Abstract
Results of a combined Auger electron spectroscopy (AES), UV photoelectron spectroscopy (UPS), low energy electron diffraction (LEED), work function and flash desorption study on the adsorption of xenon on a Pd(110) at ∼ 100 K are presented. Above a steady state pressure of 3 × 10 −8 Torr xenon is found to give a LEED superstructure which is interpreted in terms of a centered rectangular packing model, whereafter the xenon atoms are forming rows within the Pd(110) surface troughs. This packing model is compatible with a hexagonally close-packed overlayer if the xenon radius is taken to be 4.49 Å. The excellent agreement of this value with other xenon/metal systems indicates a perfect fit between the xenon atoms and the Pd(110) surface geometry. With the according monolayer capacity of 5.8 × 10 14 Xe atoms/cm 2 the sticking coefficient is estimated to be unity up to a relative coverage of ν = 0.6. The work function Δφ is found to decrease linearly by 0.92 eV up to saturation coverage, from which a constant dipole moment of 0.42 D is deduced for the adsorption complex. From flash desorption traces an initial activation energy of desorption E des ≈ 9.5 kcal/mole is calculated using the Redhead analysis. Up to saturation coverage E des decreases by about 1 kcal/mole, which is fully explained in terms of electrostatic repulsion between the induced adsorbate dipoles. The UPS measurements reveal two results of fundamental novelty. Firstly, adsorption induced variations of the Pd valence band emission are regarded as the first direct experimental evidence for chemisorptive bonding contribution in noble gas-metal adsorption interaction. Secondly, a continuous variation of the ionization potential of Xe 5 p 3 2 and Xe 5 p 1 2 electrons as a function of xenon coverage is detected, which has never been observed with any other gas-metal adsorption system. This latter shift together with a greater width of the Xe 5 p 3 2 signal as compared to Xe 5 p 1 2 and a reduced spin-orbit splitting between these two peaks is only mentioned and will be dealt with in a separate paper. An extra-atomic relaxation shift of 1.3 eV at saturation coverage is measured between adsorbed and gaseous xenon.
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