State selective photodissociation dynamics of à state ammonia. II

Abstract

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.

Formation of Vibrationally Excited Methyl Radicals Following State-Specific Excitation of Methylamine

“…Methylamine is the simplest alkyl-substituted analog of ammonia, which provides an almost textbook example of the influence of exit-channel conical intersections on photodissociation dynamics. [25][26][27][28][29][30] The absorption spectra and the electronic states of ammonia and methylamine share several important characteristics and lead to the expectation of similar behavior. The photochemistry of methylamine, however, is far richer than that of ammonia; five dissociation pathways have been experimentally identified after excitation in the structured long-wavelength absorption band: The thermochemical data shown above are for electronic ground state products and have been derived from standard enthalpies of formation at 298 K tabulated in the NASA-JPL evaluation.…”

Near ultraviolet photolysis of phosphine studied by H atom photofragment translational spectroscopy

“…The time-of-flight (TOF) spectra of the H atoms resulting from photodissociation of NH3 following excitation to each of a number of vibronic levels of the A'A; state showed well resolved struture. Analysis of thjs structure revealed a strong propensity for the partner NH2(X) fragments to be formed with little vibrational excitation, but with large amounts of rotational excitation, specifically concentrated about the a-inertial axis (28)(29)(30). Such product rotational energy disposal can be directly correlated to the effects of the conical intersection, in the H2N---H dissociation channel, through which the dissociating NH,(A) molecules must pass in order to reach the H + NH2 asymptote (28)(29)(30)33).…”

“…Analysis of thjs structure revealed a strong propensity for the partner NH2(X) fragments to be formed with little vibrational excitation, but with large amounts of rotational excitation, specifically concentrated about the a-inertial axis (28)(29)(30). Such product rotational energy disposal can be directly correlated to the effects of the conical intersection, in the H2N---H dissociation channel, through which the dissociating NH,(A) molecules must pass in order to reach the H + NH2 asymptote (28)(29)(30)33). Finally, given detailed knowledge of the identities and-energies of these high angular momentum states of the NH2(X) products it has since proved possible to assign their LIF excitation spectra and thereby deduce improved bending potential energy functions for both the ground^^^^ and first excited A2AI states of the NH2 radical (34).…”

“…Such conclusions were reiterated in a more recent study of the Doppler lineshape of the resulting H atom photofragments (26). However, any quantitative understanding of the energy disposal in this dissociation, and its specific dependence on parent quantum state, had to await the development of the technique of H atom photofragment translational spectroscopy by Welge and co-workers (27)(28)(29)(30)(31)(32). The time-of-flight (TOF) spectra of the H atoms resulting from photodissociation of NH3 following excitation to each of a number of vibronic levels of the A'A; state showed well resolved struture.…”

Recent Laboratory Photochemical Studies and Their Relationship to the Photochemical Formation of Cometary Radicals

“…These studies have been carried out by Wittig and his colleagues at 193 nm (Xu et al 1989) and Welge and his colleagues (Biesner et al 1988(Biesner et al , 1989 at a variety of different wavelengths. Energy partitioning between the H atom and the NH2 radical is not simply described.…”