Laser photofragmentation and photodesorption of physisorbed CH3Br on lithium fluoride

Abstract

Laser irradiation at 193 nm of multilayer of CH₃Br physisorbed on LiF at 30 K leads to both photofragmentation and photodesorption of molecules. One‐photon photofragmentation was observed leading to broad velocity distributions for ejected CH₃ and Br. Fragments as fast as those found in gas‐phase photofragmentation were seen as well as collisionally slowed fragments. High energy (2 eV, four times the gas‐phase kinetic energy) Br fragments were also observed due to collisions with rebounding CH₃. Collisionally ejected molecules were also seen. A small fraction of molecules desorbed via a thermal process, associated perhaps with small amounts of adsorbed particulate contaminants. This mechanism may be responsible for some of the molecular desorption seen by other groups. Thermal desorption studies show that CH₃Br (or CO₂, Xe, or H₂O) does not wet LiF, indicating a general trend important to all alkali–halide overlayer studies.