External heavy atom effects on benzene in rare gas hosts

Abstract

A study has been made of the triplet lifetimes and emission spectra of benzene and perdeuterobenzene in various sites in neon, argon, krypton, and xenon hosts. Accurate lifetime measurements show that there is no deuterium effect on the phosphorescence lifetimes in krypton and xenon. One of the sites in argon and krypton has been identified as having threefold symmetry (D_3h or C_3v), as evidenced by a complete absence of symmetric vibrations. It is shown that the vibrational structure of the phosphorescence from krypton and xenon are very similar, with both symmetric and antisymmetric modes, even though there is a change in the triplet decay rate by a factor of 30. This implies an external heavy atom effect proceeding by both a direct and a vibronic route. The vibronic structure of the phosphorescence varies substantially with the site, indicating a shallow potential in the triplet state which can be easily influenced by the environment.