Vibrational excitation in the DECENT approximation: The large-angle scattering of Li+ from H2

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Abstract
We have applied the DECENT model, which exploits the correspondence between the classical and quantal equations of motion for a forced harmonic oscillator, to the calculation of quantum vibrational transition probabilities and differential cross sections for large−angle scattering of Li+ from H2, using the Lester potential energy hypersurface. The results are in good agreement with the experimental data of David, Faubel, and Toennies, but they show the necessity for taking into account concurrent rotational excitation, which has not yet been resolved experimentally in this energy and scattering angle regime. If the trajectories are calculated using the spherically averaged potential, rather than the results obtained first using the full anisotropic potential and then orientation−averaged, the same angle dependence of the classical differential cross section is obtained but the vibrational transition probabilities are severely underestimated. A collinear trajectory also gives very small vibrational excitation energy. We show that large vibrational excitations are associated with a particular class of noncollinear trajectories in which the vibrational and rotational motions are strongly coupled. The vibrational excitation process can therefore be adequately described only by a three−dimensional calculation using the complete potential energy hypersurface, including anisotropic terms.