Charge-overlap effects on electronic transitions in moderate-energy M⁺(= Li⁺, Na⁺, and K⁺)-He collisions

Excitation mechanisms in M⁺(= Li⁺, Na⁺, and K⁺)-He collisions at moderate laboratory energies of E_lab = 0.5-5 keV were studied by calculating interaction potentials of V(R) = 10-1000 eV at small distances of R = 0.13-3.5 au. To discuss electronic transitions through radial couplings, potential crossing distances R_C were evaluated using an empirical charge-overlap model, which was revised by considering the mixings of wave functions for interacting electronic states. The model potentials at V(R) > 100 eV were refined by taking the screened Coulombic interactions into account. The potential crossings for the M⁺-He systems evaluated by the charge-overlap model reasonably interpreted the system dependences on the one- and two-electron (1e and 2e) excitations observed in differential scattering, photon emission, and ejected electron spectroscopy experiments. For 1e excitations, direct He atom excitations and charge transfers were observed in all of the systems, while apparent M⁺ ion excitations were observed only in K⁺-He collisions. For 2e excitations, prominent He atom excitations were observed in Li⁺-He collisions, and both He atom and Na⁺ ion (Na atom) excitations were observed in Na⁺-He collisions, while only M⁺ ion (M atom) excitations were observed in K⁺-He collisions. (author)