Growth and electronic structure of ultrathin palladium films on Al(111) and their interaction with oxygen and carbon monoxide

Pd films vapor deposited onto Al(111) were studied by Auger-electron spectroscopy, low-energy electron diffraction (LEED), angle-resolved ultraviolet photoemission spectroscopy, and work-function measurements. At 300 K, Pd grows in the Stranski-Krastanov mode. For thicknesses greater than eight monolayers the Pd film exhibits order which was detected with LEED. The work function shows a minimum at the monolayer coverage and a rise over a transition region of about ten monolayers. The electronic structure of the monolayer was found to be an atomiclike 4$d^{10}$5$s^0$ configuration with the 4d emission centered at 4 eV below $E_F$. This configuration evolved into the bulklike 4$d^{9.6}$5$s^{0.4}$ one with d states at the Fermi edge for a film thickness above eight monolayers. The adsorption of CO and ${\mathrm{O}}_2$ on Pd monolayers is reversed with respect to the adsorption on bulk Pd: The Pd monolayer reacts strongly with oxygen but only weakly with CO. This is explained by differences in overlap between the Pd 4d and the O 2p and CO 2${\pi }^{\mathrm{*}}$ states, respectively.