Room temperature adsorption of Na on Si(1 1 1)-7×7 as studied by resonant optical second harmonic generation

Resonantly enhanced optical second harmonic generation at a pump laser energy of 1.17 eV was applied to investigate the Na-adsorption process on the 7×7 reconstructed Si(1 1 1) surface at room temperature. The change of second harmonic (SH) intensity as a result of Na deposition at normal incident pump radiation varies significantly from that measured at 45° angle of incidence. The observed difference can be explained in terms of electronic transitions from the rest-atom state to the adatom state of the 7×7 reconstructed Si(1 1 1) surface as well as of the features of Na growth on the silicon surface. By measuring the SH polarization dependence for different stages of Na deposition, a change of the interface symmetry in both directions, namely, parallel and perpendicular to the (1 1 1) plane, was clearly observed at the very beginning of the adsorption process.