Site-Selective Reaction of ${\mathrm{Br}}_2$ with Second Layer Ga Atoms on the As-rich $\mathrm{GaAs}\left(001\right)-2\times 4$ Surface

The top layer of the $\mathrm{GaAs}\left(001\right)-(2\times 4)$ surface consists of rows of As-As dimers. However, our scanning tunneling microscopy study shows that, in the initial adsorption stage, monoenergetic ${\mathrm{Br}}_2$ molecules (0.89 eV) react exclusively with the second layer Ga atoms exposed in trenches or at defects on the surface. A simple molecular orbital argument was used to explain the dynamics of forming gallium bromide species at various surface Ga sites.