ROOM-TEMPERATURE ADSORPTION OF CS ON SI(111)-(7X7) STUDIED BY OPTICAL2ND-HARMONIC GENERATION

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