Charge state effect on K-shell ionization of aluminum by 600–3400 keV xenonq+ (12 < q < 29) ion collisions

K-shell X-ray spectra of Al were measured by the interaction of 600–3400 keV Xe q+ (q = 12–29) ions with Al surface. The X-ray yields per incident ion were deduced and the K-shell ionization cross-sections were obtained from the experimental yield data. With the same incident energy, the K-shell ionization cross-sections of Al excited by Xe q+ (q < 26) ions were of the same order of magnitude, while for q = 26 and 29 Xe ion collisions, they were, respectively, about two and ten times larger. Taking into account the binding-energy-modification and the recoil effect of target atoms, the binary encounter approximation (BEA) theory was consistent with the experimental data for q < 26 Xe ion collisions, but it underestimated those excited by q = 26 and 29 Xe ions. This indicates that the K-shell ionization of target induced by Xe q+(q < 26) ions was mainly due to the direct Coulomb excitation. However for q = 26 and 29 Xe ions collisions, the transfer of 3d vacancies of projectile to the 1s orbital of target via rotational coupling of the 3dπ, δ-3dσ molecular orbitals, which were formed in the ion-atom quasi-molecule, may cause a considerable contribution to the enhancement of ionization. In addition to the well known Auger and X-ray transition, our experiments proved that the molecular orbital transition (“side-feeding”) mechanism is also a significant channel for de-excitation of hollow atoms formed below the surface.