DISPERSION OF OPTICAL 2ND-HARMONIC GENERATION FROM SI(111)7X7

The dispersion of the rotationally anisotropic contribution to optical second-harmonic generation from Si(111)7X7 has been measured for pump photon energies from 1.0 to 1.8 eV. Three distinct resonance peaks ar e 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 stat es. The third peak at 1.7 eV corresponds to the so-called strain reson ance previously observed for the isotropic contribution to second-harm onic generation by Daum et al.