THE ADSORPTION OF O2 MOLECULES ON CU(100) - EVIDENCE FOR ENHANCED DISORDER AND ANHARMONICITY

The structural and dynamical properties of molecular oxygen adsorbed on Cu(100) were investigated by means of O K-edge NEXAFS and SEXAFS. In an ''almost-physisorbed'' state, the O-O distance is 1.21 angstrom, the O-Cu distance is 2.05 angstrom, and the 2nd and 3rd cumulants of the pair distribution function (PDF), C2 and C3, are much larger than those of the atomic oxygen ''precursor'' state. In a ''strongly-chemisorbed'' state, the O-O distance is elongated to 1.45 angstrom, the O-Cu distance is shortened to 2.01 angstrom, and C2 and C3 are reduced significantly, although these values are still larger than those of the ''precursor'' atomic state where the O-Cu distance is 1.86 angstrom. For the adsorption geometry of these molecular states, a tilt model is most probable, where one oxygen atom adsorbs on the 4-fold hollow site and the other locates close to the bridge site.