Electronic excitation in collisions of H2+ on He.

Abstract

Ab initio cross sections have been calculated for electronic excitation resulting from single collisions in the keV energy range between ${\mathrm{H}}_{2}^{+}$ initially in the ground electronic and vibrational state incident on He targets. These collisional excitation studies are based on ab initio calculations of the three lowest adiabatic states of the (${\mathrm{HeH}}_{2}$${)}^{+}$ quasimolecular system and ab initio calculations of the nonadiabatic gradient coupling matrix elements between these three adiabatic states. With use of straight-line trajectories in the classical trajectory approximation, cross sections have been obtained for (1) charge exchange into the ground state of He and (2) collisional dissociation via electronic excitation 1${\mathrm{\ensuremath{\sigma}}}_{\mathit{g}}$\ensuremath{\rightarrow}1${\mathrm{\ensuremath{\sigma}}}_{\mathit{u}}$ of the ${\mathrm{H}}_{2}^{+}$ projectile. Furthermore, with a knowledge of the diabatic structure of the (${\mathrm{HeH}}_{2}$${)}^{+}$ energy levels, (3) the sum of the cross sections for Rydberg excitation of the He target or Rydberg excitation of the ${\mathrm{H}}_{2}^{+}$ projectile or charge exchange into the repulsive state of ${\mathrm{H}}_{2}$ was also obtained. The calculated cross sections are in reasonable agreement with available experimentally measured cross sections. The energy thresholds for these cross sections are found to be determined by excitation at level crossings between the ground electronic state and the ${\mathrm{H}}_{2}^{+}$(1${\mathrm{\ensuremath{\sigma}}}_{\mathit{u}}$)+He(1${\mathit{s}}^{2}$) at R\ensuremath{\simeq}1.2 a.u. and the ${\mathrm{H}}_{2}^{+}$(1${\mathrm{\ensuremath{\sigma}}}_{\mathit{g}}$)+He(1s,2l) at R\ensuremath{\simeq}0.7 a.u.