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

The absorption and resonance Raman excitation profiles of ethylene followin g pi --> pi* excitation and taking full account of anharmonicity and Duschi nsky rotation effects are calculated from first principles molecular dynami cs using the ab initio multiple spawning (ALMS) method and a correlation fu nction approach. The AIMS method solves the nuclear and electronic Schrodin ger equations simultaneously and it associates a unique nuclear wave functi on with each electronic state. The computed absorption spectrum has a full width at half maximum of 9800 +/- 1300 cm(-1) (in agreement with the experi mental value, 9500 cm(-1)) and a high-frequency structure spaced by 800 +/- 10 cm(-1) attributed to C=C stretching. The resonance Raman excitation pro file 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 observat ion that the first excited state is twisted and one of the CH2 groups is py ramidalized. 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 coordina tes. However, within the approximations used in this paper this dependence does not influence the spectra significantly and the Condon approximation i s quite accurate.