B³Σ_u^- predissociation and relaxation processes in matrix isolated S₂

Spectroscopy of S₂ in rare gas solids is re-examined. Excitation of the B³Σ_u^- state populates by predissociation-recombination sequence several excited electronic states, which can then be characterized by their fluorescence spectra. The observed spectra are quite analogous to those observed in "thermoluminescence" experiments. Analysis of the emission and laser excitation spectra yields the spectroscopic constants of several electronic states of S₂ in solid Ne. In particular, the data provide direct evidence that the lowest a ¹Δ_g state is located in this matrix 4395 cm ^-1 above the ground state. Vibrational relaxation within the B ³Σ_u- state was examined. It exhibits a very steep rate dependence on temperature and enhanced relaxation efficiency with decreasing mass of the host atoms. It is proposed that relaxation in S ₂ is due to a direct multiphonon process, and that unlike relaxation in light hydride molecules, the molecular rotation is unimportant.