The electronically excited states of furan, thiophene, and pyrrole have been studied by electron impact at scattering angles from 0° to 80°, and impact energies of 30 and 50 eV. Low-lying features at 3.99 and 5.22 eV in furan, 3.75 and 4.62 eV in thiophene, and 4.21 eV in pyrrole are identified as singlet → triplet transitions. The locations and, for furan and thiophene, the energy splittings of these excitations suggest that they are analogous to the lowest π → π* singlet → triplet transitions in benzene, and that these heterocycles have appreciable aromatic character. A weak feature observed in pyrrole at 5.22 eV is attributed to an optically forbidden singlet → singlet transition. In all three molecules, transitions to several superexcited states are observed.

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The electron impact excitation of 1,3-butadiene has been studied experimentally at impact energies of 20, 35, and 55 eV and scattering angles from 10° to 85°. The energy and angular dependences of the cross section ratios are used to identify the nature of the excited states. Two transitions with maxima at 3.22 and 4.91 eV are identified as singlet→triplet transitions. Comparison with theoretical calculations indicates that these are due to the 13Bu and 13Ag states, respectively. Their significance for the photochemistry of this molecule is discussed. The optically allowed X̃ 1Ag → 11Bu(N → V1) transition is observed with a maximum at 5.92 eV. An additional transition appears between 6.9 and 7.8 eV with vibrational features at 7.09, 7.28, and 7.46 eV. The optical absorption in this region was originally attributed to a 1A1 state of the s -cis molecule and subsequently to a Rydberg state or to a 1Ag state of the s -trans molecule. On the basis of intensity arguments and the angular dependence of the cross section ratios, we suggest that it may instead be due to the X̃ 1Ag → 21Bu transition of the s -trans molecule. Rydberg transitions are observed at 8.00 and 8.18 eV. Two broad transitions are also seen beyond the first ionization potential with maxima at 9.50 and 11.00 eV. The results of this study are in good agreement with recent ab initio configuration interaction (CI) calculations, and give support to the analysis of the valence excited states in terms of a ``molecules-in-molecules'' approach. This is consistent with recent interpretations of the resonance energy and reactivity of this molecule and differs from the older classic model of extensive delocalization in the π electron system.

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.