Spin-forbidden transitions in amorphous and crystalline thin films of benzene Original Research Article

Electron-energy-loss spectra of the lowest singlet–triplet transition in thin films of amorphous and crystalline solid benzene have been recorded at a temperature of 32 K. The structure of the solid films depends on the substrate temperature at which the molecules are deposited. At 32 K films of amorphous character are formed whereas crystalline films 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 isotropic scattering behavior, the crystalline films are characterized by a distinct enhancement of dipole-allowed vibrations under specular conditions. The 0–0 transition to the lowest triplet state is found at 3.676 eV in crystalline benzene and at 3.665 eV in the amorphous film. This is the first time that the shift of a spin-forbidden band can be related to changes in the bulk structure of the molecular solid. The reason for this shift is explored by discussing the available experimental and theoretical information about effects of the surrounding medium on the transition energies of aromatic hydrocarbons.