Mechanisms of Photodesorption of Br Atoms from CsBr Surfaces

We investigate desorption of Br atoms from the α-CsBr(110) and β-CsBr(100) surfaces induced by 6.4 and 7.9 eV ultraviolet laser irradiation. The mechanisms of Br-atom desorption were modeled using density functional theory (DFT) calculations. Together the experimental data and theoretical predictions demonstrate that the sub-bandgap irradiation at 6.4 eV predominantly excites the CsBr surface, leading to desorption of neutral Br atoms with a hyperthermal kinetic energy distribution. Excitation above the bandgap at 7.9 eV leads to desorption of Br atoms with both thermal and hyperthermal energies. Our theoretical modeling suggests that desorption of Br atoms with thermal velocities originates from the decay of subsurface excitons, which produces interstitial Br atoms that subsequently diffuse to the CsBr surface. Hyperthermal desorption can be explained by the surface-exciton-based desorption model. The computed maximum kinetic energy of desorbed Br atoms agrees well with the experimental observables.