XPS UPS INVESTIGATION OF NO ON PD(111) IN THE TEMPERATURE-RANGE BETWEEN 20-K AND 300-K

In continuation of a recent multimethod investigation, XPS and ARUPS measurements were performed for NO adsorbed on Pd(111). Quantitative analysis of the XPS spectra were performed in the attempt of determinating the coverage for the three NO desorption states α, β and γ known from previous TDS results. We feel that the XPS results favour the values θα = 3 4, θβ = 1 2, θgg = 1 3, that had been used in our structure models for the three states, rather than other authors proposals, although the alternative value θgg = 1 4 cannot be completely excluded. In order to gain information on the adsorption geometry, the σ shape resonance has been investigated and found at ν = 42 eV for the 4σ level of NO. From the angular distribution of the emitted photoelectrons it can be concluded that the NO molecular orientation is basically perpendicular even at the saturation coverage θsat = 3 4 for the chemisorbed phase, suggesting a partial revision of our model for the 2 × 2 structure. Some comment is made on the energy of the σ shape resonance. XPS spectra were also recorded for NO adsorbed on Pd(111) cooled to 20 K. The intensity, position, and shape of the O(1s) and N(1s) levels provide further evidence for the formation of the thick layer in which the NO molecules pair to form (NO)2 dimers. © 1990.