Abstract
The S₁–S₀ fluorescence excitation and dispersed fluorescence spectra of 9-methylanthracene are measured in a supersonic free jet expansion. Several low-frequency bands assigned to the transitions between the internal rotational levels of the methyl group can not be fitted to the calculated energy levels obtained by the one-dimensional rotor model. We have introduced a two-dimensional model incorporating the coupling of the methyl rotation and an out-of-plane bending motion of the anthracene ring. The calculations based on this model reproduce the intensity distributions in the fluorescence excitation and dispersed fluorescence spectra as well as the frequencies of the internal rotational levels in the S₀ and S₁ state.