Boundary Conditions for Electron Distributions at Crystal Surfaces

We derive a boundary condition (b.c.), differing significantly from the well-known Fuchs' b.c., for the distribution function for conduction electrons at a crystal surface. The new b.c. reduces to the simple Fuchs' form under physical conditions which are not fulfilled in certain important cases. The Fuchs' reflectivity parameter is shown to differ in physical significance and in magnitude from the kinetic specularity, with which it is commonly associated. The connection between the b.c. and surface-scattering models is clarified.