Adsorption of CS on RH(100)-Phases and Phase Transitions

Abstract

Low-energy electron diffraction (LEED) monitoring reveals that cesium develop several superstructures dependent on coverage when adsorbed on a clean Rh(100) surface at T≈ 120 K. This chapter shows schematic LEED patterns and real space models of Cs/Rh(100) with increasing Cs-coverage. The models proposed wait for confirmation by a LEED structure determination. Intensity spectra were taken for different structures, but only the analysis of the c(4×2) structure is discussed in the chapter. Using a computer controlled video camera, the measurement was performed at 120 K and normal incidence with subsequent averaging of symmetrically equivalent beams. Because of the large interlayer distances, RFS could be applied for inter layer diffraction, while intralayer scattering was handled by matrix inversion. Up to 10 phase shifts were considered for both Cs and Rh. The structures shown in the chapter were observed at 120 K and distinct coverages. To construct the phase diagram, T and θ were varied continuously. Simultaneously, integral spot intensities as well as spot profiles in reciprocal space were recorded. The physics of the structures observed is dominated by the interaction of adatoms. Alcali atoms are well known to transfer electronic charge to the substrate creating dipoles. In fact, assuming dipole–dipole interaction (0.2 eV for nearest neighbors) MC calculations on a hollow site lattice gas model reproduces the correct phase boundaries for θ ≤ 0.2S.