Electron-impact spectroscopy of the fluoroethylenes

Abstract

The electron-impact excitation spectra of the six fluoroethylenes and chlorotrifluoroethylene have been investigated at impact energies of 60, 40, and either 20 or 25 eV, and at scattering angles from 0° to 80°. The energy and angular dependence of the relative differential cross sections was determined for several features in the energy-loss range 0–16 eV. This information was used to identify transitions as either spin forbidden or spin allowed. In each molecule, the lowest observed inelastic transition is a spin-forbidden excitation with maximum intensity between 4.18 and 4.68 eV. The locations of these transitions, which are analogous to the N→T transition in ethylene, can be used to interpret the results of some photochemical electronic energy transfer experiments. A second weak singlet→triplet transition occurs in vinyl fluoride with a maximum intensity at 6.4 eV. The other features observed in the 6–10 eV energy-loss region of these molecules agree well with optical spectra. In particular, the large positive shift in the N→V transition energy of tetrafluoroethylene is confirmed. However, no such shift occurs in the N→V maximum intensity transition energy of chlorotrifluoroethylene (7.80 eV), indicating that the steric resistance to torsion is probably not the cause of the observed shift in tetrafluoroethylene. Transitions to many superexcited states lying above the first ionization potential are observed in each molecule. Using the term value method, many of these transitions, as well as lower-lying ones, are assigned to Rydberg series. The average term values obtained in this study agree well with those determined from other series of molecules.

Low-energy electron collisions with tetrafluoroethene, C2F4

“…As noted earlier, our cross sections for the T and V states are smaller than the reported experimental values, 16 assuming ͑as seems likely͒ that no great error has arisen in normalizing the latter via the elastic cross section. On the other hand, one often finds that few-channel calculations overestimate the scattering cross section; to take a pertinent example, calculated cross sections 31,39 for the T and V states of C 2 H 4 were about a factor of 2 larger than measured 40 values.…”

“…Corresponding differential cross sections are shown in Figs. 5 and 6, which also show, at 25 and 40 eV, the measured results of Coggiola et al; 16 as just discussed, we have normalized their relative measurements by equating their elastic cross section at 25 eV and 30°to our ours.…”

“…3. At 25 and 40 eV, we may compare to the measurements of Coggiola et al, 16 which we have placed on an absolute scale by normalizing to our computed DCS at 25 eV and 30°. As may be seen, there is good qualitative agreement between the experimental and theoretical cross sections, and similar results would have been obtained from normalizing at any other angle except 20°, which would have raised the normalized values by about 30%.…”

“…14 A direct search for resonances in the C 2 F 4 cross section was undertaken by Chiu et al 15 using electron transmission spectroscopy; their technique measures the derivative of the total cross section, rather than the cross section itself, and is thus sensitive to rapid variations with energy caused by sharp resonances ͑though it may fail to see weak or broad features͒. Coggiola et al 16 measured the excitation threshold energies for several states and obtained relative differential cross sections for elastic scattering and for two excitation processes at 25 and 40 eV. Their limited data appear to be the only elastic or electron-impact excitation cross sections for C 2 F 4 available to date.…”

Electron transport properties and collision cross sections in C2F4

“…Excitation of the T state is important because of its very low threshold, 4.68 eV. 15 The V state has a considerably higher threshold, measured values being 8.84 eV ͑Ref. 15͒ and 8.88 eV, 16 but it has a large oscillator strength ͑photoexcitation cross section͒, which typically implies that the electron cross section is also large.…”