The adsorption of bromine on Pt(111): Observation of an irreversible order-disorder transition

Abstract

The adsorption of bromine on Pt(111) was investigated using LEED, Auger spectroscopy, thermal desorption and work function measurements. A sharp (3 × 3) LEED pattern is observed at room temperature at θ/θ max ≈ 0.6–0.9. Incorporation of additional, more weakly bound, bromine introduces some disorder into this structure without completely destroying it. A complicated behavior of the work function with exposure is observed; the work function first decreases and then increases during the build-up of the adsorbate layer. The work function results together with thermal desorption and Auger evidence suggest adsorption in atomic form with relatively little charge transfer between adsorbate and substrate. An irreversible order-disorder transition of the (3 × 3) LEED structure is reported around 250°C. The phase transition was studied by monitoring LEED intensities of integral and fractional order beams as a function of temperature. Irreversible work function changes and changes in Auger intensities are observed due to the disordering process. Rationalization of these results is attempted in terms of a transition from an ordered, normal overlayer at room temperature to a disordered, reconstructed overlayer at elevated temperatures. LEED intensity measurements revealed beside the disappearance of fractional order beams an increase of the (00) beam intensity induced by the order-disorder transition, whereas the (01) beam intensity is almost unaffected.