P.M. Stefan

The temperature dependence of the interaction of oxygen with Pd(111); A study by photoemission and auger spectroscopy

Abstract The temperature dependence of the different binding states of oxygen on Pd(111) has been studied using photoemission spectroscopy and Auger spectroscopy. By examining the oxygen 2s binding energies, three distinct states are observed, which are identified as surface chemisorbed oxygen, oxygen incorporated into the first Pd layer, and subsurface oxygen. Changes in the relative oxygen Auger peak heights for 1000 K exposure temperature are explained if the oxygen is assumed to become more negatively charged as it penetrates the Pd surface. A model for the interaction of oxygen with the Pd(111) surface is proposed.

Oscillations in the compositional depth profile of Cu/Ni alloys: A study by UPS

Abstract The composition in the surface region of a (110) oriented single crystal of Cu/Ni alloy was studied by ultraviolet photoemission spectroscopy (UPS) in the energy range hv = 32–240 eV. The data indicate a surface enrichment of Cu and that the composition, as a function of depth below the surface, does not approach the bulk value in a monotonic manner but shows at least one oscillation. The depth sampled by the UPS experiment was varied by changing the photon energy and thus the escape length of the photoemitted electrons. Two surfaces of a single bulk crystal (bulk atomic composition 90% Ni and 10% Cu) were studied. One was the high temperature equilibrium surface (AES measurements yielded a surface composition of ~65% Cu and 35% Ni); the second surface was higher in Ni concentration (AES measurements gave ~65% Ni and 35% Cu) and was obtained by annealing at a lower temperature. Theories based on the quasi-chemical and regular solution models predict a surface enrichment of Cu for the alloy, but do not predict any oscillation in composition below the surface.

An angle-resolved photoemission study of the chemisorption of chalcogens on Cu(100) III. Cu(100) + p(2 x 2)S

Abstract Starting with the three-step direct-transition model of ARPES for bulk materials, which was examined in the preceding paper, we propose a framework for describing changes in the photoemission spectra due to chemisorption. Normal emission ARPES data for Cu(100) with a c(2 × 2)O overlayer were obtained in the photon energy range hv = 11 to 34 eV. These spectra have been compared within the proposed framework with those obtained from clean Cu(100). Changes were found in the Cu emission features which could be explained by the relaxation of momentum conservation perpendicular to the surface in the optical excitation step and by the relaxation of momentum conservation parallel to the surface in the escape step. These changes include a photon energy dependent broadening of the d-band peak and the preferential attenuation of the sharp direct-transition feature associated with the sp-band. Some evidence for a surface resonance at the top of the d-bands has been obtained. Changes in the spectrum of scattered electrons were related to modifications of evanescent final states. A 1.3 eV wide band derived from the oxygen p x,y -orbitals was deduced from spectra obtained at normal emission and along the Γ X and Γ M lines of the surface Brillouin zone. On the other hand, no emission was clearly detected from the oxygen p z -orbitals. Oxygen induced emission above the Cu d-bands was observed and attributed to antibonding states. This emission was directed towards the bulk [011] directions.

Photoemission studies of W(100)-(5 × 1)C, 65 ⩽ hν ⩽ 200 eV and a comparison with WC(0001)

A photoemission study of the W(100)-(5 × 1)C carburized tungsten surface, performed in the photon energy range 65 ⩽ hν ⩽ 200 eV, is reported. Comparison with W(100) and WC(0001) spectra permit detailing of changes in electronic structure upon carburization, as well as identification of primarily C 2s, C 2p, and W 5d-derived regions of the valence band. A separate band forms at −12.2 eV of states essentially pure C 2s in character. States in the upper valence band, particularly near EF, have more W 5d character, while those in the lower valence band have more C 2p character. Structures are altered within the valence band upon carburization, including a significant decrease in emission near EF. Detailed comparison with WC(0001) spectra reveal many similarities: similar binding energies of structures, similar C 2s band, and similar variation of normalized valence band area with hν.

CO chemisorption on PtCu surfaces: I. CO on (1×3) Pt0.98Cu0.02(110)

Abstract Thermal desorption and photoemission spectroscopy (PES) have been used to investigate the chemisorption of CO on an annealed Pt 0.98 Cu 0.02 (110) surface. The clean surface shows 9.1 ± 2.6% Cu within the top 4 A, and is (1 × 3) reconstructed. Thermal desorption of CO has revealed the existence of various adsorption states with these respective heats of adsorption: (α) 35.2 to 37.8 kcal/mol and (β) 24.5 to 26.3 kcal/mol on Pt sites, (γ) 16.0 to 17.2 kcal/mol on PtCu “mixed” sited, and (δ) 12.9 to 13.9 kcal/mol on Cu sites. PES observation of Cu 3d-derived states (using hv = 150 eV) and the Cu 2p 3 2 core levels (using Mg Kα radiation) shows that the electronic structure of the Cu constituent is changed only when CO adsorbs on the Pt-Cu “mixed” sites or the Cu sites. Furthermore, the CO states associated with Pt sites reflect the structural difference between the (1 × 3) alloy surface and the (1 × 2) pure Pt(110) surface: α-CO on the alloy surface desorbs at a temperature 17 to 21 K. higher than the maximum desorption temperature of CO from pure Pt(110), and the ratio of β-CO to α-CO desorption from the alloy surface is larger than the ratio of low temperature to high temperature peaks in the desorption of CO from pure Pt(110).

Chemisorption-induced Pt 4f surface core level shifts

Soft X-ray photoemission experiments have led to the unambiguous observation of a metal surface core level (Pt 4f72) shift, due to an adsorbate (CO), to a binding energy larger than the bulk binding energy. The 4f72 clean (110) surface component, with a binding energy 0.35 ± 0.02 eV lower than the bulk, is shifted by 1.06 ± 0.04 eV towards higher binding energy upon CO chemisorption. The lack of significant changes in the bulk component indicates the localized nature of the CO-Pt surface bonding.

Surface segregation and surface electronic interactions in PtCu

Photoemission and Auger electron spectroscopy on Pt0.98Cu0.02 show that the (110) face has over twice as much Cu surface segregation as the (111) face. The Cu 3d-derived surface “density of states” differ strikingly in peak shape and in width (by 0.5 eV). The centroids, compared with bulk Cu d states, are shifted by more than 0.3 eV towards the Fermi level. This is the first experimental correlation between surface segregation and surface bonding.

Photoemission studies of clean and oxidized Nb and Nb3Sn

In conjunction with Auger electron spectroscopy measurements, we have developed a procedure for obtaining a clean Nb surface using an electron bombardment heating technique. We have then studied the valence band photoemission as a function of photon energy. We find that calculations for the bulk theoretical density of states are in basic agreement with the ultraviolet photoemission spectroscopy data. We have also studied the initial oxidation of the clean Nb surface. Structure in the energy distribution curves suggests three stages in the oxidation process. Furthermore, the valence band energy distribution curves for clean Nb3Sn prepared ’’in situ’’ were measured and showed a strong resemblance to pure Nb as has been observed previously in x‐ray photoemission. Evidence is found for the existence of Nb2O5 and SnO2 in the surface oxides of Nb3Sn.

Co chemisorption on PtCu surfaces III. CO on PtCu(111)

Abstract Selected thermal desorption and valence band photoemission data on the chemisorption of CO on PtCu(111) surfaces are presented. The main objective is to make a comparison with CO chemisorption on an annealed (1 × 3) reconstructed Pt0.98Cu0.02(110) surface. The (111) alloy surfaces are unreconstructed (1 × 1) surfaces, with average near-surface Cu concentrations ranging from ≳ 7.5% to ≳ 20% as indicated by the Cu 920 eV Auger signal. It is observed that the effect of alloying Pt(111) with Cu is to progressively lower the desorption peak temperature and hence the free energy of CO desorption from Pt sites. A second observation is that the energy distribution of the Cu 3d-derived states is little affected by CO adsorption on Cu sites at 155 K. Both these results offer a contrast to the results for CO/Pt0.98Cu0.02(110) reported earlier.

Magnetic behaviour of ultra-thin iron overlayers on palladium (111)

Abstract A LEED, Auger photoemission study of ultra-thin overlayers of iron on palladium (111) is reported. At room temperature and below, the overlayer grows epitaxially in a simultaneous multi-layer mode. On annealing a sub-monolayer coverage to create flat two-dimensional platelets it was found that the density of the Fe 3d states at the Fermi level was reduced and the local iron magnetic moment could not be detected.