Electronic excitation in impact scattering of low-energy He+ from solid surfaces

Inelastic scattering and electron-exchange processes in low-energy (0.11 keV) He+ scattering from surfaces of ionic compounds and metallic elements have been investigated. The probability for ionization of neutral He0 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 He0 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 He0 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 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.