On the “anomalous” flourescence of 3,4-benzypyrene in the vapour phase

An analysis of the fluorescence of 3,4-benzpyrene in the vapour phase shows two contributions to the “anomalous” fluorescence: (i) the emission from the second excited state to the ground state and (ii) a vibronically induced S₁ → S₀ emission originating from the +520 cm^?1 vibrational level. A comparison between the intensities of the emissions indicates that in the vapour phase the vibrational redistribution from the 520 cm^? vibrational level of S₁ to modes of lower frequencies is relatively slow.