State selective photodissociation dynamics of à state ammonia. I

Abstract

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.

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.…”