SINGLET-TRIPLET CONVERSION EFFICIENCY OF METASTABLE HE ATOMS DEPENDENT UPON STRUCTURES OF ADSORBED ALKALI-METALS ON SI(100)2X1

The singlet-triplet conversion efficiency for metastable He atoms.has been studied. It depends on the structure of adsorbed alkali metals on a surface. At a coverage of less than half a monolayer of K and Cs ad sorbed on Si(100)2 x 1, the singlet-triplet conversion efficiency is h igh and it decreases upon reaching the saturation coverage at room tem perature. On the other hand, it stays high up to the saturation covera ge of Na. These facts suggest that the electronic bands of monolayer-c overed Cs and K on Si(100)2 x 1 are semiconducting or insulating (band is fully filled), while that of Na is metallic (band is half-filled).