He0 on D2 collisions at keV energies and the HeH2 energy surface

Abstract

An experimental and theoretical study of ${\mathrm{He}}^0$ on ${\mathrm{D}}_2$ collisions at energies 1.0, 1.5, and 2.0 keV has been carried out to probe and to understand the energy surface of the ground electronic state of the ${\mathrm{HeH}}_2$ triatomic molecule and of the intersections of this surface with those of low-lying electronically excited states. At each collision energy, doubly differential energy-loss spectra have been obtained and the results have been analyzed in terms of a parametric fit to an ab initio calculated ground-state energy surface. The scaled energy loss for quasielastic collisions (electronically elastic collisions with vibrational-rotational excitation) are shown to constitute a sensitive probe of the region of the ground-state energy surface in which the proximity of the He projectile breaks the ${\mathrm{H}}_2$ (or ${\mathrm{D}}_2$) bond. Sigmund scaling has been experimentally demonstrated to hold for the quasielastic channel in He on ${\mathrm{D}}_2$ collisions despite the strong presence of electronically inelastic processes, a finding of particular significance, since the scaling law was derived under the assumption that there are no accessible electronically inelastic channels in the collision system. The theoretical study confirms this behavior for collisions in which electronic excitation is velocity independent and occurs in well-defined surface intersection regions. Cross sections differential in angle but integrated over all vibrotational-rotational inelastic energy losses have been both calculated and experimentally measured for the quasielastic channel, and the two are found to be in good agreement.