Theory of fluorescence excitation spectra using anharmonic-coriolis coupling in S1 and internal conversion to S0. I. General formalism

A treatment of oneor two-photon fluorescence excitation spectra is described using the vibration-rotation coupling of zeroth order states in the excited electronic state and nonadiabatic coupling to the ground state. Using perturbation theory, experimental harmonic frequencies, an anharmonic force field, and various theoretical Coriolis coupling constants, a quasistationary molecular eigenstate in an excited electronic state S, is first calculated. The S, eigenstate is then coupled via the nonadiabatic nuclear kinetic energy operator (internal conversion) to rovibronic states in the ground state manifold, the latter states approximated in a simple manner. A search algorithm is used to select the S, dark states and the Sc states. Both the perturbation theory coefficient and the Franck-Condon factors are employed in the evaluation function used in the search. The results are applied in part II to the channel three problem in benzene.