E. Umbach

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.

A multimethod study of the adsorption of NO on Ru(001): I. XPS, UPS and XAES measurements

Abstract The adsorption of NO on the basal plane of ruthenium at temperatures between 80 and 700 K has been investigated using XPS, UPS and XAES. The combination of these spectroscopies allows the differentiation of molecular and dissociated states and the in situ analysis of their quantitative evolution with exposure at various temperatures and with heating. The main results are: Below ∼200 K, NO adsorbs on Ru(001) in two different molecular states termed v 1 and v 2 ; their relative quantities depend on the adsorption temperature. At adsorption temperatures above 200 K, a dissociative s-state is formed first, followed by molecular adsorption into v 1 and/or v 2 . Heating of low temperature layers shows that in diluted layers dissociation starts again at ∼200 K without desorption, while from saturated layers NO must be desorbed to make dissociation possible, due to spatial requirements. Around 450 K all molecular NO is removed; all N atoms have left the surface by ∼600 K, leaving behind only O atoms. Arguing from the measured binding energies and other spectral features, and by comparison with published ELS data, v 1 and v 2 are assigned to bridge-bonded and linearly bound NO.

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.

An electron spectroscopic investigation of the adsorption of NO on Ni(111)

Abstract The adsorption, decomposition, and desorption of NO on the close packed Ni(111) surface have been investigated by XPS, XPS satellites, XAES, UPS, and LEED between 125 and 1000 K. At adsorption temperatures below 300 K a single molecular species (v) is formed with about unit sticking coefficient, which is interpreted as bridge-bonded; its saturation coverage is about 85% of that of CO, i.e. 0.5 relative to surface Ni atoms. Adsorption at 300 to 400 K yields dissociative adsorption (β) followed by molecular adsorption; above 400 K only dissociated species are formed. Upon heating, a full molecular layer dissociates only after some NO desorption (at 380–400 K), while dilute layers (below half coverage) dissociate already above 300 K without NO desorption. Together with quantitative findings this shows that for dissociation of one v-NO, the space of two is required. N2 desorption from the β-layer occurs above 740 K; the oxygen staying behind diffuses into the crystal above 800 K. Readsorption of NO onto a β-layer or onto an oxygen precoverage at 125 K leads, besides to an α1-state similar to v-NO, to another molecular state (α2) which is interpreted as linearly bound. The resulting total coverage is considerably higher than in a virgin layer. This shows that the blocking of dissociation in a full v-layer is probably not due to β requiring the same sites, but to kinetic hindrance; an influence of β-induced surface reconstruction cannot be excluded, however. The LEED results agree with a previous report and are well compatible with the other results.

A multimethod investigation of the adsorption of NO on Ru(001) : II. Δφ, thermal desorption and LEED results

Abstract In continuation of earlier work using spectroscopic techniques, NO adsorption on the basal plane of Ru has been investigated using thermal desorption, Δφ (in adsorption and desorption) and LEED measurements between 86 and 800 K. The results can be well correlated with the earlier findings and give evidence of the molecular v1 and v2 and the dissociated β species defined before. From a full layer two NO and a structured N2 desorption peaks are observed which are accompanied by strong φ changes. A (2 × 2) LEED structure can be correlated with the presence of the v2 state. Dissociation mainly occcurs from the v1 molecular state. A model is derived which implies mutual stabilisation of v1 and v2, and of v2 and β, in agreement with their opposite dipole moments and their desorption behaviour.

An XPS study of desorption and dissociation kinetics of CO on W(110)

The purpose of this paper is to demonstrate the usefulness of XPS as a quantitative in-situ technique for species-resolving studies of adsorbate reactions and for the investigation of surface kinetics. The desorption and conversion (dissociation) processes in the well-known adsorption system CO/W(110) are chosen for this test. Some details of experimental procedures and necessary precautions are given. CO layers on W(110) prepared at 100 K are molecular (v-CO). Desorption from saturated layers occurs in three main steps. In the first, the compressed CO overlayer is depleted between 225 and 300 K by desorption with low energies and preexponentials. The main desorption from the molecular layer takes place in a second step between 300 and 375 K with Ed = 86kJ/mol and k0 ≅ 1011 s−1. The competing dissociation process is hindered by site blocking by the v-CO species. For lower coverages where site blocking becomes negligible, dissociation without desorption occurs at lower temperatures (between 200 and 300 K). The activation energy (≈21 kJ/mol) and the (“first order”) preexponential (≈ 2 × 102 s−1) for dissociation are very small, in agreement with observations made for room temperature adsorption. Finally, the dissociated layer (β-CO) desorbs between 800 and 1200 K. The desorption and dissociation results of CO/W(110) are discussed, and plausibility arguments are given for the observed kinetic parameters.

Electronic structure and orientation of NO on Ni(111) studied by arups using synchrotron radiation

Abstract The electronic structure and the orientation of NO adsorbed on Ni(111) at 120 K was studied by angle resolved UPS using linearly polarized synchrotron radiation and a multichannel angle resolving electron analyzer. The layer investigated corresponds to a coverage of 0.5 ML and exhibits a c(4 × 2) LEED pattern with only twofold bridging sites being occupied. The binding energies for 4 gs, 5 gs and 1 gP at the ḡG point are 15.1, 9.3 and ~7.5 eV, respectively; the 1 gP level is very broad which may be due to splitting. The 2D band structure shows a dispersion of 0.3 eV for the 4 gs and 0.6 eV for the 5 gs level, indicative of lateral interactions within the adsorbed NO layer. The photon energy dependence of the photoionization cross sections of the 4 gs, 5 gs and 1 gP NO molecular orbitais has been studied in the energy range 26 eV ⩽ ℏ gw ⩽ 60 eV. The 4 gs level shows a pronounced maximum at ℏ ω = 36 eV ( E kin = 15 eV) that is peaked in the direction of the surface normal and is interpreted as a shape resonance. For the 5 gs and 1 gP levels no shape resonances are observed. Using dipole selection rules, it is demonstrated independently by initial state and final state arguments that in a c(4 × 2) layer the NO molecules are adsorbed with their molecular axis perpendicular to the surface.

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.

Angle-dependent valence photoemission from adsorbed molecular nitrogen a comparison of measurements and model calculations

Abstract He I and He II photoelectron spectra for N2 adsorbed on W (110) have been measured at three different polar emission angles. The behaviour of the adsorbate-induced features with angle is compared to the angle- dependence of dipole matrix elements for oriented N2, Ni-N2, and Ni9-N2 clusters obtained by SCF-Xa-SW calculations. The agreement is very good for He I and acceptable for He II, if the N2 axis is assumed normal to the “surface” and the assignment of peaks proposed earlier is used. For He II the use of the big cluster is important to arrive at good agreement.