PNNL

Abstract

We have studied desorption of Br atoms from thin crystalline CsBr films grown on KBr(100) and LiF(100) induced by ultraviolet laser irradiation at photon energies of 6.4 and 7.9 eV. The mechanisms of Br atom desorption were modeled using Density Functional Theory (DFT) calculations. The experimental data and theoretical calculations demonstrate that the sub-bandgap photoexcitation at 6.4 eV excites predominantly 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. The results of the theoretical modeling suggest that desorption of Br atoms with thermal velocities originates from subsurface exciton decay which produces interstitial Br atoms that subsequently diffuse to the CsBr surface. Hyperthermal desorption can be explained by the exciton-based desorption model. The calculated maximum kinetic energy of emitted Br atoms agrees well with the experimental data.