J.C. Fuggle

Photoelectron spectroscopic studies of adsorption of CO and oxygen on Ru(001)

Abstract XPS and UPS have been used for a detailed study of the adsorption and coadsorption of CO and oxygen on a clean Ru(001) single crystal. The measured substrate and adsorbate core level binding energies and valence levels are discussed. The O 1s XPS peak intensity has been used for kinetic studies of adsorption and coadsorption. Some studies of the angular dependence of adsorbate and substrate peak intensity ratios are presented. We also present data on the shifts of XPS peaks and changes in UPS spectra as a function of adsorbate coverage. The data are correlated with the results of earlier measurements with other methods.

XPS, UPS AND XAES studies of the adsorption of nitrogen, oxygen, and nitrogen oxides on W(110) at 300 and 100 K: II. Adsorption of NO

The adsorption layers formed by exposure ofW(110) to NO gas at about 300 and 100 K have been investigated by XPS, UPS and XAES. At room temperature only dissociative adsorption takes place. At 100 K, the initial adsorption is also dissociative, but there is some indication of the formation of a transient NO species. Higher NO exposures lead to the formation of adsorbed N2O, which then partly desorbs and partly dissociates before 150 K is reached. This shows that N2O can be formed from N(ad) + NO. The relevance of our data to catalytic NO decomposition is discussed.

XPS, UPS and XAES studies of the adsorption of nitrogen, oxygen, and nitrogen oxides on W(110) at 300 and 100 K

Abstract The adsorption of N2, N2O, and NO2 on W(110) at 300 and 100 K and the behaviour of the resulting adlayers upon heating have been investigated using XPS, UPS, and XAES. For O2 adsorption, some results of importance to the present investigation (sticking coefficients, XPS peaks and satellites, UPS, and XAES spectra) are also reported. The determinations of absolute coverages of O and N are based on these data. All nitrogen oxide layers are found to be deficient in N, as compared to the gaseous species. The occurrence of dissociative or molecular adsorption under the various conditions is determined. It is found that at 300 K, N2O and NO2 adsorption are completely dissociative, while at 100 K initial dissociation is followed by the formation of condensed layers of N2O and N2O4, respectively, with different growth characteristics. Adsorbate line widths in UPS and line shapes in XPS are discussed. The usefulness of the various methods for fingerprinting is compared.

Electron induced dissociation of CO on Ru(001): A study by thermal desorption and electron spectroscopies

Abstract CO adsorbed on Ru cannot convert thermally into a β-state of the kind observed on W and Mo — which is most probably dissociated CO — under CO pressures below 10 −5 Torr. Such a state can, however, be produced by electron impact onto a virgin CO layer adsorbed on Ru. This is shown using temperature-programmed desorption, UPS, XPS, XPS satellites, and XAES. A comparative discussion of the β-states on W and Ru yields rough estimates of the energies for desorption and conversion of CO on these metals; possible implications for catalytic reactions are mentioned.

Adsorbate characterisation by correlation of various photoelectron spectroscopies: CO on W(110)

Abstract As a test of the value of various electron spectroscopies and their combination for the characterisation of adsorption states, UPS valence spectra, XPS core spectra (O (1 s ) and C (1 s )) and core satellite spectra (O (1 s )), and X-ray induced Auger spectra (O KLL) were measured for various adsorption layers of CO on W(110) prepared at and above room temperature and, for comparison, of oxygen on the same surface. Virgin- and β-CO can readily be distinguished in all four kinds of spectra, while α-CO shows spectra very similar to those of virgin-CO. The conversion of virgin- to β-CO and their desorption can be followed in some detail. For all four techniques, the oxygen derived spectra of β-CO are identical to those of adsorbed oxygen, at about half the intensity. This makes it very likely that CO is dissociated in the β-layer on W(110). Virgin- and α-CO show the typical features of molecularly-adsorbed CO.

Angular dependence of UV photoemission spectra from clean Ru(001) and from adsorbed oxygen and CO

Abstract We report HeI and HeII excited spectra from a clean Ru(001) surface and surfaces covered with oxygen of CO 1 recorded at a variety of electron take-off angles. The angular dependence of the emission from the Ru valence band is stronger for HeI and HeII excitation. The adsorbate valence band peaks shift less than 0.4 eV as a function of take-off angle and fractional coverage of the surface but the relative intensity of the 4δ and (5δ + 1π) peaks from adsorbed CO is extremely sensitive to electron take-off angle. A simple model calculation can account for the main trend in the angular dependence of CO peaks.

Coverage dependent shifts of XPS peaks during chemisorption on metals

Abstract Shifts of substrate and adsorbate XPS core level peaks and of some Auger peaks have been measured as a function of electron emission angle and of coverage in the adsorption systems oxygen on W (100) and (110), Ru (001), and Ag (110), and CO on Ru (001). The substrate peaks are mostly observed to shift to higher binding energies with increasing coverage, while for adsorbate peaks shifts to higher and lower energies have been observed. Low electron emission angles can be used to maximize the contribution of the first layer in substrate core peaks and valence emission. The shifts show no simple relation to work function changes. Possible contributions to the observed effects are discussed.

X-ray photoelectron satellites from adsorbed species

Abstract The OIs XPS peaks from monolayers of CO and oxygen adsorbed on the basal (001) face of a ruthenium single crystal are observed to have satellites whose position and intensity are sensitive to the chemical environment.

High-resolution auger spectra of adsorbates

Abstract X-Ray excited Auger spectra (XAES) from adsorbates and small condensed molecules containing oxygen and nitrogen are presented. The data collected show the utility of XAES for identification of surface species. The interpretation of the shapes of O KLL and N KLL spectra is discussed. The role of screening or relaxation processes is illustrated with the example of the O KLL Auger spectra of oxides and adsorbed oxygen.

X-ray excited auger spectra (XAES) from chemisorbed species

Abstract Using X-ray excitation, well-resolved oxygen KLL Auger spectra have been obtained from chemisorbed layers of oxygen and carbon monoxide on Ru(001). These spectra are markedly different, while the spectrum of CO on Ru(001) is quite similar to the Auger spectrum from gaseous CO. The shift between the spectra of gaseous and adsorbed CO is much larger than for the corresponding XPS spectra, a fact understandable in terms of image charge screening. The use of XAES as a fingerprinting method for adsorbed species is recommended.