Probing metallization transitions in two-dimensional alkali metal films by metastable He-atom scattering

The survival probability of metastable He* atoms scattered from low work function materials depends sensitively on the electronic surface density of states which was utilized to characterize the degree of metallicity of ultrathin alkali metal (AM) films. By combining metastable He-atom scattering (MHAS) with He-atom scattering (HAS) to monitor the film growth, the coverage dependence of the metallization was studied. In this paper the metallization transition obtained for various AM (Na, K, Rb, Cs) and Ba films grown on Cu(100) and GaAs(100) surfaces are compared. It is shown that the onset of metallization observed for K, Cs and Ba on Cu(100) is in close agreement with a critical coverage estimated from a simple 2D Herzfeld model. Moreover, MHAS was also used to investigate the de-metallization of ultrathin AM films upon adsorption of O2 or CO