Microwave detected, microwave-optical double resonance of NH3, NH2D, NHD2, and ND3. II. Predissociation dynamics of the A state

Abstract

Using microwave detected, microwave-optical double resonance, we have measured the homogeneous linewidths of individual rovibrational transitions in the à state of NH3, NH2D, NHD2, and ND3. We have used this excited state spectroscopic data to characterize the height of the dissociation barrier and the mechanisms by which the molecule uses its excess vibrational and rotational energies to help overcome this barrier. To interpret the observed vibronic widths, a one dimensional, local mode potential has been developed along a N-H(D) bond. These calculations suggest the barrier height is roughly 2100cm-1, approximately 1000cm-1 below the ab initio prediction. The observed vibronic dependence of levels containing two or more quanta in nu2 is explained by a Fermi resonance between nu2 and the N-H(D) stretch. This interaction also explains the observed trends due to isotopic substitution. The rotational enhancement of the predissociation rates in the NH3 2(1) level is dominated by Coriolis coupling while for the same level in ND3, centrifugal effects dominate.