The adsorption of 0.15- eV and 0.89- eV Br-2 onto the As-rich GaAs(001)-2 x 4 surface at 300 K has been investigated on the atomic scale over a wide range of total Br coverage using scanning tunneling microscopy (STM). It is found that the strained As-As dimer bonds and the As-Ga back bonds are significantly weakened and chemically activated by the Br atoms site-selectively adsorbed onto the second-layer Ga atoms. Consequently, at higher Br coverages, incident Br-2 will preferentially react with these As-As and As-Ga bonds, removing (etching) As- As dimers and forming AsBr(s) or AsBr2((s)) species, GaBr(s) chains, and etching pit islands. The incident energy of Br-2 has a strong effect on the adsorption mechanisms.