Bromine adsorption, reaction, and etching of Cu(100)

The interaction of Br2 with Cu(100) was characterized using temperature-programmed desorption (TPD) and low-energy electron diffraction (LEED). Initial exposure to Br2 resulted in the formation of a c(2 × 2) LEED pattern and CuBr desorption peaks at 870 and 1000 K. These desorption peaks saturated at a dose of approximately 5 L and were attributed to Br chemisorption. Continued exposure to Br2 resulted in the growth of Cu3Br3 desorption peaks associated with the formation of bulk CuBr. The Cu3Br3 desorption peaks exhibited a strong coverage dependence. At low CuBr coverages, desorption peaks at 450 and 485 K were observed, while at intermediate coverages a third peak at 540 K was observed, and at high coverages a single broad peak at 500 K was observed. Similar results were obtained whether the CuBr layer was formed by exposure to Br2 or by deposition of Cu3Br3 onto a c(2 × 2) layer. The two higher-temperature peaks were shown to be consistent with sublimation of α and β CuBr, while the lowest-temperature peak could not be associated with sublimation of any bulk phase of CuBr. The lowest-temperature peak was attributed to either a grain-size effect or to the interaction of CuBr with chemisorbed Br. At 325 K, growth of CuBr resulted in a hexagonal LEED pattern that disappeared upon annealing to 400 K. Again, the same results were obtained for CuBr formed by reaction with Br2 and by vapor deposition of Cu3Br3, suggesting that the reaction produces near-equilibrium structures. The hexagonal LEED pattern was attributed to compressed epitaxial CuBr(111). The reaction of Br2 with Cu(100) was characterized by a constant sticking coefficient, indicating that the reaction is adsorption-rate limited over the range studied. The sticking coefficient was found to depend on temperature in two distinct ways: (i) annealing the c(2 × 2) layer irreversibly increases the sticking coefficient, and (ii) the sticking coefficient reversibly decreases with increasing reaction temperature. The first effect was attributed to structural changes in the chemisorbed layer.