Generating Function Approach to Single Vibronic Level Fluorescence Spectra.

An efficient time-dependent generating function method to compute vibronic emission and absorption spectra arising from transitions from a singly excited vibrational initial state is presented. In contrast to existing finite temperature approaches that intrinsically contain these transitions weighted by a Boltzmann factor, the current approach allows one to calculate these transitions individually. Using vibrational frequencies and normal modes computed by the second-order approximate coupled cluster (CC2) method, this formalism is used to compute the single vibronic level (SVL) fluorescence spectra of anthracene. Calculated spectra are in excellent agreement with spectra measured in jet-cooled expansion experiments. Duschinsky mixing is necessary to explain intensities of certain peaks. In a few cases, CC2, however, underestimates Duschinsky mixing, leading to too low peak intensities. An empirical correction of the Duschinsky matrix is presented. The presented method has the potential to facilitate the assignment and interpretation of SVL fluorescence spectra.