Bromine atom concentrations in Br2discharges were measured by Br2absorption spectroscopy. At 3.7 MHz, the dissociation of Br2increased with power, reaching a maximum of ∼40%. The aluminum etch rate was proportional to the bromine atom concentration. In the discharge, atoms etched aluminum 20 times faster than molecules. The etch product molecule appears to be reversibly physisorbed on the brominated surface with an apparent binding energy of ∼0.2 eV/molecule.