Site Hopping of Single Chemisorbed Oxygen Molecules on Si(111)-(7×7)Surfaces

We observe directly the site hopping motion of a molecular ${\mathrm{O}}_2$ species on Si(111)- $(7\times 7)$ surfaces using a variable-temperature scanning tunneling microscopy. At the temperature range of our observation, the hopping is mainly confined to one-half of the $7\times 7$ unit cell between adjacent $7\times 7$ adatom sites. Through Arrhenius plots, the activation energies for the hopping between different atomic sites are derived. We also resolve two short-lived intermediate states which mediate the site hopping. An atomic mechanism is proposed to explain the molecular hopping process.