Electron-Energy-Loss Spectroscopy of Spin-Forbidden Transitions with Vibronic Resolution: The Lowest Triplet State of Fluorobenzenes

Low-energy electron-energy-loss spectra in the range of the lowest triplet state are presented for solid benzene, fluorobenzene, and p-difluorobenzene deposited at a temperature of 30 K. Vibronic structure within the triplet bands of fluorobenzene and p-difluorobenzene is observed for the first time. The spectrum of fluorobenzene closely resembles that of benzene, whereas p-difluorobenzene shows characteristic differences. The spectra of all three molecules are dominated by pure progressions of the totally symmetric ring-breathing vibrational mode v₁. In addition, less intense transitions to vibronic levels relating to the e_2g modes in benzene are observed and their assignment is given. The occurrence of e_2g modes in all three spectra underlines that the vibronic coupling pathways between the lowest and the second triplet state are similar in benzene and its derivatives. In p-difluorobenzene a splitting of the extremely low frequency mode that relates to the v₈ (e_2g) vibration in benzene is observed. This is in agreement with theoretical predictions of an asymmetric shape of the lowest triplet potential energy surface along a symmetry coordinate relating to v₈ in substituted benzenes.