Second-harmonic generation from sodium covered Si(111)7x7 surfaces

We investigate the variation of the reflected second-harmonic (SH) intensity from Si(111)7 x 7 surfaces as they are being covered with sodium at surface temperatures of 140 and 326 K. The SH is produced by irradiating the sample with femtosecond laser pulses at lambda(infinity) = 570 and -197 nm. As the sodium coverage increases, the SH is generated first by isolated Na-induced dipoles and then by a rough Na film with metallic character. The observed characteristic variations of the SH intensity as a function of Na coverage can be explained by different adsorption sites on the surface, changes in the sticking coefficient, and by a frequency shift of the adsorbate surface plasmon resonance. (C) 1998 Elsevier Science B.V. All rights reserved.