Electronic Absorption and Resonance Raman Spectroscopy from Ab Initio Quantum Molecular Dynamics

Abstract

The absorption and resonance Raman excitation profiles of ethylene following π → π* excitation and taking full account of anharmonicity and Duschinsky rotation effects are calculated from first principles molecular dynamics using the ab initio multiple spawning (AIMS) 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 CC stretching. The resonance Raman excitation profile exhibits fundamental activity in all totally symmetric modes with the CC 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 wi...