Unimolecular dissociation dynamics of highly vibrationally excited DCO (X̃ 2A) . I. Investigation of dissociative resonance states by stimulated emission pumping spectroscopy

The vibrational level structure and unimolecular dissociation dynamics of highly vibrationally excited X̃ 2A DCO were investigated using the method of stimulated emission pumping spectroscopy (SEP). Single vibration-rotation states were probed with excitation energies up to E(X̃)=18 200 cm−1, ≈12 700 cm−1 above the asymptotic D-CO dissociation limit. The vibrational level structure of the molecule was found to be determined by distinctive polyads arising from a 1:1:2 resonance between the CD stretching, CO stretching, and DCO bending vibrations. Anharmonic coupling mechanisms give rise to considerable level mixings, especially regarding the CD and CO stretching motion. Thus, only a minority of vibrational states can be unambiguously assigned. The spectral line shape profiles of ≈100 highly excited “resonance states” in the continuum above the D-CO dissociation limit were measured at high resolution. The profiles are homogeneously broadened. The unimolecular decay rates, obtained from the observed line widths, were observed to fluctuate by more than two orders of magnitude in a strikingly state specific manner. The decay rates on average increase with increasing vibrational excitation energy. The state resolved experimental data are compared to predictions of the microcanonical specific unimolecular rate coefficients calculated from different statistical models. Serious problems were encountered considering the calculation of the density of states of the molecules in the continuum region of the potential energy surface regarding the contribution of the disappearing oscillator. Despite tentative corrections, the calculated rate coefficients were to too high by one to two orders of magnitude. Overall, the unimolecular dynamics of DCO appears to conform to an intermediate case between the strictly vibrationally “mode specific” and the “statistical” limits.