The adsorption of Fe on W(110) and of O and H on Fe-covered W(110)

Abstract

The adsorption of Fe monolayers on W(110) and adsorption of oxygen and hydrogen on the Fe layers have been examined via work-function, XPS, Auger, LEED, and thermal desorption measurements. Two distinct forms of Fe 1 W(110) exist, although both show sharp p(1 × 1) LEED patterns. For the monolayer formed at 90 K, Δф = −836 meV , while it is −530 meV for the layer formed or annealed at 300–600 K. Heating the 90 K layer converts it irreversibly to the high-temperature form. Adsorption of H or O on these layers gives different ф and Δф values. Initial sticking coefficients for H 2 at 90 K are s 0 = 0.36 for the 90 K Fe layer and 0.3 for the 300/600 K layer. The amounts adsorbed at 90 K after 40 Langmuir exposure (where s<0.05) are (in units of H W , i.e. unity corresponding to 1.42 × 10 15 atoms cm −2) 0.69 for the 90 K and 0.63 for the 600 K layer. H 2 thermal desorption spectra for H/Fe 1/W(110) and for Fe 1/H 1/W(110) are similar in shape and peak at 320 K, much lower than on W(110) or on Fe(110). O is adsorbed to the same extent and with virtually identical sticking coefficients on both Fe layers, with s 0 ≈ 0.88 at 90 K. Fe 2p peak-shifts indicate that O uptake leads to chemisorption for O W<0.5 , followed by oxide formation. The maximum O uptake corresponds to O W ≈ 1.35 , and suggests that Fe 3O 4 is formed at saturation, as also indicated by the Fe 2p peak shift. Work function increments are different for the two layers, being 713 meV for the 90 K layer and 773 meV for the 600 K layer for chemisorption, i.e. for O W = 0.42 . On heating saturated layers to T> 1200 K, Fe, WO 2 and traces of FeO are desorbed with peak temperatures at 1340 K. For O 0.5/Fe 1/W(110) or Fe 1/O 0.6/W(110), oxidation of Fe does not occur; in the former case O becomes chemisorbed on the W surface on heating to 450 K. In both cases this is followed by segregation on heating to 800 K, as indicated by decreases in Fe Auger intensities and formation of a p(1 × 1) LEED pattern, corresponding to close-packed oxygen islands, as observed previously with other metal-oxygen coadsorption systems on W(110). A brief discussion of the similarities and differences of the present results with those for Cu, Ag, Hg, Ni, and Pd on W(110) is given.