State-to-state unimolecular reaction dynamics of HOCl near the dissociation threshold: The role of vibrations, rotations, and IVR probed by time- andeigenstate-resolved spectroscopy

We use infrared-visible double resonance overtone excitation to prepare HOC l molecules in single, well-characterized rotational levels of high OH stre tching states just above the dissociation threshold on the ground potential energy surface. Combined with time-resolved laser induced fluorescence (LI F) detection of the OH product, this approach allows us to monitor the depe ndence of unimolecular dissociation rate on the angular momentum (J,K-a,K-c ), total energy, and vibrational character of the state of the reactant mol ecule as well as on number of dissociation channels available to the OH pro duct. Dissociation rates from single states of the parent molecule are dist ributed over more than two orders-of-magnitude in a fashion that appears la rgely independent of the excess energy and the total angular momentum. In s everal instances we observe a one-order-of-magnitude difference in dissocia tion rate between states that are nearby in rotational quantum number and/o r energy. Superimposed on these state-to-state rate fluctuations is a gener al trend toward decreasing unimolecular dissociation rate with increasing K -a quantum number. Moreover, the measured rates, which range from 1 to 300 mu s(-1), are much slower than the predictions of statistical theories. We present a simple model calculation to explain the observed phenomena. (C) 1 999 American Institute of Physics. [S0021-9606(99)02640-9].