On the semiclassical calculation of molecular absorption and fluorescence spectra

Abstract

A semiclassical expansion is developed for the direct calculation of molecular electronic (absorption and fluorescence) spectra without having to calculate the molecular vibrational eigenstates. The expansion is based on expressing the necessary dipole correlation functions (which do not have a straightforward classical analog) in terms of a hierarchy of other intramolecular correlation functions which do have a simple semiclassical expansion. To lowest order in h/ the spectra may be calculated by running classical trajectories on a zero-order Hamiltonian Hc. The present semiclassical expansion is not unique and depends on our choice of Hc. This arises since the spectra are not analytic functions of h/. Applications are made to the absorption and to the fluorescence spectra of polyatomic molecules (single- and two-photon processes). The present method may be easily generalized to other multiphoton processes.