Analysis of the CH-chromophore spectra and dynamics in dideutero-methyliodide CHD2I1

Abstract

We report the infrared spectra of dideutero-methyliodide CHD2I in the range from 500 to 12 000 cm−1. Twenty two vibrational bands were assigned to the coupled CH-stretching and bending vibrations of the CH-chromophore. They were analysed in terms of an effective Hamiltonian including strong anharmonic resonances up to the polyad N = 4 corresponding to a maximum of four quanta of stretching excitation. The ab initio potential hypersurface for the three-dimensional CH-chromophore subspace was calculated at the full MP2 level. This surface was finally scaled to fit experiment with vibrational energy levels being calculated by an adiabatic successive truncation technique. The resulting spectrum was represented by the same effective Hamiltonian as used for the experimental data, the results from both approaches agreeing well. The time-dependent quantum dynamics were obtained from this Hamiltonian adjusted to experiment. It showed very fast but incomplete energy transfer from pure CH-stretching excitation with four or five quanta to partial CH-bending excitation in about 100 fs. These results are discussed in terms of a doorway to further excitation transfer to the lower frequency modes measured before by femtosecond pump–probe experiments in our group. 1Dedicated to Hubert van den Bergh on the occasion of his 65th birthday.