MEASUREMENT OF THE OVERLAYER SUBSTRATE SPACING FOR POTASSIUM ON NI(111) AS A FUNCTION OF COVERAGE

The overlayer-substrate distance for potassium adsorbed on Ni{111} was measured for four different coverages by measuring the low-energy electron diffraction (LEED) specular beam intensities and analyzing them using a constant-momentum-transfer averaging (CMTA) technique. The overlayer-substrate spacing was found to be 2.7 +/- 0.1 angstrom for the four coverages measured, namely 0.13, 0.20, 0.25 and 0.275. This spacing corresponds to a potassium radius of 1.46 +/- 0.10 angstrom for the 0.25 coverage measurement, which is between the ionic and metallic radii for potassium of 1.33 and 2.38 angstrom, respectively. This result suggests that the nature of the bonding of the potassium overlayer atoms to the substrate does not change appreciably in this coverage range. We propose an alternative criterion to replace the labels "ionic", "covalent" and "metallic" for describing the bonding of the alkali metal adatoms to the substrate which does not depend on the labeling of electrons and which is easy to compare to calculations of the charge densities for these systems.