A SHG-study of Na-induced Si (111) 7×7 surface symmetry changes

We demonstrate via measurement of normal-incidence, near-resonant, angular dependent second harmonic generation a strong change of Si(111)7×7 surface symmetry as a consequence of submonolayer adsorption of sodium atoms. The results are interpreted by assuming that a new surface structure with lower symmetry coexists on the Na/Si(111)7×7 interface with the 3m-symmetry of the clean Si(111) surface. This new surface structure could be due to an Na-induced surface reconstruction.