Intensity Patterns in Absorption and Fluorescence Spectra of Molecular Dimers: A Comparison between the Vibronic and the Born–Oppenheimer Approach

After showing in a previous paper that the Born–Oppenheimer zeroth-order approximation can produce accurately the vibronic energies of a model of an excited molecular dimer for a wide range of parameters, the same approach is now used to calculate the vibrational patterns in absorption and fluorescence spectra. Two different levels of approximation depending on the degree of adiabaticity of the electronic wavefunctions are compared with the vibronic treatment. The progressions in the spectra are interpreted in terms of Franck–Condon factors, vibrational borrowing, and vibronic coupling between Born–Oppenheimer product wavefunctions.