Vibronic activity in the fluorescence and absorption spectrum of pyrene

The vibronic coupling terms responsible for the intensity of the b 3g modes in the absorption and emission spectra of pyrene and pyrene-d 10 have been calculated taking into account both the Herzberg-Teller (HT) mechanism and the Born-Op-penheimer (BO) breakdown mechanism. The calculations have been carried out in the orbital-following scheme, implemented by the INDO/S Hamiltonian. The results, reproducing the main spectral features, such as the large difference observed in the intensity patterns of the two spectra, are discussed in terms of interference between HT and BO mechanisms, isotopic redistribution and the Duschinsky effect.