Surface structure determinations of crystalline ionic thin films grown on transition metal single crystal surfaces by low -energy electron diffraction

The surface structures of NaCl(100), LiF(100) and α-MgCl2 (0001) adsorbed on various metal single crystals have been determined by low energy electron diffraction (LEED). Thin films of these salts were grown on metal substrates by exposing the heated metal surface to a molecular flux of salt emitted from a Knudsen cell. This method of investigating thin films of insulators (ionic salts) on a conducting substrate (metal) circumvents surface charging problems that plagued bulk studies, thereby allowing the use of electron-based techniques to characterize the surface.;The surface layer of NaCl(100) was found to have a 0.12 ± 0.03 Å corrugation due to movement of the Na+ ions towards the bulk. Additional deviations from the bulk structure were not seen in the deeper atomic layers. In order to determine if the metal substrate had an influence on the films' growth characteristics, multilayer films of NaCl(100) were grown on Pd(100) and Pt(111). An tensor LEED (TLEED) analysis of the diffraction beam intensities from the NaCl(100)-(1 x 1) on Pd(100) and NaCl(100)-(1 x 1) on Pt(111) LEED patterns showed that the multilayer NaCl(100) film had the same structure on both substrates to a depth sampled by the electrons in the individual experimental energy ranges. Additionally, these two films exhibited the same adsorbate-substrate interaction on Pd(100) and Pt(111) as evidenced by the lone multilayer (zero order) desorption peak observed in the temperature programmed desorption (TPD) spectra of NaCl.;The surface structure of LiF(100) was studied to correlate the surface corrugation of the alkali halide (100) surface to the ions' polarizability. The LiF surface is less polarizable than the NaCl surface. TLEED calculations determined that the LiF(100) surface had a 0.24 ± 0.04 Å corrugation with a 0.06 ± 0.04 Å corrugation of the second layer. Both corrugations were due to the cations moving deeper into the bulk.;Unlike NaCl and LiF, MgCl2 adsorbed on Pd(111) and Pt(111) exhibited a first order desorption feature in its TPD spectra, indicating a significant adsorbate-substrate interaction. This interaction led to the formation of two distinct monolayer LEED patterns: Pd/Pt(111)-(4 x 4)-MgCl 2 and Pd/Pt(111)-( 13×13 ) R 13.9°-MgCl2 (where a monolayer is defined as one Cl-Mg-Cl molecular layer). In addition to the monolayer patterns, an ordered multilayer was grown on Pd/Pt(111). For the adsorbed MgCl2 multilayer on Pd(111), a fully dynamical TLEED analysis was performed. The surface structure of α-MgCl2(0001) is very similar to its bulk structure with the exception of a 0.10 ± 0.06 Å contraction of the first Cl-Cl interlayer spacing.;TPD spectra showed that the strong adsorbate-substrate interaction between MgCl2 and Pd/Pt(111) disappeared when the substrate was changed to Pt(100), Pd(100) and Rh(111). Although ordered MgCl2(0001) multilayers were formed on all three of these metal crystals, no monolayer patterns were formed which indicated a strong adsorbate-substrate-interaction. The absence of monolayer patterns is consistent with the TPD results.