RADIATIVE AND NONRADIATIVE DECAY OF THE NH(ND) A 3-PI ELECTRONIC STATE - PREDISSOCIATION INDUCED BY THE 5-SIGMA- STATE

Lifetimes for excited ND A 3-PI-upsilon' = 1-3 and NH-upsilon' = 2 in specific rotation/fine-structure levels were determined by excitation on isolated rotational lines in the A 3-PI-X 3-SIGMA- DELTA-upsilon = + 1 sequence, using molecules prepared in a pulsed supersonic beam. The observed lifetimes for NH-upsilon' = 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 3-PI state is predissociated by spin-orbit coupling to the repulsive 1 5-SIGMA- state which correlates with the ground-state asymptote, N(4S) + H(2S). The mechanism is also expected to be responsible for the previously observed predissociation of high N' levels in NH-upsilon' = 0 and 1. By contrast, no significant evidence for predissociation was found for the decay of excited ND-upsilon' = 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 3-PI-OMEGA and 1 5-SIGMA-OMEGA- 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 3-PI --> X 3-SIGMA- 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.