Second-harmonic generation from alkali-metal overlayers

The frequency dependence of the linear and nonlinear electronic response of alkali-metal overlayers adsorbed on metal surfaces is calculated within the time-dependent density-functional approach. The nonlinear dipole moment which characterizes the perpendicular surface component of the second-harmonic polarization shows a pronounced resonance near 2ω≊0.8${\omega }_p$(ads), where ${\omega }_p$(ads) is the bulk plasma frequency of the adsorbed alkali metal. This resonance can enhance the second-harmonic intensity by several orders of magnitude. This surface local-field effect might partly explain the huge enhancements that have been observed for optical second-harmonic generation from alkali-metal overlayers on Rh and Ag surfaces.