  Using microwave detected, microwave-optical double resonance, we have
measured the homogeneous linewidths of individual rovibrational transitions in
the \~A 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.
