SPECTROSCOPIC STUDY ON THE ADSORPTION AND DISSOCIATION OF CH3I ON PD(100) - THERMAL AND PHOTO EFFECTS

The adsorption of methyl iodide on a Pd(100) surface has been investigated by temperature programmed desorption (TPD), work function change DELTAphi, 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 CH3I is characterized by a significant decrease in the work function (1.54 eV at monolayer) indicating that adsorbed CH3I has a positive outward dipole moment. UPS, XPS and TDS measurements suggest that CH31 adsorbs dissociatively at submonolayer even at 85-90 K yielding adsorbed CH3 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 1(3d5/2) core electrons. The decomposition of adsorbed CH3 started above 160 K and is completed below 250 K The primary product of the decomposition is CH4 which desorbs with T(p) = 170 K. C2H6, C2H4 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.