Site hopping of single chemisorbed oxygen molecules on Si(111)-(7x7) surfaces

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 tunneling microscopy. At the temperature range of our observation, the hopping is mainly confined to one-half of the 7 x 7 unit cell between adjacent 7 x 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.