Mass-Spectrometric Study of the Photoionization of C2F4 and CF4

Abstract

Photoionization-efficiency curves for CFa+ from CF 4 and for the parent ion and selected fragment ions from C2F4 are reported for the energy region from onset of ionization to about 20 eV. The ionization-efficiency curves for both parent and fragment ions from C2F4 demonstrate a number of very pronounced autoionization peaks. Appearance potentials are made more reliable by using a cooled ionization chamber to remove most of the thermal vibrational energy of the molecules. From the difference between the appearance potentials of CF+ and CFa+ from C2F 4 , it is concluded that J.P. (CF) =J.P. (CFa) +0.06 eV. Therefore the J.P. (CF) =9.23±0.08 eV can be calculated from the J.P. (CFa) =9.17±0.08 eV derived in this paper. The onset of CFa from CF4 is asymptotic, and there is no clear threshold. An upper limit for this threshold was set at 15.35 eV.

Simultaneous Adjustment of Experimentally Based Enthalpies of Formation of CF3X, X = nil, H, Cl, Br, I, CF3, CN, and a Probe of G3 Theory

“…One should take notice of the fact that the difference can be fully corroborated by simple superposition of suitably scaled spectra, thus avoiding any possible influences arising from the interpretation of the threshold shapes and their individual fitting with model functions. The results of Asher and Ruscic 6 are in very good agreement with the slightly coarser measurements by Walter et al 55 who give 13.70 ( 0.02 and 13.76 ( 0.01 eV for the two appearance energies and 0.06 eV for the difference. Since the two fragmentation processes differ only by the final location of the positive charge, the difference between the two appearance energies simply corresponds to the difference in the adiabatic ionization potentials of CF 3 and CF, EI(CF 3 ) -EI(CF).…”

“…With the well-known ionization energy of CF determined by Dyke et al 56 as 9.11 ( 0.01 eV, Asher and Ruscic 6 obtain EI(CF 3 ) ) 9.05 5 ( 0.01 1 eV. A very similar result of 9.05 ( 0.02 eV was derived by Berkowitz 57 using the fragment appearance energies given by Walter et al 55 One should also note that this path to IP(CF 3 ) does not involve any additional thermochemical values, such as ∆H f°( C 2 F 4 ), ∆H f°( CF 3 ), or ∆H f°( CF) nor does it depend on direct measurement of the adiabatic threshold for direct ionization, which suffers from very unfavorable Franck-Condon factors 58,59 and is, hence, prone to produce only an upper limit. In fact, at least technically, this approach produces a lower limit to the ionization energy of CF 3 .…”

Reactions of fluorocarbon ions in C2F6. Implications for the radiolysis

“…At the low pressure limit, where charge exchange does not occur to any appreciable extent, some C2F t is produced by direct photoionization of C 2F 6. More interesting conclusions can be derived from the results given in figure 3 which shows the data obtained in the xenon-C 2F 6 mixture irradiated with 16.67-16.84 eV photons in the photoionization mass spectrometer. Here the Xe +(2P 1/2) ions (apparently about 30% of the Xe + ions formed under these conditions) undergo reaction (3) to produce CFt ions in C 2F 6 . The interesting point here is that if one accepts a heat of formation of CFt of 414 kllmol (99.1 kcal/mol) [3], then the CFt ions formed in reaction (3) involving Xe +(2P1/2) and C 2F 6 can only have a maximum of 8.4 kllmol (2 kcal/mol) excitation energy. Even so, these CF t ions undergo reaction (1) to from C 2F t product ions ( fig.…”

“…From the generally accepted heats of formation of these species [3][4] (for C2F t only an estimated heat ·Work suppo rted in part by the U.S. Atom ic Energy Co mmi ssion.…”

“…(2) the charge exchange reactions of the CF t ion formed in reaction (2) were investigated in a tandem mass spectrometer [2bl-In these experiments, CFt ions having an average kinetic energy of 0.3 eV 2 , were ' I eV = 96.485 kJ 'mol -'_ reacted with a variety of compounds having ionization potentials above and below the reported value for CF 3 (9.17 e V) [3]. The results suggested that some fraction of the CF t ions had as much as 2.9 eV internal energy under the conditions of these experiments, since charge exchange was found to occur with CzH4 (I.P.…”