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 upon adsorption of O2 and CO has been monitored in situ by metastable helium (He∗ 3S) atom scattering. He∗ can be used to investigate this transition because the variation in the surface density of states (SDOS) induced by demetallization leads to an increase in the He∗ survival probability by a factor of more than 100. It is shown that, for adsorption of O2 and CO at low temperatures (120 K), demetallization is completed by the time exposures of 10 L and 100 L, respectively, are reached.