Laser catalysis and transition state spectra of the H+H₂ exchange reaction

Electronic excitation "transition-state" spectra of the H + H₂ exchange reaction are computed by a uniform semiclassical approximation, at a number of collision energies. The spectra, which compare well with the coupled channels computations of Engel et al. [J. Chem. Phys. 82, 4844 (1985)] are shown to yield unique information pertaining to ultrashort time dynamics. The transition amplitudes are then incorporated in a general formulation by which the laser catalysis scheme, suggested recently by Shapiro and Zeiri [J. Chem. Phys. 85, 6449 (1986)] is treated exactly. According to this scheme, reaction barrier crossings can be achieved through resonant light scattering via a bound upper electronic state. The laser acts as a catalyst, since no net photons are absorbed or emitted. When the process is coherent, interference between "natural" (nonradiative) tunneling and the optical process is shown to lead to "Fano-type" dependence of the reactive probabilities on laser frequency: The reaction is stopped on the red side and enhanced on the blue side of the absorption line. For an ensemble of reactants with thermal-like distribution of kinetic energies, laser catalysis is shown to depend linearly on the laser power. For H + H₂, the effect is most pronounced at threshold and subthreshold energies.