ADSORBATE-INDUCED CHANGES OF LINEAR AND NONLINEAR ELECTRONIC-RESPONSEPROPERTIES AND 2ND-HARMONIC GENERATION AT SIMPLE METAL-SURFACES

We present a formalism to calculate the change of linear and nonlinear response properties induced by single atoms adsorbed on simple metal surfaces in the adiabatic limit. The atom plus jellium model of Lang a nd Williams for adsorbate chemisorption was extended to incorporate th e presence of an external static field. On the basis of these first-pr inciples calculations in the frame of density-functional theory the me chanisms for the increase or decrease of second-harmonic generation (S HG) by a number of representative adsorbate atoms on free-electron-lik e jellium substrates are discussed and correlated with the various ord ers of the adatom and substrate response. The dipole field of the adso rbed atom causes an important long-range contribution to the response. These dipole fields also give external-field screening and depolariza tion and lead to a quantitative model for adatom-adatom interactions i n the low-coverage regime. The results of recent SHG experiments are i nterpreted with our approach.