INTERACTION OF TETRACHLOROETHYLENE WITH PD(100) STUDIED BY HIGH-RESOLUTION X-RAY PHOTOEMISSION SPECTROSCOPY

Adsorption and reaction of tetrachloroethylene (C2Cl4) On a clean Pd(1 00) surface have been investigated at room and cryogenic temperatures. The 300 K saturation of Pd(100) with C2Cl4 gas gave rise to a well-or dered p(2 x 2) overlayer structure. High-resolution X-ray photoemissio n spectroscopy established that the C-CI bonds in the p(2 x 2)C2Cl4 ov erlayer were dissociated while retaining the stoichiometry 2C: 4Cl, an d the amounts of carbon and chlorine on the surface were 0.125 and 0.2 5 monolayer (ML), respectively. At 131 K, the exposure of the clean Pd (100) surface to C2Cl4 resulted in predominantly molecular adsorption, evidenced by the binding energies (BEs) of the Cl 2p and C 1s core le vels. A detailed core level scan in the Cl 2p region revealed two sate llite Cl peaks: one shifted from the molecular C2Cl4 peak by Delta BE = -2.7 eV and the other by Delta BE = -1.4 eV, corresponding to atomic Cl and partially dissociated C2Cl4 species, respectively. As the temp erature increased, the partially dissociated C2Cl4 gradually converted to adsorbed Cl atoms until T = 291 K, at which temperature all Cl on the surface formed atomic Cl of 0.25 ML. The carbon species, while pre sent in stoichiometric amounts, did not give rise to additional struct ural features, but they indirectly affected the Cl ordering in forcing the p(2 x 2) structure, which does not form upon dissociative adsorpt ion of elemental chlorine.