The adsorption of CO on Fe-covered W(110)

Abstract

The adsorption of CO on Fe monolayers on W(110), with Fe deposited at 90 K, or so deposited and then annealed at 600 K, was studied by thermal desorption, XPS, UPS, work-function and electron-impact measurements. Although the CO-free 90 K and 600 K Fe layers show very different work functions, they behave very similarly in CO adsorption. The latter is qualitatively similar to what is seen on clean W(110). CO is adsorbed without dissociation at 90 K to a coverage of CO W = 0.8–0.85 on both layers. Heating leads to desorption of part of the CO with peaks in the TPD spectrum at 320 K for the 600 K Fe layer and 250 and 340 K for the 90 K layer, compared to 360 K for CO W(110) , and conversion of an amount CO W = 0.27 to dissociated CO (i.e. β-CO), as indicated by a shift in the O 1s peak from E b = 531.8 eV for virgin CO to 530.1 eV for β-CO (both relative to E F). These values are the same as for CO on clean W(110). Work-function increments Δø relative to the clean Fe layers are 1.46 and 1.33 eV for the 90 and 600 K layers, respectively, slightly higher than for CO on clean W(110) (1.1 eV at CO W = 0.72 ). UPS spectra for the two Fe layers are very similar and do not differ appreciably from that of CO on W(110). Electron-stimulated desorption yielded neutral CO and also CO +, the latter with a much smaller cross-section than on clean W(110). The total desorption cross-sections for the two Fe layers were very similar. Neutral desorption from both Fe layers showed two regimes. The first has cross-sections of ∼ 1.3 × 10 −17 cm 2 if both regimes coexist ab initio and ∼ 5 × 10 −18 cm 2 if the second regime is created by electron impact. The second regime has a cross section of ∼ 10 −18 cm 2 for both layers. These results indicate that the 90 and 600 K Fe layers have very similar properties despite their work-function differences.