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.

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