Leed debye-waller analysis of vibrational hybridization for p(2 × 2)-O/Pd(100)

The temperature dependence of the 75 eV specular LEED beam intensity was measured between ～100–300°C and ~50–200°C for clean p(1 × 1) and quarter-monolayer O-covered p(2 × 2) Pd(100), respectively. The experiments were performed in a near-normal-incidence geometry to increase sensitivity to vibrations perpendicular to the surface, which include the high-energy, HRELS-active O-stretch mode. The measurements indicate that the effective meansquare displacements were ～40% larger for the clean surface than for the bulk, in qualitative agreement with expectation, and were increased another 20% for the O-chemisorption structure. This observed increase in the Debye—Waller effect upon chemisorption is interpreted to suggest hybridization between the substrate phonon continuum and the high-energy, O-stretch modes. As a result of the hybridization oxygen would have a vibrational component within the substrate continuum that would dominate the magnitude of the oxygen mean-square displacements. A bulk impurity model due to Mannheim is invoked for numerical simulation of such hybridization effects and to serve as an analogy with the surface problem. It is shown that this analogy can be extended to describe resonant modes on a clean surface, and the force-constant increases necessary to create local modes at step defects, in agreement with more realistic treatments. It is shown also that vibrational hybridization can significantly influence the choice of the appropriate adsorbate Debye temperature — an important contrast parameter — that enters into dynamical LEED calculations.