Ion-stimulated desorption of O+ from the oxygenated TiC(111) surface: The role of nonadiabatic electronic transitions and the band effect

Abstract

The mechanism of secondary O+ ion emission from the oxygenated TiC(111) surface has been investigated by noble-gas ion irradiation. The O+ ion is ejected from the surface by He+ irradiation due to an electronic transition without violent collisions. The O+ ion occurs preferentially from the weakly bound oxygen species, but very little O+ arises from the strongly chemisorbed oxygen on the threefold hollow site. The O+ emission is not caused by two localized valence holes resulting from the Auger decay of the primary ions, but is rather initiated by the formation of a long-lived O 2s core-hole state with an antibonding character. The O 2s hole is created by He+ via the nonadiabatic electronic transition during moderate impact-parameter collisions (1.0–3.0 a.u.), which is accompanied by very little kinetic energy transfer (<1 eV) to the target oxygen atom. The probability for O 2s hole creation increases with increasing velocity of the primary ions. The ionization of oxygen occurs due to the Auger decay of the O 2s hole on the way out from the surface after breakage of the chemisorptive bond; this is essential for the emission of very low-energy (<10 eV) O+ ions without resonant neutralization. This mechanism is consistent with that developed in low-energy ion scattering and successfully explains the O+ emission even from metal surfaces without the need for any assumption of the localization of valence holes.