Supersonic molecular beam study of the trapping of Cl2 on GaAs{110}. Dissociative chemisorption and desorption

Angle-resolved supersonic molecular beam scattering and time-of-flight (TOF) techniques are used to investigate the dissociative chemisorption and scattering behavior of molecular Cl2 from GaAs{110} at room temperature. TOF spectra of the scattered Cl2 molecules are recorded as a function of final polar angle and of Cl-adatom coverage (θCl) for a Cl2 incident beam angle of 45° and incident kinetic energy of 0.51 and 0.74 eV, respectively. When the initially clean surface is exposed to Cl2 only a direct-inelastic scattering component is observed. The TOF distributions recorded after filling Cl2 dissociation sites to a coverage of θCl 0.5 monolayer are clearly bimodal consisting of direct-inelastic and trapping-desorption components. These observations are shown to be consistent with a precursor-mediated Cl2 dissociative chemisorption mechanism. The angular distribution of the desorption component is broader-than-cosine exhibiting a dip in intensity along the surface normal. The in-plane angular distribution of the direct-inelastic scattering lobe is peaked at a supra-specular angle and the average final kinetic energy of the direct inelastic scattered Cl2 is only approximately 28% of the incident kinetic energy (Einc) for Einc = 0.74 eV indicating a substantial exchange energy in the gas/surface collision. The dependence of kinetic energy exchange on incident angle is generally discussed in relationship to the probability of trapping an incident species on a highly corrugated surface.