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

The structural and dynamical properties of molecular oxygen adsorbed o n 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, t he O-Cu distance is 2.05 angstrom, and the 2nd and 3rd cumulants of th e pair distribution function (PDF), C2 and C3, are much larger than th ose of the atomic oxygen ''precursor'' state. In a ''strongly-chemisor bed'' 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 sign ificantly, 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 mo st probable, where one oxygen atom adsorbs on the 4-fold hollow site a nd the other locates close to the bridge site.