Interactions of methyl halides (Cl, Br and I) with Ag(111)

Abstract

The adsorption of methyl halides (Cl, Br and I) on a Ag(111) surface has been investigated by temperature programmed desorption (TPD), work function change (ΔΦ), ultraviolet photoelectron spectroscopy (UPS), and X-ray photoelectron spectroscopy (XPS). All three compounds adsorb with high sticking probability at 100 K, even up to multilayer coverages. The absolute coverage at one monolayer of CH 3X is independent of X and is (4.6 ± 0.3) × 10 14 molecules/cm 2. UPS and XPS spectra suggest that the adsorption of methyl halides is molecular at 100 K and with little distortion of the corresponding gas-phase molecular electronic structures. Submonolayer adsorption is accompanied by a significant work function decrease (ΔΦ between −0.83 and −1.25 eV) indicating a dipole with the positive end pointed away from the surface. At high exposures, multilayers form and desorb at 113 (Cl), 121 (Br), and 136 K (I). Whereas chemisorbed monolayer coverages of CH 3C1 and CH 3Br desorb, with T p = 126 and 142 K, without detectable decomposition, a significant fraction (≅ 35%) of monolayer CH 3I dissociates between 130 and 190 K to give adsorbed CH 3 and I. The dissociation of CH 3I is accompanied by a decrease in the binding energy of the I(3d 5 2 ) core electrons. The adsorbed CH 3 does not dehydrogenate on Ag(111), but recombines above 190 K to yield C 2H 6 which immediately desorbs.