Direct Chemisorption Site Selectivity for Molecular Halogens on the Si(111)-(7×7)Surface

Direct chemisorption site preferences within the Si(111)- $(7\times 7)$ unit cell are examined for ${\mathrm{F}}_2$, C${\mathrm{l}}_2$, and B${\mathrm{r}}_2$. 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.