Structure determinations of SO2 and its decomposition product SO adsorbed on Cu(100) by use of X-ray absorption fine-structure measurements

The adsorption of SO2 on Cu(100) has been studied by X-ray absorption fine structure measurements above both the oxygen and sulfur K edge. The analysis of the data for the (SO2+O) phase formed after heating a condensed SO2 layer to 180 K shows that SO2 is adsorbed with its molecular plane parallel to the surface normal and that the co-adsorbed atomic oxygen is located in bridge sites. The data are not consistent with a single SO2 adsorption geometry. The molecules are bonded to the surface through the S atom and at least one O atom resulting in S-Cu bond lengths of about 2.3 Angstrom and O-Cu bond lengths close to 2.0 Angstrom. The polarisation-dependent O-S distances of 1.42 Angstrom for near-grazing and of 1.48 Angstrom for normal X-ray incidence indicate an at least partial lengthening of the S-O bonds on adsorption. The analysis of the data for the (SO+2O) phase obtained after heating a condensed SO2 layer to about 280 K shows that the SO species are lying flat on the surface and bond to the substrate through both the S and O atom. The S atoms are located in hollow sites with a S-Cu bond length of 2.34+/-0.05 Angstrom whereas the O atoms are nearly bridging two Cu atoms with an O-Cu distance of 1.93+/-0.05 Angstrom. The intramolecular O-S bond length measures 1.41+/-0.03 Angstrom. Co-adsorbed atomic oxygen is located in bridge sites.