Determination of NO adsorption sites on Pd(1 0 0) using core level photoemission and low energy electron diffraction

The adsorption of NO on Pd(1 0 0) is investigated with high resolution core level spectroscopy and low energy electron diffraction (LEED). Several ordered NO overlayers were observed, in agreement with earlier studies. Our data clearly show that NO adsorbs in fourfold hollow sites at coverages up to 0.25–0.30 monolayer (ML) whereas at 0.5 ML only bridge sites are occupied. By a reinterpretation of previous electron energy loss spectroscopy (EELS) investigations we show that the new site assignments are in agreement the EELS data. Based on the photoemission results for the N 1s and the Pd 3d core levels we propose new structure models for the (2√2×2√2)R45° and the p(4×2) LEED patterns found at coverages of 0.5 and 0.25 ML, respectively. In the latter case, it is suggested that the p(4×2) LEED pattern is formed from domains having p(2×2)-NO and c(4×2)-2NO unit cells.