Electronic spectroscopy of benzene and the fluorobenzenes by variable angle electron impact

Electron-impact spectra of benzene and 11 fluorine-substituted derivatives have been obtained at impact energies of 75, 50, and either 25 or 30 eV, and scattering angles from 50 to 800. Each molecule shows an absorption maximum at about 3.9 eV corresponding to a singlet→triplet, π→π*, transition. In benzene, fluorobenzene, o- and m-difluorobenzene, and 1,3,5-trifluorobenzene, an additional singlet→triplet excitation was detected at about 5.7 eV. Three singlet→singlet transitions analogous to the 4.90, 6.20, and 6.95 eV benzene excitations are seen in each of the fluorine-substituted molecules. The more highly substituted compounds exhibit an additional singlet→singlet transition, which we designate as the C band system, that is most clearly observed in the hexafluorobenzene spectrum, where it has a peak at 5.32 eV. We briefly discuss the effects on relative transtion intensities due to the different molecular symmetries of the various fluorobenzenes. We also report numerous superexcited states for each molecule studied