Structure analysis of alkali metal adsorption on single crystal nickel surfaces

Abstract

Studies of alkali metal adsorption on single crystal Ni surfaces were undertaken to determine surface crystallography, electronic structure and atomic binding in these adsorption systems as a function of absolute surface coverage. A detailed analysis of surface crystallography using low energy electron diffraction (LEED) is presented and discussed in this paper. The work function and thermal desorption energy studies will be presented elsewhere. Significant information concerning the nature of surface forces was deduced from the surface crystallography. It is shown that alkali metal adatoms on Ni repel one another at all coverages forming a single layer. Furthermore, the forces between alkali metal adatoms on Ni (110) are highly anisotropic. These non-classical anisotropic forces are probably related to the indirect inter-adsorbate interactions via the metal's conduction electrons treated by Grimley 6). The strong repulsive forces between adsorbed alkali metal atoms on Ni (111) and Ni (100) resulted in the adatoms being uniformly spaced. By uniformly spaced, it is meant that the standard deviation of the average distance between nearest neighbor adatoms is small. Incoherent hexagonal close-packed surface structures are demonstrated to form with Na on Ni (111) and Ni (110) at high coverages in order to maximize the packing of adatoms. The distance between nearest neighbor adatoms in these structures decreases continuously with coverage. The existence of these incoherent structures shows that the repulsive forces among alkali metal adatoms on Ni (111) and Ni (110) are more important in determining their positions at high coverages than are the variations in potential of the adatom due to substrate surface geometry. The distance between nearest neighbor alkali metal atoms adsorbed on Ni at one physical monolayer coverage is shown to be less than the diameter of bulk alkali metal atoms. The occurrence of a reduced alkali adatom diameter coupled with the existence of incoherent, close-packed surface structures raises significant uncertainty about the validity of conventional calculations of surface roughness factors from the density of adsorbed alkali metal atoms.