Interpretation of the surface boundary conditions in the diffusion model for NEA photoemission

The recombination velocity parameter S in the boundary conditions for diffusion of photoexcited electrons is expressed in terms of the quantum-mechanical reflectance of the interface and transport parameters for the interfacial region. The recombination velocity at the photoemitter-substrate interface in a transmission photocathode is expressed in terms of material parameters. Recent experimental data are analysed and factors favouring low recombination velocities are discussed. The ratio of the empirical surface escape probability P to the theoretical surface transmittance T may vary above or below unity. Physically, P/T>1 arises because an electron reflected back from the surface into the bulk has a significant probability of re-entering the bandbending region.