Direct chemisorption site selectivity for molecular halogens on the Si(111)-(7x7) surface

Direct chemisorption site preferences within the Si(111)-(7 x 7) unit cell are examined for F-2, Cl-2, and Br-2. Abstractive chemisorption, which produces a single chemisorbed halide atom, always prefers center adatom adsorption, indicating that the site with the lowest electron density is the more reactive. Dissociative chemisorption, which produces a pair of chemisorbed halide atoms, shows an extreme preference for the closest neighboring pair of center adatoms. The notion of a stepwise direct chemisorption mechanism is upheld by these results.