State-resolved unimolecular reactions: The vibrational overtone initiated decomposition of nitric acid

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Abstract
We have determined the internal state distribution of the OH fragment from the unimolecular dissociation on nitric acid initiated by excitation in the regions of the fourth (5νOH) and fifth (6νOH) overtones of the O–H stretching vibration. The two excitations correspond to average excess energies of 350 and 2700 cm−1, respectively. In the case of 5νOH excitation, where states near the threshold energy for dissociation dominate the decomposition, phase-space theory (PST) correctly predicts the observed populations of both the rotational and spin–orbit states of the OH fragment. This observation, which is consistent with there being no barrier to the unimolecular dissociation on the ground electronic state potential-energy surface, illustrates the ability of simple statistical theory to describe spin–orbit state populations, at least in near-threshold decomposition of nitric acid. In contrast, the same model does not reproduce the quantitative details of the decomposition initiated by excitation of the fifth overtone vibration, although it does reproduce the general shape of the OH rotational state distribution. The observed rotational state distribution for the 6νOH initiated decomposition is less energetic than the PST prediction, and the measured spin–orbit state population ratio P(2Π3/2)/P(2Π1/2) is larger than the calculation.

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