Asymmetric surface structure of SO2 on Pd(111) studied by total-reflection X-ray absorption fine structure spectroscopy

S K-edge surface X-ray absorption fine structure measurements reveal that SO2 is adsorbed on Pd(111) with the molecular plane normal to the surface, via the S atom and one of the O atoms. This is in clear contrast to the SO2/Ni cases where the molecule lies flat on the surfaces. Inequivalent intramolecular S–O bonds are clearly identified with the distances of 1.43 Å for the upright one and 1.48 Å for the surface-parallel one. The total-reflection measurements play an important role in discriminating between the two S–O bond lengths with asymmetric surface structure.