State selective photodissociation dynamics of à state ammonia. II

Abstract

The photodissociation dynamics of à state ammonia molecules (both NH3 and ND3) has been further investigated using the technique of H(D) atom photofragment translational spectroscopy. The resulting NH2(ND2) fragments are observed to carry high levels of internal excitation, the precise disposition of which is sensitively dependent upon the parent v′2 level excited. Dissociation from the v2=0 level of the à state yields ground state NH2(ND2) fragments, primarily in their zero-point level, but with high levels of rotational excitation specifically concentrated about the a-inertial axis; the population distribution over the energetically accessible product rotational levels with N≂Ka appears near to statistical. In contrast, dissociation from the parent v′2=1 level yields a markedly inverted fragment internal energy distribution. These different energy disposals have been rationalized via classical trajectory calculations employing the best available ab initio potential energy surfaces for the à and X̃ states of the ammonia molecule. The energy disposal following excitation to the parent v2=2 and 3 levels is found to mimic that observed for, respectively, the v′2=0 and 1 levels. These results provide clear evidence for the importance of anharmonic coupling (whereby an even number of bending quanta are redistributed into stretching motions) in promoting the fragmentation of parent levels with v2≥2. The threshold energy for producing electronically excited NH2(Ã2A1) fragments is 6.02 eV [∼6.16 eV for ND2(Ã)]. The present studies of NH3 photolysis suggest that this fragmentation channel opens at threshold and clearly indicate that branching into this channel occurs with much higher quantum yield than hitherto believed.