Surface kinetics of a nonlinear oxygen-induced (1×5)→(1×1) phase transition on Ir{100}

Abstract

The interaction of oxygen with the stable Ir{100}-(1×5) and the metastable (1×1) surfaces has been studied using supersonic molecular beams in the surface temperature range 200–1080 K. Starting from the clean (1×5) substrate, the adsorption kinetics are dominated by the adsorbate-induced lifting of the reconstruction. The formation of (1×1) islands occurs between two limiting oxygen surface coverages, as confirmed by helium scattering and low-energy electron diffraction (LEED) measurements. Two distinct temperature regimes are observed in the sticking probability measurements; between 350 and 600 K the local oxygen coverage on the (1×1) phase is about 0.28 monolayers (ML) during the prevailing phase transformation, whereas it is 0.20 ML in the temperature range 700–900 K. This “biphasic” behavior is explained by the enhancement of surface diffusion of adsorbed oxygen atoms at sample temperatures above 650 K and has been investigated further using thermal energy atom scattering (TEAS). In contrast to the (1×5) phase, TEAS measurements show that random adsorption of O2 takes place on the clean metastable (1×1) surface. At 1080 K a pronounced flux dependence of the sticking probability is observed due to a nonlinear growth law for the formation of (1×1) islands, r=c(θO1×5)4.5. Thermal desorption measurements accompanied by LEED show that the desorption rate is strongly influenced by the (1×1) to (1×5) surface phase transition; repulsive lateral interactions exist between adsorbed oxygen atoms on the (1×1) substrate. We present a mathematical model which takes these effects into account in reproducing the salient features of the temperature programmed desorption (TPD) spectra. Sticking probability, TEAS, and TPD data are all consistent with a defect concentration of 0.03 ML on the clean (1×5) surface annealed at 1400 K.