Measurement of surface diffusion by laser-beam- localized surface photochemistry

Abstract

The surface migration of a weakly adsorbed molecular species has been measured by studying the kinetics of a highly spatially localized laser-beam-driven photoreaction in an adsorbed layer. The measurement technique, which is based upon a saturation in photodeposition rate due to limitations on mass transport by surface migration, has been applied to a study of tetraethyllead (TEL) adsorbed on sapphire substrates. A theoretical analysis has been made in which the diffusion equation is solved by a Green's function integral technique. The solution is numerically integrated in order to calculate the spatial and temporal evolution of the adsorbate surface number density of parent and product molecules in a beam-localized adsorbate reaction. Comparison between experiment and theory indicates a room-temperature surface diffusivity of D surf=3×10 −7 cm 2/s and gives the room-temperature rate and activation energy of TEL desorption, at ∼1 monolayer coverage, as k des=7×10 −3 s −1 and E des=0.13 eV, respectively. The method of measurement may be broadly applicable to studies, impractical by high-vacuum techniques, of the surface-migration kinetics of other fragile or weakly adsorbed molecules.