Unimolecular dissociation of HOCl near threshold: Quantum state and time-resolved studies

The unimolecular dissociation of HOCl has been examined by accessing several rotational resonances of the 6 nu(OH) vibrational level using overtone-overtone double resonance. An examination of the energetic thresholds for opening various OH fragment rotational states allows us to determine the dissociation energy for the Cl-O bond to be D-0 = 19 288.8 +/- 0.6 cm(-1) as well as confirm the absence of any appreciable barrier (less than or equal to 2 cm(-1)) along the reaction coordinate. Time-resolved measurements of the unimolecular dissociation rates arising from excitation to selected \J,K-a] quantum states of the 6 nu(OH) vibrational level are found to be substantially slower than that expected on the basis of RRKM theory, and as a function of energy, these rates exhibit recurrent maxima that approximately correlate with the rotational energy level spacings of the OH((2) Pi(3/2)) manifold. The OH fragment product state distributions resulting from near-threshold unimolecular dissociation are found to be hotter than those predicted by phase space theory.