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

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 ³Π_Ω 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.