PHYSISORPTION ON AN EPITAXIAL NACL(100) DOUBLE-LAYER - SF6 AND XENON

The adsorption behaviour and initial layer growth of SF6 and Xe on an epitaxial, (100)-oriented NaC] double-layer on Ge(100) has been characterized by means of TPD, XPS, UPS, and XAES. Adsorption as well as desorption kinetics are quite different in both cases. SF6 on NaCl(100) shows a layer-by-layer growth for the first three layers and a two-phase desorption behaviour in the monolayer regime. This indicates the coexistence of a two-dimensional (2D-) condensed and a 2D-gas phase in the monolayer with different activation energies for desorption. The TPD results agree well with theoretical calculations by Payne and Kreuzer. For xenon on NaCl(100) a completely different, non-wetting growth mechanism is observed (Volmer-Weber growth for xenon compared to Stranski-Krastanov growth for SF6). This is mainly derived from photoemission and Auger spectra which exhibit drastic changes of adsorbate and substrate intensities, comparing the coverage dependence of SF6 and xenon adsorption, The different growth modes are due to the relative interaction strength between adsorbates and between adsorbate and substrate, respectively. Photoemission and Auger peaks show small energy shifts from mono- to multilayer, which indicate screening only by polarization effects.