Electronic and vibrational energy transfer and relaxation in crystalline ϱ-dibromodenzene

Abstract

Electronic and vibrational energy transfer and relaxation mechanisms have been studied in crystalline ϱ-dibromobenzene. Variations in linewidths in the phosphorescence spectrum have been related to the vibrational exciton density-of-states functions. Migration by ground state vibrational excitons is found to be generally more probable than by triplet excitons in these crystals. Broadening in the absorption spectrum has been attributed to fast vibrational relaxation (≈10 −13 s) enhanced by exciton—phonon coupling. The phosphorescence quantum yield is found to be constant throughout the S O → T 1 absorption spectrum. External spin—orbit coupling has been found to inhance the activity of the totally symmetric fundamentals, whereas Herzberg—Teller vibronic coupling dominates in the forbidden part of the spectrum. The Herzberg—Teller active b 2g mode is assigned as shifting in frequency from 277 cm −1 in the ground state to 104 cm −1 in the excited state.