Deexcitation of metastable He atoms has been used to probe the existence of a partially occupied $4s$-derived level close to the Fermi level of K atoms adsorbed on a Cu(110) surface, even at very low coverages. The width of this level decreases from about 3 eV at low coverage to 2 eV at high coverage, reflecting the transition from the chemisorbed to the metallic state of the adsorbate.

• Received 17 June 1985

DOI:https://doi.org/10.1103/PhysRevLett.55.1231