Monitoring of electronic poisoning of ultrathin alkali metal films by oxygen and carbon monoxide adsorption

The demetallization of ultrathin films of sodium, potassium and caesium upo n adsorption of O-2 and CO has been monitored in situ by metastable helium (He* S-3) atom scattering. He* can be used to investigate this transition b ecause the variation in the surface density of states (SDOS) induced by dem etallization leads to an increase in the He* survival probability by a fact or of more than 100. It is shown that, for adsorption of O-2 and CO at low temperatures( 120 K), demetallization is completed by the time exposures of 10 L and 100 L, respectively, are reached. (C) 2000 Elsevier Science B.V. All rights reserved.