X-ray absorption fine structure of adsorbates on metal surfaces

Abstract

The structural techniques of SEXAFS. NEXAFS and photoelectron diffraction have been applied to several adsorption systems. In combination they have allowed detailed surface structural studies to be achieved and the usefulness of these complementary techniques (which employ the same experimental geometry) has been demonstrated. SEXAFS studies have been carried out for atomic and molecular adsorption systems. It has been found that for several systems the (usual) Fourier filtering analysis method is not applicable due to the superposition of EXAFS from different shells. Some systems have been successfully analysed by the Fourier method, however. In particular, EXAFS from the Cu(111) [ √3x√3] R30°-Cl adsorption system yield the same result when analysed by both the Fourier filtering method and a multiple shell simulation routine. Adsorption occurs in the three-fold hollow with a Cl-Cu bond length of 2.39±0.02Ǻ. This is in contrast with earlier analysis of I chemisorbed on Ni(100) where it was found that only the multi-shell method was applicable. Photoelectron diffraction measurements for both normal and off-normal geometries confirmed the site found by SEXAFS and further revealed that adsorption was in the fcc-like three-fold hollow (as opposed to the hcp-like one). A study of the formate intermediate on Cu(110) using NEXAFS revealed that the molecule is adsorbed with it’s plane parallel to the [1ī0] azimuth. Single shell SEXAFS analysis determined the adsorption site to be atop a Cu atom on the 'ridges' with the 0 atoms in the pseudo bridge sites. The average O-Cu distance being l.98±0.O7Ǻ. Reanalysis of the SEXAFS data of formate adsorbed on Cu(110) and Cu(100) using the multi-shell method revealed that the 'anomalous chemisorption bond' proposed for the latter was erroneous. A misleading result had been obtained with the Fourier filtering method because of the superposition of EXAFS from two similar bond lengths. NEXAF studies of the reaction of ethanol and oxygen on Cu(110) revealed that ethanol and an ethoxy intermediate were coadsorbed with the 0-C bond effectively parallel to the surface in the ethanol and perpendicular in the ethoxy