A near-edge X-ray absorption fine structure and photoelectron spectroscopic study of the structure of acetylene on Pd(111) at low temperature

The structure of acetylene adsorbed on Pd(111) below ∼ 200 K is probed using near-edge X-ray absorption fine structure (NEXAFS) and ultraviolet and X-ray photoelectron spectroscopy. The NEXAFS spectra are calculated using a one-electron cluster formalism corrected by an energy-dependent exchange contribution. Experimental spectra agree well with results calculated for an acetylene molecule adsorbed in a three-fold hollow site with a CCH angle of ∼ 117°, a C-C bond length of ∼ 1.3 A ̊ and with its H-C-C-H plane tilted slightly at ∼ 22° from normal to the Pd(111) surface. The thermal removal of carbon from the surface is measured using X-ray photoelectron spectroscopy by monitoring the C 1s signal intensity. These results indicate that ∼ 35% of the surface carbon is removed by heating to 600 K. This is the temperature range over which benzene desorption is detected in thermal desorption spectroscopy implying that ∼ 35% of the acetylene initially adsorbed at ∼ 90 K converts to benzene. In addition, a shift in the C 1s peak position on heating to between 200 and 300 K implies the conversion of the adsorbed acetylenic molecule into another surface species.