The photodissociation dynamics of 1 state ammonia molecules (both NH3 and ND 3 ) has been further investigated using the technique ofH(D) atom photofragment translational spectroscopy. The resulting NH2 (ND 2 ) fragments are observed to carry high levels of internal excitation, the precise disposition of which is sensitively dependent upon the parent v~ level excited. Dissociation from the v~ = 0 level of the 1 state yields ground state NH2 (ND 2 ) 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~ = 1 level yields a markedly inverted fragment internal energy distribution. These different energy disposals have been rationalized via classical traje~tory calculations employing the best available ab initio potential energy surfaces for the 1 and X states of the ammonia molecule. The energy disposal following excitation to the parent v~ = 2 and 3 levels is found to mimic that observed for, respectively, the v~ = 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 fra~entation_o!parent levels ~ith v; ;;'2. The threshold energyJor producing electronically excIted NH2 (A AI) fragments IS 6.02 e V [ -6.16 e V for ND2 (A) ]. 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.

The photofragmentation dynamics of ammonia molecules following pulsed laser excitation to the two lowest levels (vi = 0 and 1) of their A tA ; excited state has been investigated by monitoring the time-of-flight spectra ofthe nascent H-atom products. These spectra show well resolved structure. Analysis of this structure confirms recent revised estimates of the quantity DJ (H-NH2) (4.645 ± 0.01 eV) and reveals that the majority ofthe accompanying NH2 (X 2 B t ) 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 (X) fragments resulting from photodissociation via the vi = 1 level of NH3 (A) 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. McC~rthy et al., J. Chem. Phys. 86, 6693 (1987)] ab initio potential energy surfaces for theA and X 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.

The photofragnentation dynamics of ammonia molecules following pulsed UV laser excitation of ground state to the two lowest levels ν′2 = 0 and 1 (ν2 vibrational mode) of their ?1(A"2) electronic excited state has been investigated by monitoring thetime-of-flight spectra of the nascent H-atom products. The spectra confirm recent revised estimates of the quantity D00(H-NH2) = 4.645 eV and reveals that the majority of the accompanying NH2 (X2B1) fragments are formed vibrationally unexcited, but with high levels of rotational excitation specifically concentrated about the a-inertial axis. The NH2(X) fragments resulting from photodissociation via the ν′2 = 1 level of NH3(?) carry a higher level of internal excitation and show an inverted population distribution over the N = Ka rotational levels.