The mechanism of HNO predissociation

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Abstract
Laser-induced fluorescence has been observed with excitation in five vibronic transitions of the à 1 A″ - [Xtilde] 1 A′ band system of HNO. The onset of predissociation has been detected by breaking-off in the rotational structure of many of the branches. Rotationally induced predissociation has also been observed for two vibronic levels lying above the dissociation limit. Potential energy functions for the [Xtilde]1, A′, ã 3 A″ and Ã1 A″ states have been constructed using spectroscopic data, thermochemical data, and quantum chemical calculations as an aid to the understanding of the predissociation mechanism. It is shown that at threshold dissociation occurs by crossing from the à to the [Xtilde] state. The observed breaking-off energy is a function of J′, but not of K′ or the vibrational quantum numbers. This dependence is in quantitative agreement with calculations using the potential surface of the [Xtilde] state. The observed breaking-off energy is a function of J′, but not of K′ or the vibrational quantum numbers. This dependence is in quantitative agreement with calculations using the potential surface of the [Xtilde] 1 A′ state. This permits the derivation of an improved dissociation energy for HNO → H + NO of 16 450 ± 10 cm-1, which is 550 cm-1 lower than the previously determined upper limit.