SPIN-FORBIDDEN TRANSITIONS IN AMORPHOUS AND CRYSTALLINE THIN-FILMS OFBENZENE

Electron-energy-loss spectra of the lowest singlet-triplet transition in thin films of amorphous and crystalline solid benzene have been rec orded at a temperature of 32 K. The structure of the solid films depen ds on the substrate temperature at which the molecules are deposited. At 32 K films of amorphous character are formed whereas crystalline fi lms are produced at a deposition temperature of 100 K and a thickness of more than 5 layers. This is shown by the angular dependence of the vibrational spectra. While amorphous films display an essentially isot ropic scattering behavior, the crystalline films are characterized by a distinct enhancement of dipole-allowed vibrations under specular con ditions. The 0-0 transition to the lowest tripler state is found at 3. 676 eV in crystalline benzene and at 3.665 eV in the amorphous film. T his is the first time that the shift of a spin-forbidden band can be r elated to changes in the bull; structure of the molecular solid. The r eason for this shift is explored by discussing the available experimen tal and theoretical information about effects of the surrounding mediu m on the transition energies of aromatic hydrocarbons. (C) 1998 Elsevi er Science B.V.