Spectroscopic study on the adsorption and dissociation of CH 3I on Pd(100): thermal and photo effects

Abstract

The adsorption of methyl iodide on a Pd(100) surface has been investigated by temperature programmed desorption (TPD), work function change Δφ, ultraviolet and X-ray photoelectron (UPS and XPS) spectroscopies at 85–425 K. By means of TPD only one adsorption state is identified, T p = 129 K, which is formed only at high exposures and attributed to multilayer. Adsorption of CH 3I is characterized by a significant decrease in the work function (1.54 eV at monolayer) indicating that adsorbed CH 3I has a positive outward dipole moment. UPS, XPS and TDS measurements suggest that CH 3I adsorbs dissociatively at submonolayer even at 85–90 K yielding adsorbed CH 3 and I species. The dissociation of the monolayer is complete below 200 K. This process is accompanied by a decrease in the binding energy of the I(3d 5 2 ) core electrons. The decomposition of adsorbed CH 3 started above 160 K and is completed below 250 K. The primary product of the decomposition is CH 4 which desorbs with T p = 170 K. C 2H 6, C 2H 4 and H 2 were detected only in trace amounts. Illumination of the adsorbed layer at 85–90 K enhanced the extent of the dissociation which is ascribed to the generation of photoelectrons.