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 tri plet state are presented for solid benzene, fluorobenzene, and p-diflu orobenzene deposited at a temperature of 30 K. Vibronic structure with in the triplet bands of fluorobenzene and p-difluorobenzene is observe d for the first time. The spectrum of fluorobenzene closely resembles that of benzene, whereas p-difluorobenzene shows characteristic differ ences. The spectra of all three molecules are dominated by pure progre ssions of the totally symmetric ring-breathing vibrational mode v(1). In addition, less intense transitions to vibronic levels relating to t he 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 v (8) (e(2g)) vibration in benzene is observed. This is in agreement wit h theoretical predictions of an asymmetric shape of the lowest tripler potential energy surface along a symmetry coordinate relating to v(8) in substituted benzenes.