Electronic excitation in impact scattering of low-energy ${\mathrm{He}}^{+}$ from solid surfaces

Inelastic scattering and electron-exchange processes in low-energy (0.1–1 keV) ${\mathrm{He}}^{+}$ scattering from surfaces of ionic compounds and metallic elements have been investigated. The probability for ionization of neutral ${\mathrm{He}}^0$ in the ground state is minimized for target elements which have filled d orbitals located in a shallower energy position than the He 1s level. Inelastic scattering other than reionization of ${\mathrm{He}}^0$ is also clearly observed in the spectra from a large number of ionic compounds. These excitations are observed provided that the surface p levels are located at lower energies than the He 1s level and that ionization of ${\mathrm{He}}^0$ takes place with a large probability. On the basis of the quasimolecular framework, it is found that the inelastic scattering is caused by excitation of surface p electrons along the σ orbital, which is promoted due to the antibonding interaction with the He 1s orbital. The quantum-mechanical interference between bonding and antibonding orbitals, which results in oscillatory-yield–versus–kinetic-energy curves because of the quasiresonant charge exchange, is found to be broken down by the intervening occurrence of the inelastic process.