By combining scanning tunneling microscopy (STM) and ultraviolet photo
electron spectroscopy, we show that the adsorption of single O-2 molec
ules on Si(111)-(7 x 7) produces essentially molecular sites, seen as
bright individual sites in STM topographs, which are stable at room te
mperature. Atom-resolved STM studies also reveal that these molecular
sites react very efficiently with further O-2 and H2O molecules. This
is shown to account for the saturation at low exposure of the molecula
r site density and for the apparent decay lifetime of these molecular
sites.