X-ray photoelectron spectroscopic study of the physical adsorption of xenon and the chemisorption of oxygen on tungsten (111)

Abstract

X-ray photoelectron spectroscopy (ESCA) has been used to study the physical adsorption of Xe and the chemisorption of oxygen by W (111). An ultrahigh vacuum ESCA spectrometer has been modified such that thermal desorption behavior from the W (111) crystal can be directly compared with ESCA spectra of the adsorbed species. In addition, since the work function of a W (111) crystal covered with one monolayer of Xe is accurately known from previous work, the binding energy of the Xe (3 d 5 2 ) adsorbate level can be accurately compared to the gaseous Xe (3 d 5 2 ) level. When Xe is physisorbed to 1 monolayer the Xe (3 d 5 2 ) level exhibits a binding energy (relative to the vacuum level) which is 2.1 eV below that found for Xe (g). At lower Xe coverages the shift becomes monotonically greater, approaching 2.6 eV at a Xe coverage of 0.05. This 0.5 eV shift downward is accompanied by an increase of only 0.05 eV in adsorption energy as coverage decreases, and may be partially caused by the presence of ~ 10–20 % of extraneous adsorption sites other than W (111) which adsorb Xe with higher adsorption energy. The adsorption energy of Xe may also be increased by coadsorption of oxygen and the Xe (3 d 5 2 ) binding energy exhibits a corresponding shift downward as adsorbed oxygen coverage is increased to θ o = 0.5. Electronic relaxation processes affecting the final state are dominant factors in determining the magnitude of the chemical shift upon adsorption, in agreement with the predictions of Shirley. The magnitude of the relaxation effect seems to be very sensitive to small changes in Xe adsorption energy. Similar effects have been seen for chemisorption of CO. The adsorption of O 2 at 120 K by W (111) yields a single broad O(1s) peak whose line-width decreases with increasing coverage. The final spectra at θ o = 1 monolayer are very similar to those obtained at temperatures of 300 K or above on polycrystalline tungsten.