2ND-HARMONIC GENERATION AND EFFECT OF ADSORBATES FOR FREE-ELECTRON METAL AND SEMICONDUCTOR SURFACES

We discuss aspects of a developing microscopic theory of SHG from simp le metal and semiconductor surfaces. For semiconductors calculations o f the dynamical nonlinear susceptibility on the basis of realistic tig ht-binding parametrizations of the electronic Hamiltonian provide a pr actical scheme, In the resulting spectra the effect of the dangling bo nds on SHG is clearly seen together with a strong decrease upon satura tion with H atoms, In the metal case the adsorbate induced changes of the static nonlinear electron density can be calculated self-consisten tly by applying density functional theory to the jellium model, The se cond-order dipole moment determines the effect of adsorbates on the SH G intensity in the adiabatic limit, Quite general a correlation with t he nature of the adsorbate expressed by its electronegativity and the characteristic charge transfer, adsorption dipole and polarizabilities in first and second order is found.