Dispersion of optical second-harmonic generation from Si(111)7×7

The dispersion of the rotationally anisotropic contribution to optical second-harmonic generation from Si(111)7×7 has been measured for pump photon energies from 1.0 to 1.8 eV. Three distinct resonance peaks are found in this range. Two partly overlapping peaks found at 1.15 and 1.3 eV involve transitions from rest atom to adatom dangling-bond states. The third peak at 1.7 eV corresponds to the so-called strain resonance previously observed for the isotropic contribution to second-harmonic generation by Daum et al.