The effect of residual oxygen impurity on ionization processes of Si+ and Si2+ has been studied quantitatively. In this study, ion sputtering experiments were carried out for a Si(1 1 1)- 7 x 7 surface, irradiated with 9 - 11 keV Ar-0 and Kr-0 beam. Even if the oxygen concentration is less than the detection limit of Auger electron spectrometry, SiO+ and SiO2+ ions have been appreciably observed. Moreover, as the SiO+ and SiO2+ yields increases, the Si+ yield is slightly enhanced, whereas the Si2+ yield is significantly reduced. From the incidence angle dependence of secondary ion yields, it is confirmed that Si+* (Si+ with a 2p hole) created in the shallow region from the surface exclusively contributes to Si2+ formation. By assuming that the SiO+ and SiO2+ yields are proportional to the residual oxygen concentration, these observations are reasonably explained: The increase of Si+ with the increase of residual oxygen is caused by a similar effect commonly observed for oxidized surfaces. The decrease of Si2+ yield can be explained by the inter-atomic Auger transition between the residual oxygen impurity and Si+*, which efficiently interferes the Si2+ formation process. (C) 2008 Elsevier B. V. All rights reserved.