Alkali Metal Adsorption on Metals Studied by Hiqh Resolution Core Level Spectroscopy and Low Energy Electron Diffraction

Sammanfattning: Alkali metal adsorption on metals is studied by High Resolution Core Level Spectroscopy (HRCLS) and Low Energy Electron Diffraction (LEED). The studies can be divided into two parts: adsorption of alkali multilayers and alkali adsorption in the submonolayer regime. In the case of the adsorption of alkali multilayers, the interface region between a thin alkali film of Na,K,Rb and Cs on different single crystal surfaces has been studied by HRCLS. It is shown that the photoelectrons emitted from different alkali layers can be resolved. The layer resolved alkali core level binding energy shifts are found to depend systematically on the atomic number of the alkali and the substrate metal. The surface index of the substrate is shown to influence the layer resolved binding energy shifts. In the case of alkali adsorption on 4d-metals, measurements of the substrate core levels reveal that interpretations in terms of a simple initial state charge transfer picture do not explain the observed changes in the core level blnding energies. Values for thermo-dynamical parameters such as adhesion energies and segregation energies are extracted from the layer resloved core level binding energy shifts. Conversely, the various core level binding energy shifts measured for the different systems are estimated using thermo-dynamical models, and difficultles when using these models are discussed. For alkali films on sp-metal substrates a surprisingly regular relation is found between the alkali core level binding energy shifts and the difference in r_s of the sp-metal substrate and the adsorbate. Turning to submonolayer alkali adsorption, Na,K,Rb, and Cs have been adsorbed on Al(lll) at Room Temperature (RT) and at 100 K. It is shown that alkali adsorption at RT leads to a disruption of the outermost Al layer in the Al(lll) surface. Island formation is found for Na,K, and Rb at 100 K and for Na and K at RT. In the case of Na adsorption on Al(lll) at RT, the Na and the Al atoms at the interface are found to intermix and a multilayer surface alloy is formed. It is demonstrated how such a surface alloy can be analyzed by HRCLS. The surface geometry of the Al(111)-(2x2)-Na is determined by quantitative LEED. The local geometry is shown to be different when adsorbing K and Rb on Al(lll) and Na on Al(100) at RT compared to adsorption at 100 K, even though the long range order is the same at RT and at 100 K as observed by LEED. The geometry of the Al(lll)-( v 3x v 3)R30 -Rb structure is determined by quantitative LEED for adsorption at 100 K and at RT. An on top site is found at 100 K and a substitutional site at RT adsorption. The details of the irreversible transformation between these two sites are studied with HRCLS. In addition, Na and K on Al(100) at 100 K have been investigated. It is shown that both Na and K condense into islands on the Al(100) surface.