We observe directly the site hopping motion of a molecular O-2 species
on Si(111)-(7 x 7) surfaces using a variable-temperature scanning tun
neling microscopy. At the temperature range of our observation, the ho
pping is mainly confined to one-half of the 7 x 7 unit cell between ad
jacent 7 x 7 adatom sites. Through Arrhenius plots, the activation ene
rgies for the hopping between different atomic sites are derived. We a
lso resolve two short-lived intermediate states which mediate the site
hopping. An atomic mechanism is proposed to explain the molecular hop
ping process.