Tensor low-energy electron diffraction analysis of the surface structure of NaCl(100) thin films grown on Pd(100) and Pt(111)

Abstract

Thin films of NaCl were molecularly deposited and subsequently ordered on Pd(100) and Pt(111). These films exhibited the same adsorbate–substrate interaction on Pd(100) and Pt(111) as evidenced by the lone multilayer desorption peak observed in the temperature programmed desorption spectra of NaCl on these two substrates. The ordered structures of the films were produced by exposing a heated substrate to a flux of NaCl emanating from a Knudsen cell and investigated through the use of low-energy electron diffraction (LEED). On Pd(100), the adsorbed multilayer yielded a (1×1) NaCl(100) LEED pattern. The (1×1) pattern was seen as the result of the film's thickness being greater than the sampling depth of the elastically scattered electrons used in the LEED experiment, thus removing the contribution of the substrate's lattice to the observed pattern. For the (1×1) pattern of NaCl(100) on Pd(100), it was found that the NaCl film grew along the [010]-type surface directions of Pd(100) to take advantage of a near 1:1 lattice match. On Pt(111), the ordered NaCl multilayer gave a (1×1) NaCl(100) LEED pattern also, but, unlike Pd(100), one of the overlayer's unit cell vectors lies parallel to that of the substrate's with the other rotated by 90° from the previous one to preserve the geometry of the NaCl(100) lattice. Multiple intensity versus electron energy ( IV) data sets were gathered for the NaCl(100) multilayer film on both Pd(100) and Pt(111). An analysis of the I– V curves of the multilayer NaCl(100) pattern on both substrates showed that the multilayer has the same structure on both substrates to a depth sampled by the electrons in the experimental energy range. Data from both systems were used in a fully dynamical LEED calculation. In the final structure, which refined down to a Pendry R-factor ( R P) of 0.16, the largest deviation from the ideally terminated NaCl(100) structure was the movement of the surface Na + towards the bulk, thereby causing a 0.12±0.03 Å corrugation of the surface layer. This shift of the surface cations was not followed by a corresponding displacement of the underlying anions, so the first interlayer spacing, d(Cl 2–Na 1), was reduced to 2.72±0.03 Å from its bulk value of 2.82 Å. Deviations from the bulk were smaller in subsequent layers, e.g. the second interlayer spacing, d(Na 3–Cl 2), was 2.77±0.03 Å, and there was essentially no buckling of the second layer.