State selective photodissociation dynamics of <i>A</i> state ammonia. I

Abstract

The photofragmentation dynamics of ammonia molecules following pulsed laser excitation to the two lowest levels (v′2 =0 and 1) of their à 1A″2 excited state has been investigated by monitoring the time-of-flight spectra of the nascent H-atom products. These spectra show well resolved structure. Analysis of this structure confirms recent revised estimates of the quantity D00 (H–NH2) (4.645±0.01 eV) and reveals that the majority of the accompanying NH2 (X̃ 2B1) fragments are formed vibrationally unexcited, but with high levels of rotational excitation specifically concentrated about the a-inertial axis. The detailed energy disposal is sensitive to the initially excited parent vibronic (and even rovibronic) level: the NH2 (X̃) fragments resulting from photodissociation via the v′2 =1 level of NH3 (Ã) carry a higher level of excitation of the N=Ka rotational levels, which show an inverted population distribution. We also describe the results of trajectory calculations employing the recently reported [M. I. McCarthy et al., J. Chem. Phys. 86, 6693 (1987)] ab initio potential energy surfaces for the à and X̃ states of ammonia. These provide a detailed rationale for the experimentally observed energy disposal and highlight the massive influence on the eventual fragmentation dynamics of the conical intersection between these surfaces along the H–NH2 dissociation coordinate.