Coverage dependence of surface optical second-harmonic generation from CO/Ni(110): Investigation with a nonlinear-interference technique

A nonlinear-optical-interference technique in surface second-harmonic generation is developed to measure directly the variation of surface nonlinear susceptibility of CO/Ni(110) as a function of the CO coverage. It is found that some of the susceptibility elements vary linearly with the coverage and others nonlinearly. A simple model suggests that the former are dominated by contributions from nearly free electrons, while the latter by contributions from the more localized electrons.