Photofragment spectroscopy with lasers in the vacuum UV energy region.

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Two-photon dissociation of nitrogen dioxide and carbon disulfide at 308 nm has been studied by means of multiphoton ionization of photofragments. The angular distributions of fragments are well represented by a form of 1 + <beta>_2P_2(cos<theta> + <beta>_4P_4 (cos<theta>). Because the intermediate state of NO_2 (^2B_2) generated by the first one-photon process is dissociative, the coefficient <beta>_4 for the fourth degree Legendre polynomial is appreciable. The coefficient <beta>_4 for CS_2 is, however, not appreciable because the intermediate CS_2(^1B_2) state has a long fluorescence Iifetime. The angular distributions not only reveal the symmetry and lifetimes of the two-photon excited states of NO_2 and CS_2 but also information on the two-photon excitation process of these molecules.The photodissociation of methylnitrite in the molecular beam has been studied by laser irradiation at 308-364 nm. The translational energy distribution for the photofragment indicates that there are two distinct paths by which CH_3O and NO radicals are produced; one is a one-photon dissociation, and the other is a two-photon dissociation. The angular distribution of fragment intensities peaks in a direction orthogonal to the elctric vector of the photolysis light indicating that the transition moments for both the first and the second one-photon absorptions lie perpendicular to the O-N bond that breaks.Methyliodide ions prepared by two-photon resonant three-photon ionization via Rydberg states are photodissociated by a one-photon process to result in the formation of methyl fragment ions by laser irradiation at 335-403 nm. The parallel angular distrubutions of these fragment ions have rather small anisotropy parameters, suggesting that dissociation is predissociative via the A(^2E_<1/2>) state of CH_3I^+.