Anti-phase domain formation during cesium adsorption on Ru(0001)

Abstract

The adsorption of cesium on the Ru(0001) surface at T = 300 K has been studied by LEED, Auger spectroscopy, work function measurements, thermal desorption and optical simulation techniques. Work function measurements indicate a dipole-dipole interaction ordering adsorbates in the low coverage regime which is in agreement with other data published for similar systems. In the high coverage region the work function curve reaches a local maximum at 1.9 eV before its value decreases to that characteristic of bulk cesium. Auger intensity measurements show an apparent change in the slope of the uptake curves for Cs(47 eV) and Ru(231 eV) for the same cesium dose at which the local maximum in the work function curve occurs. Three peaks are found in the TDS results at desorption temperatures 1130 K, 690 K and 380 K, respectively. The LEED patterns obtained show that some of the initially observed (2 × 2) diffraction spots at the coverage θ = 0.25 start splitting as the cesium dosing continues. Optical diffraction experiments lead to a model of the cesium overlayer near monolayer coverage that explains the spot splitting as due to the existence of separate (2 × 2) domains, out-of-phase with respect to each other. The analytical formula for electron diffraction spot positions in the presence of such anti-phase domains is developed. Detailed examination of the model shows that the borders between anti-phase domains must lie along three specific directions on the substrate, defined by the criterion of minimum energy of the attractive interaction between bordering domains. This also implies that there are open areas (not covered by cesium atoms) at domain boundaries near the completion of the first layer with the minimum spacing between adjacent domains equal to ( 7 2 )a ( a is the 2 × 2 mesh size), which explains the local maximum in the work function at this coverage.