Stability and structure of O/Pt/Rh(1 0 0): a LEIS and LEED study

Low energy ion scattering (LEIS) has been used to study the effects of O on ultrathin layers of Pt deposited on a Rh(1 0 0) substrate. The Pt/Rh(1 0 0) surface forms a p(3 × 1) structure when heated in O2 with a minimum of 0.8 ML Pt needed for long range order to be established. The p(3 × 1)-O/Pt/Rh(1 0 0) surface shows a strong Rh enrichment when compared to the clean 0.8 ML Pt/Rh(1 0 0) surface. The form of the reconstruction is in agreement with a shifted-row model proposed for the p(3 × 1)-O/Pt25Rh75(1 0 0) surface. The experimental azimuthal Rh intensity is only consistent with 3-D simulations of the surface if it is composed of domains of unreconstructed fcc Rh atoms, as well as domains of the shifted-row reconstruction. It is also shown that surface Pt atoms are not involved in the reconstruction. Finally, the O/Pt/Rh(1 0 0) surface is investigated after being exposed to H2. This liberates the adsorbed O with the LEED image revealing a p(1 × 1) structure. The surface chemical concentrations of Rh in the first and second layer are measured to be 90 ± 10% and 20 ± 10% respectively. It is also shown for this surface that Pt has not diffused into deeper layers of the crystal, but resides predominantly in the second layer.