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 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.