Fluorescence of Benzene Vapor at Low Pressures

Abstract

The intensity and spectral distribution of fluorescence from the first excited singlet state ${(}^1{B}_{\text{2u}})$ of benzene in the pressure range 0.05–7 × 10⁻⁵ torr has been studied by monitoring a number of prominent bands. Fluorescence was excited by Hg 2537‐Å radiation. At pressures below 0.01 torr, the intensity of every band monitored is directly proportional to benzene pressure. This indicates that both the quantum yield and the spectrum of benzene fluorescence are invariant in this pressure domain, at least to within the 10% scatter of the data. The constant quantum yield is estimated to be near 0.4 so that significant non‐radiative relaxation occurs. These pressures extend by more than two orders of magnitude into the low‐pressure region where hard‐sphere collisions cannot effect relaxation of ${}^1{B}_{\text{2u}}$ molecules. The implications of these data with respect to radiationless relaxation processes in isolated molecules are discussed. Collision‐induced relaxation (probably vibrational relaxation) of some of the ${}^1{B}_{\text{2u}}$ benzene vibronic states excited by Hg 2537‐Å absorption can be observed at 0.05 torr, and this has been studied in pure benzene and in mixtures of benzene and CO. The efficiency of ground‐state benzene in depopulating these states is about one in two hard‐sphere collisions. CO is about half as effective. The rate of vibrational relaxation by CO is seen to differ among the several excited states populated by Hg 2537‐Å absorption.