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 6ν 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 (≤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 6ν 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 Π 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.