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 measure the S K-edge soft X-ray absorption fine structure spectra of submonolayer thiophene on Pd(111) and Pd(100) surfaces. The total-reflection mode allows us to obtain reliable spectra with a much higher signal-to-background ratio compared to that given by the conventional non-total reflection method. Similar surface structures of adsorbed thiophene are consequently determined on both surfaces. Thiophene molecules are found to lie flat accompanied by slightly elongated S–C bonds due to the charge transfer of 1.1–1.2 electrons from the Pd valence bands to the thiophene π* orbital. The S atom in the thiophene adsorbate is located at an intermediate position between bridge and atop sites. The displacement is estimated to be 0.61±0.05 A away from the bridge site towards the atop one. The structural and electronic properties of adsorbed thiophene on Pd(111) and Pd(100) are discussed in comparison with the previous results on Ni(100) and Cu(100).