INFLUENCE OF A POLARIZABLE MEDIUM ON THE NONLOCAL OPTICAL-RESPONSE OFA METAL-SURFACE

The nonlocal optical response of a metallic surface is evaluated for a two-component s-d electron system with the aim of achieving a qualita tive representation of the electronic surface excitations of Ag. The s electrons are treated as a semi-infinite homogeneous electron gas whi le the effect of the fully occupied d bands is simulated by a polariza ble medium. According to the more localized nature of the d states, th is polarizable medium is assumed to extend only up to a certain distan ce from the surface. Since the s electrons spill out farther into the vacuum, the s-d screening interaction in the surface region is less pr onounced than in the bulk. Using time-dependent local-density calculat ions, it is shown how the profile of induced surface charge density an d the dispersion of the surface plasmon change with the amount of s-d screening allowed. A key to these calculations is the derivation of a sum rule expression for the centroid of the induced surface charge in the s-d system.