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We observe directly the site hopping motion of a molecular O 2 species on Si(111)-͑7 3 7͒ 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 3 7 unit cell between adjacent 7 3 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. [S0031-9007(97)

Adsorption and diffusion of single hydrogen atoms on Si͑111͒-͑7 3 7͒ surfaces at elevated temperatures have been studied using scanning tunneling microscopy. Hydrogen atoms adsorb preferentially atop rest atoms. An adsorbed H atom can hop between two neighbor rest atoms via an adatom, i.e., via a metastable intermediate state. Below 340 ± C, the hopping is mostly confined within a half-cell, but at higher temperatures, they can hop across the cell boundary. The activation energies for different hopping paths were measured. The binding energy difference between rest-atom and adatom sites and that between corner and edge adatom sites were also determined to be ϳ0.2 and ϳ0.05 eV, respectively.[S0031-9007(98)06382-0] PACS numbers: 66.10. Cb, 68.35.Fx, 82.65.My

Site-specific adsorption and diffusion of atomic hydrogen on the Si͑111͒-7ϫ7 surface at elevated temperatures are studied by scanning tunneling microscopy. Hydrogen atoms are found to adsorb preferentially on rest-atom sites rather than adatom sites with a binding-energy difference of ϳ0.2 eV. The adsorption causes a reverse charge transfer from rest atoms back to adatoms. Above ϳ280°C, atomic hopping between two rest-atom sites within a half-cell can occur which is mediated by an adatom site. Above ϳ330°C, H atoms start to hop across the cell boundary via two adatom sites, or they can diffuse across the surface. The activation barrier for hopping from a rest-atom site to a corner adatom site is ϳ50 meV lower than that to an edge adatom site. Thus, in cross boundary jumps, they hop preferentially via two corner adatom sites. From Arrhenius plots, the hopping barriers within the cell and across the cell boundary are determined. The hopping paths, relative binding energies, and site selectivity of hydrogen atoms on the Si͑111͒-7ϫ7 surface agree in general with theoretical results, but our result are both site and path specific. The dynamic behavior of two to three H atoms inside a half-cell is also investigated. ͓S0163-1829͑98͒00939-4͔