Laser induced fluorescence from fluorobenzene radical cations in the gas phase

Laser induced fluorescence excitation spectra have been observed for a number of fluorobenzene radical cations including hexafluorobenzene, sym-trifluorobenzene and its deuterated analog, three isomers of tetrafluorobenzene, and pentafluorobenzene. In all cases vibrational structure is clearly resolved and the spectra assigned. It is shown that low frequency ring distortion modes and the C-F stretch dominate the spectra. There is an extremely strong correlation between vibrational frequencies in the neutral molecule and both electronic states of the ion involved in the transition. Convincing evidence is found for Jahn-Teller distortion of the ions with degenerate ground states.