Ab initio calculation of second-harmonic-generation at the Si(100) surface

We present a microscopic, first-principles calculation of the second-harmonic spectra of clean and Hydrogenated Si(100) surfaces. The differences between theoretical spectra obtained for different reconstructions, namely the 2 X 1 and the c(4 X 2) ones, are dramatically enhanced with respect to those obtained in linear optical response. The calculated spectral features are analyzed in detail, studying their relations with those of bulk and surface linear optical spectra. The inclusion of quasiparticle effects within the scissors operator approximation yields theoretical spectra in good agreement with the experiments.