Nonadiabatic Effects in Slow Atomic Collisions. I.He++ He

In the collision ${\mathrm{He}}^{+}$ + He→${\mathrm{He}}^{+}$ + ${\mathrm{He}}^{*}$, excitation at low incident ion energies (≲ 1 keV) proceeds via a pseudocrossing of the elastic ${}_{}{}^2{}_{}{}^{}\Sigma _g^{}{}_{}{}^{}$ diabatic potential with excited-state ${}_{}{}^2{}_{}{}^{}\Sigma _g^{}{}_{}{}^{}$ potentials. As previously reported, this excitation mechanism fails to explain diverse experimental data concerning total excitation cross sections. A new physical mechanism is hypothesized, its existence verified, and it is shown to provide good qualitative and semiquantitative interpretation of the observations. Nonadiabatic couplings among some excited states occur at pseudocrossings of the respective inelastic-channel molecular potentials at large internuclear separations, resulting in coherent phase interference in the inelastic-scattering amplitudes. A linear-combination-of-atomic-orbitals (LCAO) calculation of 18 excited-state ${}_{}{}^2{}_{}{}^{}\Sigma _g^{}{}_{}{}^{}$ potentials of the intermediate ${\left(\mathrm{He}_2^{}{}_{}{}^{+}\right)}^{*}$ system verifies the presence of these outer pseudocrossing. Such a mechanism is shown to be likely in many ion-atom collisions.