V.U.V. Excitation of benzene: C6H6 fluorescence and phosphorescence in rare gas and nitrogen matrices

Benzene isolated in rare gas or nitrogen matrix at about 5 K has been irradiated with photons of 4, 6, 8.4, and 10 eV energy. The phosphorescence $\text{a}\text{?}{}^3\text{B}{}_{\mathrm{1u}}\text{→}\text{X}\text{?}{}^1\text{A}{}_{\mathrm{1g}}$ and the fluorescence $\text{A}\text{?}{}^1\text{B}{}_{\mathrm{2u}}\text{→}\text{X}\text{?}{}^1\text{A}{}_{\mathrm{1g}}$ are observed amost in every case, from the vibrationally relaxed states. For both emissions, shifts are observed from one matrix to another. They depend upon the vibronic bands in the fluorescence case. Some features of spectra are interpreted as phonon-induced, specially the 0-0 bands electronically forbidden for both transitions. The phosphorescence versus fluorescence ratio increases when passing from a nitrogen matrix to a xenon matrix and as the photon energy increases. To account for these observations, we are led to believe that the $\text{a}\text{?}{}^3\text{B}{}_{\mathrm{1u}}$ state is not only populated from the $\text{A}\text{?}{}^1\text{B}{}_{\mathrm{2u}}$ state, but through upper singlet and (or) triplet states.