Face specificity of the Xe/Pd bond and the S-resonance model

Abstract

Measured initial dipole moments μ 0 and initial TDS derived adsorption energies E ad 0 for xenon adsorbed on the three low index planes of palladium, Pd(110), Pd(100) and Pd(111), are presented, namely μ 0(111)=0.7 D, μ 0(100)=0.61 D and μ 0(110)=0.44 D, and E ad 0(111)=8.3 kcal/mol, E ad 0(100)=9.4 kcal/mol and E ad 0(110)=10.2 kcal/mol. The reverse face specificity of μ 0 and E ad 0 is shown to be in clear disagreement with a simple image model description of the XePd interaction, whereafter μ 0 and E ad 0 should be face independent. Therefore we present a different model in this paper, which we like to call the “s-resonance model” and which is based on the assumption of the existence of a Xe(6s) derived resonance just above the substrate Fermi level ϵ F. This model incorporates Van der Waals and charge transfer effects due to partial filling of the 6s resonance and predicts the reverse trends of μ 0 and E ad 0 qualitatively correct. Direct support for our model stems from UPS valence band and Xe 5 p 3 2 , 1 2 spectra of the xenon covered Pd surfaces.