Spontaneous and electron-induced adsorption of oxygen on Au([formula omitted])-(1×2)

Abstract

The interaction of dioxygen with a gold(110)-(1×2) surface has been investigated between 28 and 700 K by means of thermal desorption spectroscopy (TDS), UV photoelectron spectroscopy (UPS), work function measurements (ΔΦ), low-energy electron diffraction (LEED), and near-edge X-ray absorption spectroscopy (NEXAFS). It proves that Au––unlike most of the other metals including its congeners Cu and Ag––does not spontaneously dissociate physisorbed molecular oxygen. Rather, additional activation of the physisorbed O2 either by electron or by photon impact is required to make the oxygen chemisorb. Below 50 K, dioxygen adsorbs readily with a binding energy <12 kJ/mol, causing a work function decrease of ΔΦ=0.22 eV at Θ=1.0 ML. TDS reveals three molecular desorption states with first-order kinetics around 51 and 45 K (first layer), and at 37 K (second layer). A zeroth-order peak around 34 K corresponds to multilayer desorption. Both UPS and NEXAFS exhibit signals typical for physisorbed O2. Irradiation of physisorbed O2 layers with low-energy electrons or UV photons produces chemisorbed oxygen, providing a convenient possibility for the preparation of chemisorbed oxygen adlayers. The chemisorbed oxygen species is characterized by a single desorption state above 500 K, with second-order kinetics at low coverages (Θ<0.25 ML) suggesting adsorbed oxygen atoms. A desorption energy of 140±3 kJ/mol was determined. Another desorption peak around 490 K and at coverages >1.0 ML is associated with the decomposition of an oxidic species. LEED observations reveal that chemisorbed oxygen destroys the long-range order of the Au substrate surface. Our results provide possible explanations for the `beam damage' often observed in UPS/LEED experiments with adsorbed dioxygen.