State-specific unimolecular reaction dynamics of HFCO. I. Dissociation rates

Abstract

Rovibrationally resolved unimolecular reaction rates of highly vibrationally excited HFCO (S0) are measured and found to be strongly state specific in the energy range of 14 500 cm−1–23 000 cm−1. HFCO molecules are excited to single rovibrational levels in the tunneling region by stimulated emission pumping (SEP) and the dissociation rate of each level is measured by monitoring the temporal evolution of its population by laser-induced fluorescence. The dissociation rates increase by a factor of 10 to 100 or more for small increases in rotational quantum number from J=0 up to J=4 and Ka=2. The dependence on Ka is the strongest. At higher energies, where dissociation lifetimes are shorter than the laser pulse duration, dissociation rates are estimated from the linewidths of well-resolved transition lines measured by high-resolution SEP spectroscopy. In this energy region, dissociation rates are also dependent upon rotational state but much less strongly than in the tunneling region. Vibrational mode specificity in the dissociation rates is observed. For states with approximately the same total energy those with higher excitation in the out-of-plane bending mode (ν6) dissociate more slowly than others. For rotationless levels in the tunneling region, the A′ states decay much more rapidly than the A″ states indicating symmetry-induced mode specificity. The dissociation rates in HFCO almost surely exhibit mode specificity because the ν6 mode is weakly coupled to the reaction coordinate. The enhancement of this coupling by Coriolis forces appears to produce the unprecedentedly large rotational level dependence of the rates. However, the spectroscopically observed coupling of ν6 to the background of vibrationally mixed levels is not significantly increased by rotation. Thus states with nearly all of the excitation energy in the reaction coordinate appear not to be strongly mixed into the background states. The large effect of rotation on the rates thus seems to result from Coriolis coupling of extreme motion in the reaction coordinate to the background levels or directly to ν6. By applying the Rice–Ramsperger–Kassel–Marcus theory to the measured dissociation rates for high rotational states, the barrier height for the molecular dissociation of HFCO to HF+CO is estimated to be 49±4 kcal/mole.