Intramolecular vibrational energy redistribution, mode specificity, and nonexponential unimolecular decay dynamics of vibrationally highly excited states of DCO (X̃ 2A′)

Abstract

The unimolecular dynamics of vibrationally highly excited states of DCO (X̃ 2A′) in the energy region up to Evib⩽9500 cm−1, beyond the D–CO (X̃) dissociation threshold, has been investigated using an effective polyad Hamiltonian obtained by fitting to the term energies from the measured B̃ 2A′←X̃ 2A′ stimulated emission pumping (SEP) spectra of the molecule [Stöck et al., J. Chem. Phys. 106, 5333 (1997); Temps and Tröllsch, Z. Phys. Chem. 215, 207 (2001)]. An added absorbing negative imaginary potential allowed for the unimolecular dissociation of the highly excited DCO via distinctive open reaction channels of the DC stretching vibration. The ensuing dynamics was explored using a wave packet propagation approach. Time profiles describing the intramolecular vibrational energy redistribution (IVR) and unimolecular decay kinetics were computed for the CO stretching zero-order basis states up to 6 quanta of excitation and the DCO bending zero-order basis states up to 12 quanta of excitation. The computed decay curves for the CO stretching zero-order basis states compare nicely with those of the respective coherent superposition states constructed directly from the measured SEP spectra (assuming the CO stretching mode as the Franck–Condon active bright zero-order mode that determines the observed transitions). A comparison of the decay curves with those of the almost isoenergetic DCO bending zero-order basis states in the respective polyads reveals large differences in the couplings of the two vibrational modes among each other and with the open dissociation channels. The obtained unimolecular decay profiles exhibit pronounced non-exponential kinetics. Comparison with statistically calculated decay rates shows a substantial degree of mode specificity of the dynamics, which can be attributed to a bottleneck in the IVR from the CO stretching vibration to the reaction coordinate. The model calculations explain the two-to-three orders of magnitude large difference between the measured eigenstate specific DCO (X̃) decay constants [Stöck et al.] and predictions by microcanonical statistical rate theories.