower limit of ca. 0.7 eV. In view of other measurements of IE ŽCF3 ., it seems unlikely that the difference between AE ŽCF3qrC 3 F8 . and the energy of the CF3qq C 2 F5 q ey dissociation channel is so great; indeed, it is probably as small as ca. 0.1–0.2 eV.
149
As described earlier, Asher and Ruscic have measured the CFq and CF3q ion yields from C 2 F4 at 298 K. The threshold region of both curves is fit by an appropriate model which allows for a thermal energy distribution of the C 2 F4 reactant at this temperature. By this method they are able to extrapolate AEs for both ions at 0 K. They then make the fundamental assumption that there are no kinetic shifts or reverse activation barriers in either of the two dissociation channels CFqq CF3 q ey or CF q CF3qq ey. In other words, the appearance energy of both ions corresponds exactly to the energy of the appropriate dissociation channel at 0 K. Under these circumstances, AE Ž CFqrC 2 F4 . 0 y AE Ž CF3qrC 2 F4 . 0
Ž s 0.055 " 0.003 eV. ' IE Ž CF. y IE Ž CF3 . Ž 12 . Using the value from Dyke et al. w18x for the adiabatic ionisation energy of the CF radical, Asher and Ruscic obtain IE ŽCF3 . s 9.055 " 0.011 eV, with most of the error arising from the error in IE ŽCF.. They justify that this value could be a lower limit by claiming that, if small kinetic shifts are present, CFq should have a slightly larger shift than that of CF3q. Then the differences in AEs, 0.055 eV, is an upper limit, and the value of IE ŽCF3 ., 9.055 eV, is a lower limit. Our main concern with this analysis arises from the assumption that there are no barriers in either exit channel to dissociation. To form both CF3q or CF3 products from C 2 F4 , a fluorine atom has to migrate across the C5C bond. This could easily lead to an activation energy barrier on the potential energy surface to products. It is true that if such a barrier exists, it is likely to be similar in the two exit channels CFqq CF3 q ey and CF q CF3qq ey. Until full details of the potential energy surfaces for these two processes are known, however, we cannot know whether the observed onsets of these ions correspond to their thermodynamic onsets, or whether there is any difference in the threshold region of the two dissociation channels. We therefore query the assumption made in Eq. Ž12.. On a minor point, whilst we would agree that any kinetic shifts in CFq and CF3q are likely to be small, we would argue that CF3q, and not CFq, should have the larger shift since
150
G.K. JarÕis, R.P. Tuckettr Chemical Physics Letters 295 (1998) 145–151
it is the ion with the larger number of internal degrees of freedom. If this is true, then technically 9.055 eV becomes an upper limit to this value of IE ŽCF3 .. In the Clay et al. study w16x, CF3q is detected from CF3 Br by PIMS using synchrotron radiation. The temperature of the jet-cooled source is not defined, but we estimate an upper limit of ca. 30 K. The CF3q signal is measured at its first onset with an appearance energy of 11.56 " 0.02 eV, and allowance was made for second-order radiation from the synchrotron source. The authors believe that this is an adiabatic value, i.e. this energy corresponds to the CF3qq Br q ey dissociation threshold with no kinetic shift or reverse activation barrier. This threshold then converts into an upper limit for IE ŽCF3 . of 8.55 " 0.08 eV. At the low temperature of their source, it is very difficult to see how ‘thermal’ effects in the CF3 Br precursor could account for a shift as great as 0.5 eV, as the Asher and Ruscic analysis of this result would suggest. Likewise, in the two early ion–molecule studies w10,11x similar values of AE ŽCF3qrCF4 . are reported. The temperature of neither experiment is explicitly defined, but we can safely assume it is ca. 298 K. Using the more accurate value from Fisher and Armentrout w11x of 14.24 " 0.07 eV, the procedure of Traeger and McLoughlin, described in Section 2, can be used to convert this value of AE ŽCF3qrCF4 . 298 into an upper limit for both D f H2988 ŽCF3q. and IE ŽCF3 .. We obtain D f H2988 ŽCF3q. F 373 " 9 kJ moly1 and IE ŽCF3 . F 8.69 " 0.13 eV. Once again, this value is significantly lower than the value obtained by Asher and Ruscic from their unimolecular PIMS study of C 2 F4 .
4. Conclusions We believe that the result of accumulated evidence from three bimolecular ion–molecule studies w10,11,13x, from a low temperature unimolecular PIMS study of CF3 Br w16x, and from our room temperature TPEPICO study of C 3 F8 w9x is that the adiabatic ionisation energy of the CF3 radical is certainly less than 8.8 eV, with its probable value being less than 8.6–8.7 eV. We agree with many of the criticisms made by Asher and Ruscic w4x about
the different methods that have been used to refine the original Lifshitz and Chupka value of 9.25 eV w7x, in particular that many PIMS studies quote first onsets which do not allow for thermal effects in the neutral precursor. However, we believe that their indirect method of photoionisation of C 2 F4 to yield CFq and CF3q cannot constitute a definitive measurement of IE ŽCF3 ., since details of the two potential energy surfaces to these product channels are not yet known. Acknowledgements We thank Dr. B. Ruscic ŽArgonne National Laboratory. for communications, Professor E.E. Ferguson ŽNOAA, Boulder. for informing us about the work of Ref. w12x, and Dr. R.A. Kennedy ŽUniversity of Birmingham. and Professor J.M. Dyke ŽUniversity of Southampton. for helpful discussions. We also thank Mr. K.J. Boyle and Mr. D.P. Seccombe for a critical reading of the manuscript. GKJ thanks NERC for the award of a Research Studentship. References w1x V.M. Donnelly, D.L. Flamm, J. Appl. Phys. 51 Ž1980. 5273. w2x J.W. Coburn, Plasma Chem. Plasma Proc. 2 Ž1982. 1. w3x A.R. Ravishankara, A.A. Turnipseed, N.R. Jensen, S.R. Barone, M. Mills, C.J. Howard, S. Solomon, Science 263 Ž1994. 71. w4x R.L. Asher, B. Ruscic, J. Chem. Phys. 106 Ž1997. 210. w5x C. Yamada, E. Hirota, J. Chem. Phys. 78 Ž1983. 1703. w6x M. Horn, M. Oswald, R. Oswald, P. Botschwina, Ber. Buns. Phys. Chem. 99 Ž1995. 323. w7x C. Lifshitz, W.A. Chupka, J. Chem. Phys. 47 Ž1967. 3439. w8x R.L. Asher, E.H. Appleman, B. Ruscic, J. Chem. Phys. 105 Ž1996. 9781. w9x G.K. Jarvis, K.J. Boyle, C.A. Mayhew, R.P. Tuckett, J. Phys. Chem. A 102 Ž1998. 3219. w10x M. Tichy, G. Javahery, N.D. Twiddy, E.E. Ferguson, Int. J. Mass Spec. Ion Processes 79 Ž1987. 231. w11x E.R. Fisher, P.B. Armentrout, Int. J. Mass Spec. Ion Processes 101 Ž1990. R1. w12x C.E. Moore, NSRDS-Nat. Bur. Stand. 35 Ž1971. Volume II. w13x A. Hansel, Ch. Scheiring, M. Glantschnig, W. Lindinger, E.E. Ferguson, J. Chem. Phys. 109 Ž1998. 1748. w14x M.T. Bowers, D.D. Elleman, Chem. Phys. Lett. 16 Ž1972. 486. w15x C.J. Noutary, J. Res. Nat. Bur. Stand. 72A Ž1968. 479. w16x J.T. Clay, E.A. Walters, J.R. Glover, M.V. Willcox, J. Chem. Phys. 101 Ž1994. 2069.
G.K. JarÕis, R.P. Tuckettr Chemical Physics Letters 295 (1998) 145–151 w17x T.A. Walter, C. Lifshitz, W.A. Chupka, J. Berkowitz, J. Chem. Phys. 51 Ž1969. 3531. w18x J.M. Dyke, A.E. Lewis, A. Morris, J. Chem. Phys. 80 Ž1984. 1382. w19x G.K. Jarvis, K.J. Boyle, C.A. Mayhew, R.P. Tuckett, J. Phys. Chem. A 102 Ž1998. 3230. w20x D.M. Smith, R.P. Tuckett, K.R. Yoxall, K. Codling, P.A. Hatherly, J.F.M. Aarts, M. Stankiewicz, J. Chem. Phys. 101 Ž1994. 10559. w21x J.C. Traeger, R.G. McLoughlin, J. Amer. Chem. Soc. 103 Ž1981. 3647. w22x R.K. Boyd, J.H. Beynon, Int. J. Mass Spec. Ion Phys. 23 Ž1977. 163.
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