Potassium adsorption on metal and semiconductor surfaces studied by low-energy D+ scattering.

Abstract

We report on neutralization and inelastic scattering of low-energy ${\mathrm{D}}^{+}$ ions at the K-adsorbed Si(100), Pt(111), and W(110) surfaces. The ${\mathrm{D}}^{+}$ ions scattered from the K adatoms are neutralized almost completely on the Pt(111) and W(110) substrates, although they are observed intensively with a much smaller neutralization probability on the Si(100) surface. These results strongly suggest that K is ionically adsorbed on the Si(100) surface but is essentially neutral on the Pt(111) and W(110) surfaces, which is derived from the fact that the diffusion of the D 1s hole leading to neutralization of ${\mathrm{D}}^{+}$ is extremely enhanced if the target bonds to ligands via metallic or covalent orbital hybridization, while the contrary is true if the bonding is ionic. The ionic K-Si bond results from the preferential donation of the K 4s electron to the active dangling-bond states or the bulk conduction-band states of the Si substrate, whereas the K adatoms are in the polarized neutral state rather than ionized on the metal surfaces due to charge redistributions. It is also found that the oxygen coadsorption results in the formation of the ionic K-O bond on the K/Si(100) surface, but the covalent K-Pt bond is retained irrespective of oxygenation on the K/Pt(111) surface. The fact that oxygen does not strongly interact with K implies that the catalytic promoter effect of K is less poisoned during reaction on the Pt(111) substrate.