We have investigated the molecular precursor of oxygen on Si(111) 7 × 7 surface at 1 and 2 L coverage using high resolution electron energy loss spectroscopy (HREELS) at room temperature. The time dependence of the energy loss intensity due to the O-O stretching mode at 154 meV shows that the lifetime of the molecular precursor of oxygen is 530 min. Taking into account the ESDIAD pattern of the molecular precursor and the same lifetime measured by Sakamoto et al. [Surf Sci. 306 (1994) 93], we conclude that the molecular precursor adsorbs at the on-top site of the Si(111) surface. We also discuss the origin of the intrinsic lifetime.
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