Monte Carlo trajectory calculation of state-to-state cross sections for vibrational-rotational-translational energy transfer in Ar-H₂ collisions

We have carried out quasiclassical trajectory calculations of the energy transfer cross sections for five initial states of H₂ [(υ,j) = (0,6), (0,18), (2,18), (4,6), and (6,0)] in collisions with Ar at a fixed total energy of 1.0 eV (with respect to the energy of Ar + 2H). The first of these states has an internal excitation energy that is 7% of the dissociation energy D₀, and the other four states have internal excitation energies from 45 to 63% of D₀. The calculations are based on the most accurate available potential energy surface. The results are presented as tables of state-to-state cross sections and as contour maps of these cross sections as functions of the final quantum numbers. For the four highly excited initial states these maps show large changes of υ and j, extensive vibrational-rotational energy transfer, and the population of 57-142 final states. The total vibrational-change cross sections for these states are 3.7 to 7.4 times larger than for the (0,6) state. No simple functional form gives a quantitative fit to all the pure-rotational-translational energy-transfer cross sections, and a quantitative fit to all the cross sections poses an even more severe theoretical challenge.