Vibrational energy dependence of radiationless conversion in aromatic vapours

Under collision-free conditions the fluorescence lifetimes of naphthalene, 1-2 benzanthracene, chrysene and pyrene decrease with increasing excitation energy within the first excited singlet state. In anthracene the fluorescence lifetime is constant within the first excited singlet state. An explanation of these findings in terms of bond length changes between the excited singlet state and the first triplet state, and also the symmetry of the first singlet state is given. The effect of added foreign gas on fluorescence lifetimes and triplet yields is discussed in terms of a model which allows the non-radiative decay probability to vary with the degree of vibrational excitation. Excellent agreement is obtained with the experimental results on naphthalene fluorescence. The decrease of triplet yield as foreign gas pressure is reduced is also interpreted in terms of this model.