THIOPHENE ADSORPTION ON PD(111) AND PD(100) SURFACES STUDIED BY TOTAL-REFLECTION S-K-EDGE X-RAY-ABSORPTION FINE-STRUCTURE SPECTROSCOPY

The total-reflection grazing-incidence technique has been applied to m easure the S K-edge soft X-ray absorption fine structure spectra of su bmonolayer thiophene on Pd(lll) and Pd(100) surfaces. The total-reflec tion mode allows us to obtain reliable spectra with a much higher sign al-to-background ratio compared to that given by the conventional non- total reflection method. Similar surface structures of adsorbed thioph ene are consequently determined on both surfaces. Thiophene molecules are found to lie hat accompanied by slightly elongated S-C bonds due t o the charge transfer of 1.1-1.2 electrons from the Pd valence bands t o the thiophene pi orbital. The S atom in the thiophene adsorbate is located at an intermediate position between bridge and atop sires. The displacement is estimated to be 0.61+/-0.05 Angstrom away from the br idge site towards the atop one. The structural and electronic properti es of adsorbed thiophene on Pd(lll) and Pd(100) are discussed in compa rison with the previous results on Ni(100) and Cu(100). (C) 1998 Elsev ier Science B.V. All rights reserved.