π-π∗ vibronic spectrum of ethylene from ab initio calculations of the Franck-Condon factors

The vibronic spectrum of ethylene corresponding to the π-π∗ N-V excitation has been studied using various ab initio methods. The vertical and adiabatic excitation energies, 8.1 and 5.6 eV, respectively, obtained at the highest level of theory, MRCI, are in close agreement with experiment and the best theoretical results. The ab initio calculation of Franck-Condon factors, taking into account distortion, displacement and normal mode mixing (up to four), allowed the interpretation of major features of the observed spectrum and confirmed that the π-π∗ transition is responsible for the broad continuous distribution underlying the distinct Rydberg bands.