Coverage and Chemical Dependence of Adsorbate-Induced Bond Weakening in Metal Substrate Surfaces

Strongly coverage-dependent outward relaxation and enhanced in-plane vibrational amplitudes of metal surface atoms have been observed from S- and Cl-covered Ni(001) and Cu(001) by means of temperature- and polarization-dependent surface extended-x-ray-absorption fine-structure measurements. These results explicitly demonstrate the weakening of both interlayer and intralayer metal-metal substrate bonds. A model based on adsorbate-metal charge rearrangement explains these chemisorption-induced changes in the geometric and dynamic properties of the metal surface.