COVERAGE DEPENDENCE OF SURFACE-STRUCTURE AND VIBRATION OF CL CU(100) COMPARED TO CL/NI(100)/

The coverage dependence of surface structure and vibration was investi gated for 0.12- and 0.50-ML Cl adsorbed on Cu(100) by means of tempera ture-dependent Cl K-edge surface-extended x-ray-absorption fine struct ure. The first-nearest-neighbor Cl-Cu distance was found to be 2.42+/- 0.02 Angstrom for 0.12 ML and 2.39 +/- 0.02 Angstrom for 0.50 ML. More over, the Cl-Cu chemical bond was revealed to be softer and more anhar monic in the 0.12-ML case than in the 0.50-ML one. The present finding s are completely opposite to the previous results on Cl/Ni(100) in whi ch the Cl-Ni bond was found to be elongated, softer, and more anharmon ic with the Cl coverage. For the understanding of the different nature of Cl/Cu from Cl/Ni, density-functional calculations were performed f or finite surface clusters. The theoretically estimated coverage depen dence for the equilibrium distance was very consistent with the experi mental results for both Ni and Cu. The different coverage depen dences of the bond length and strength between Cl/Ni and Cl/Cu can consequen tly be ascribed to the different Cl ionicity at the saturation coverag e, which originates from the different roles of metal 3d bands.