Diffusion of Particles Adsorbed on a Reconstructive Surface
The influence of surface reconstruction on diffusion of particles, adsorbed on the surface, is investigated in the framework of symmetrical four-position model. The analytical expressions for the free energy and chemical and jump diffusion coefficients are obtained assuming the lateral interaction between the particles is negligibly small. The coverage dependencies of the tracer, jump and chemical diffusion coefficients are calculated using the expressions and MC simulations. The results demonstrate clearly a drastic change of the behavior of all diffusion coefficients on the reconstructed surface.
Monte Carlo study of the W(001) surface reconstruction transition based on total-energy calculations.
We describe the extraction from local-density-approximation-based total-energy calculations of the potential governing the reconstruction of the W(001) surface and an extensive Monte Carlo simulation investigation thereof. Our study is successful in reproducing the qualitative features of this much-studied surface reconstruction, in particular confirming and fully characterizing its order-disorder character. We find and discuss a discrepancy with the measured transition temperature, concluding that it probably arises from remaining small inaccuracies in the total-energy calculations.
Displacive surface phases formed by hydrogen chemisorption on W{001}
Abstract A detailed LEED study is reported of the surface phases stabilised by hydrogen chemisorption on W {001}, over the temperature range 170 to 400 K, correlated with absolute determinations of surface coverages and sticking probabilities. The saturation coverage at 300 K is 19(± 3) × 1014 atoms cm−2, corresponding to a surface stoichiometry of WH2, and the initial sticking probability for both H2 and D2 is 0.60 ± 0.03, independent of substrate temperature down to 170 K. Over the range 170 to 300 K six coverage-dependent temperature-independent phases are identified, and the transition coverages determined. As with the clean surface ( 2 × 2 )R45° displacive phase, the c(2 × 2)-H phase is inhibited by the presence of steps and impurities over large distances (∼20 A), again strongly indicative of CDW-PLD mechanisms for the formation of the H-stabilised phases. These phases are significantly more temperature stable than the clean ( 2 × 2 )R45° , the most stable being a c(2 × 2)-H split half-order phase which is formed at domain stoichiometries between WH0.3 and WH0.5. LEED symmetry analysis, the dependence of half-order intensity and half-width on coverage, and I-V spectra indicate that the c(2 × 2)-H phase is a different displacive structure from that determined by Debe and King for the clean ( 2 × 2 )R45° . LEED I-V spectra are consistent with an expansion of the surface-bulk interlayer spacing from 1.48 to 1.51 A as the hydrogen coverage increases to ∼4 × 1014 atoms cm−2. The transition from the split half-order to a streaked half-order phase is found to be correlated with changes in a range of other physical properties previously reported for this system. As the surface stoichiometry increases from WH to WH2 a gradual transition occurs between a phase devoid of long-range order to well-ordered (1 × 1)-H. Displacive structures are proposed for the various phases formed, based on the hypothesis that at any coverage the most stable phase is determined by the gain in stability produced by a combination of chemical bonding to form a local surface complex and electron-phonon coupling to produce a periodic lattice distortion. The sequence of commensurate, incommensurate and disordered structures are consistent with the wealth of data now available for this system. Finally, a simple structural model is suggested for the peak-splitting observed in desorption spectra.