Vacancy-Assisted Halogen Reactions on $\mathrm{Si}\left(100\right)$- $(2\times 1)$

Scanning tunneling microscopy studies of etching of $\mathrm{Si}\left(100\right)-(2\times 1)$ show that the rate of terrace pit formation goes through a maximum for surface coverages of $\theta \left(\mathrm{Cl}\right)=0.77\mathrm{}\pm 0.05\mathrm{monolayer}$, in contrast to predictions of conventional models. Using recently calculated energies for different possible surface configurations, we show that a key component in desorption is the formation of a single-atom vacancy adjacent to a volatile ${\mathrm{SiCl}}_2$ unit. The demonstration of vacancy-assisted reaction establishes a self-limited reaction and the sequence of events leading to desorption.