Successive oxidation stages of adatoms on the Si(111)7×7 surface observed with scanning tunneling microscopy and spectroscopy

Scanning tunneling microscopy and spectroscopy reveal at least a two-stage reaction process which occurs at adatom sites on Si(111)7×{}7 surfaces exposed to oxygen. Reacted adatoms in the first stage have small ({~}1/2 eV) but specific electron-energy shifts, and show roughly a 4:1 preference for the faulted half and a 2:1 preference for the corners of the 7×{}7 unit cell. Second-stage adatoms appear similar to ``missing'' adatoms in empty-state topographic images.