Radiative and nonradiative decay of the NH(ND) A 3Π electronic state: Predissociation induced by the 5Σ− state

Abstract

Lifetimes for excited ND A 3Πv′=1–3 and NH v′=2 in specified rotation/fine-structure levels were determined by excitation on isolated rotational lines in the A 3Π–X 3Σ−Δv =+1 sequence, using molecules prepared in a pulsed supersonic beam. The observed lifetimes for NH v′=2 levels were found to be significantly shorter than those expected for purely radiative decay, indicative of additional removal of excited-state population by predissociation. The observed fine-structure dependence of the removal rates is consistent with a mechanism in which the A 3Π state is predissociated by spin–orbit coupling to the repulsive 1 5Σ− state which correlates with the ground-state asymptote, N(4S)+H(2S). This mechanism is also expected to be responsible for the previously observed predissociation of high N′ levels in NH v′=0 and 1. By contrast, no significant evidence for predissociation was found for the decay of excited ND v′=1–3 for the low J′ levels investigated. These observations were confirmed with a combination of ab initio electronic structure, and coupled electronic state dynamics, calculations. Using an extended contracted Gaussian basis and large configuration state function expansions(160 000–380 000 terms) potential energy curves for, and spin–orbit induced coupling (using the full microscopic Breit–Pauli interaction) between, the A 3ΠΩ and 1 5Σ−Ω states were determined. These electronic structure data were used as the basis for the determination of the radiationless decay rates using a semiclassical coupled state model. These results were in turn combined with radiative decay rates for the A 3Π→X 3Σ− transition to determine the total decay rates which were found to be in excellent accord with the available experimental observations, thereby confirming the mechanism of the predissociation.