Adsorption of methanol on a $\mathrm{Si}\mathrm{}\left(111\right)-7\mathrm{}\times 7$ surface was studied by means of scanning tunneling microscopy (STM). The STM image of adatoms and the tunneling spectroscopy at the rest atom position suggest that the ${\mathrm{CH}}_3\mathrm{OH}$ molecule dissociates on an adatom and rest atom pair by forming ${\mathrm{S}\mathrm{i}-\mathrm{O}\mathrm{C}\mathrm{H}}_3$ and Si-H, respectively. At saturation, half of the adatoms, three adatoms, and all three rest atoms are changed to ${\mathrm{S}\mathrm{i}-\mathrm{O}\mathrm{C}\mathrm{H}}_3$ and Si-H in every half unit cell. When the adatom changes to ${\mathrm{S}\mathrm{i}-\mathrm{O}\mathrm{C}\mathrm{H}}_3,$ the Si adatom becomes invisible at the bias potential of 2.0 V. In contrast, ${\mathrm{S}\mathrm{i}-\mathrm{O}\mathrm{C}\mathrm{H}}_3$ and the intact adatom have equal image intensity at 3.0 V, although center adatoms are brighter than corner adatoms. As a result, the saturated surface shows a characteristic uniform pattern at 3.0 V, although half of the adatoms are ${\mathrm{S}\mathrm{i}-\mathrm{O}\mathrm{C}\mathrm{H}}_3$ and the other half are intact Si adatoms. By counting darkened corner and center adatoms at the bias of 2.0 V, the site-dependent kinetics for the adsorption of ${\mathrm{CH}}_3\mathrm{OH}$ was established. The sticking probability for the formation of ${\mathrm{S}\mathrm{i}-\mathrm{O}\mathrm{C}\mathrm{H}}_3$ and Si-H was independent of the coverage, and the number of ${\mathrm{S}\mathrm{i}-\mathrm{O}\mathrm{C}\mathrm{H}}_3$ in each half unit cell is consecutively increased in a sequence of 1, 2, and 3 with increasing exposure. These facts indicate that the dissociation of ${\mathrm{CH}}_3\mathrm{OH}$ is accomplished via a precursor state in each half unit cell: that is, each half unit cell works independently as if it were a molecule.

• Received 6 March 2002

DOI:https://doi.org/10.1103/PhysRevB.66.125306