Electronic and CO chemisorption properties of ultrathin Pd films vapor deposited on Au(111)

We present a study of the electronic and CO chemisorption properties of ultrathin Pd films on Au(111). Using ultraviolet photoemission spectroscopy (UPS) we examined the valence-band electronic structure of Pd films for different thicknesses. The UPS spectrum of a Pd monolayer on Au(111) shows a narrow Pd 4d density of states at 1.6 eV below the Fermi level and a reduced density of states near the Fermi level. The valence-band spectra of a Pd monolayer annealed to increasing temperatures was also examined. As a result of Pd-Au interdiffusion, the Pd density of states is seen to sharpen and shift closer to the Fermi level. In the valence spectra of a Pd monolayer annealed above 300 K we identified a feature near 3.5 eV which we attribute to a Pd-Au hybrid state. Using UPS, temperature programmed desorption (TPD), and high-resolution electron-energy-loss spectroscopy (HREELS), we also examined the CO chemisorption properties of ultrathin Pd films on Au(111). From CO TPD experiments we estimate that the activation energy for CO desorption from a Pd monolayer on Au(111) is 32 kcal/mol. HREELS spectra of CO adsorbed on a Pd monolayer on Au(111) show that CO occupies threefold bridge sites at low CO coverage and both the twofold bridge and atop sites at saturation CO coverage. From these results we conclude that the CO chemisorption properties of a Pd monolayer on Au(111) are very similar to those of Pd(111) surfaces, which is striking in view of the large differences between the valence-band electronic structure of these two surfaces.