Electronic absorption and resonance Raman spectroscopy from ab initio quantum molecular dynamics

The absorption and resonance Raman excitation profiles of ethylene following pi --> pi* excitation and taking full account of anharmonicity and Duschinsky rotation effects are calculated from first principles molecular dynamics using the ab initio multiple spawning (ALMS) method and a correlation function approach. The AIMS method solves the nuclear and electronic Schrodinger equations simultaneously and it associates a unique nuclear wave function with each electronic state. The computed absorption spectrum has a full width at half maximum of 9800 +/- 1300 cm(-1) (in agreement with the experimental value, 9500 cm(-1)) and a high-frequency structure spaced by 800 +/- 10 cm(-1) attributed to C=C stretching. The resonance Raman excitation profile exhibits fundamental activity in all totally symmetric modes with the C=C stretching mode being the most dominant. In addition, overtone activity is observed in the torsional motion, out-of-plane wagging motions and the out-of-plane rocking motions. This activity is consistent with the observation that the first excited state is twisted and one of the CH2 groups is pyramidalized. The coordinate dependence of the electronic transition dipole is investigated, and we find that it depends very strongly on the torsional coordinate and less so on the pyramidalization and C=C stretching coordinates. However, within the approximations used in this paper this dependence does not influence the spectra significantly and the Condon approximation is quite accurate.