Single Vibronic Level Fluorescence. II. Vapor-Phase Fluorescence Spectra from the Zero-Point Level of B2u1 Benzene and Benzene-d6

Fluorescence spectra have been obtained at 300°K from C₆H₆ and from C₆D₆ vapor at 0.1 torr in which all emitting molecules are in the zero‐point vibrational level of the ${}^1{B}_{\text{2u}}$ state. Analysis of the spectra are completed using selection rules appropriate for $D_{\text{6h}}$ symmetry of both the ground ${(}^1{A}_{\text{1g}})$ and excited ${(}^1{B}_{\text{2u}})$ states. The spectra are compared with the zero‐point fluorescence obtained by the conventional method of matrix isolation of benzene at 77°K. The forbidden transitions seen in the latter are absent in isolated molecule gas‐phase spectra. The zero‐point spectra are also compared with that from a 300°K Boltzmann distribution of ${}^1{B}_{\text{2u}}$ vibronic levels.