Room temperature adsorption of Cs on Si(111)-(7x7) studied by optical second-harmonic generation

Room temperature adsorption of Cs on Si(111)-(7 x 7) has been followed continuously with second-harmonic generation during the evaporation process at several photon energies. The resonances of the clean (7 x 7) surface decay and new resonances involving Cs-induced states are observed below the monolayer coverage. The resonances at different coverages are discussed in terms of surface and interface states as they are know from photoemission, with good agreement between the techniques. Continued evaporation at monolayer coverage leads to an increasing polarizability perpendicular to the surface which - if the evaporation is stopped - rapidly decays to a lower saturation level. This is explained in terms of multipole plasmons in a thin metallic Cs film, of a thickness just above a monolayer, sustained during continuous evaporation. (C) 1997 Elsevier Science B.V.