Energy-loss calculations for medium-energy rare gas +H2collisions

Cross sections differential in energy loss and scattering angle and most-probable reciprocal-mass functions are calculated for several rare gas on ${\mathrm{H}}_2$ collisions in the low-keV collision-energy range. Specifically, the collision systems He +${\mathrm{H}}_2$, Ne +${\mathrm{H}}_2$, and ${\mathrm{Ne}}^{+}$ +${\mathrm{H}}_2$ have been considered. The classical-trajectory calculations are based on a triatomic-molecular potential-functional form suggested by ab initio calculations of the He-${\mathrm{H}}_2$ energy surface, which serves as a prototype for all rare-gas-${\mathrm{H}}_2$ systems. The parameters in that functional form have been fitted from the experimental differential energy-loss measurements for Ne on ${\mathrm{H}}_2$ collisions to yield the Ne-${\mathrm{H}}_2$ energy surface, and from the ${\mathrm{Ne}}^{+}$ on ${\mathrm{H}}_2$ differential energy-loss measurements to yield the Ne-${\mathrm{H}}_2^{+}$ energy surface. With the energy surfaces thus parametrized, the calculated differential energy-loss cross sections agree well with the experimental data. They explain the anomalously large range of scattering angle in which the collision is elastic, with no vibrational-rotational excitation.