Decay of the self-trapped exciton near the (001) surface in NaBr and KBr

The adiabatic instability of the self-trapped exciton near the (001) surface in NaBr and KBr leading to Br atom desorption is studied. It is shown that in the first two layers below a (001) surface, the exciton relaxes backward to the bulk; no desorption may result thereby. Only from deeper layers does the instability propel the Br-2(-) molecule toward the surface as well as backward. The relaxation toward the surface is strong in KBr, making energetic Pr desorption likely. Similar strong relaxation is not found in NaBr, by contrast. These results are discussed in comparison with experimental data on energetic halogen atom desorption upon electronic excitation.