Adsorption kinetics and patterning of a Si(111)-7 x 7 surface by dissociation of methanol.

CH(3)OH undergoes dissociation on a Si(111)-7 x 7 surface via a two dimensionally free precursor. The sticking probability attained by the STM (scanning tunneling microscopy) was entirely coverage independent, where the observed image represented the final state of the adsorption. CH(3)OH dissociates equally on the faulted and unfaulted halves at room temperature. However, the dissociation at the center adatom-rest atom site is four times preferential to that at the corner adatom-rest atom site in each half unit cell. Such site selectivity, center/corner, changes with the occupation of adatoms in corresponding half unit cell, that is, center/corner=4 for the half unit cell with one reacted adatom, but 2.6 and 1.8 for the half unit cells with two and three reacted adatoms, respectively. Such site selectivity is well rationalized by the dissociation depending on the local conformation of the site instead of the local density of states (LDOS). The site selectivity of center/corner is well reproduced by considering the occurrence probability of the whole dissociation pattern. As the STM image represents the final state of the adsorption, if the final step of adsorption involves dissociation of molecule or precursor, the STM image reflects the dissociation probability depending on the local structure. On the other hand, if no dissociation of molecule or precursor is involved at the final step, the adsorption probability might depend on the LDOS. The adsorption of H(2)S, H(2)O, and NH(3) is also discussed from this general viewpoint of adsorption. The concept of a two dimensionally free precursor will be important to understand the kinetics of heterogeneous catalysis.