D.T. Ling

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.

UPS and TDS studies of the adsorption of CO and H2 on Cu/Ni

The adsorption of CO and H2 on a Cu/Ni (110) surface has been studied utilizing a combination of techniques including ultraviolet photoemission spectroscopy (UPS), thermal desorption spectroscopy (TDS), Auger electron spectroscopy (AES), and low energy electron diffraction (LEED). UPS is found to be a useful chemical probe to study ensemble and ligand effects in chemisorption. The techniques of chemical shift analysis utilized in X-ray photoemission spectroscopy (XPS) are extended to the valence band region and used to detect changes in the chemical nature of the binding site. As the adsorption site for CO changes from principally Ni sites to a mixture of Cu, Cu/Ni, and Ni sites, the CO emission features are found to shift by ~0.8 eV. This is detected by comparing UPS spectra taken for CO chemisorbed at room temperature and at 160 K, and at various CO coverages, and by relating these spectra to TDS data. This is a direct reflection of the ensemble effect in chemisorption. A ligand effect was observed for CO adsorbed at Cu sites by comparing UPS spectra for CO on pure Cu and CO adsorbed at Cu sites on the alloy. The CO emission features for CO adsorbed at Cu sites on the alloy are shifted by ~0.6 eV compared to CO on pure Cu. Strong adsorbate-adsorbate interactions were observed in both the UPS and TDS data when hydrogen and carbon monoxide were coadsorbed. Preadsorbed hydrogen apparently blocks a number of CO adsorption sites, in particular, certain Ni and Cu/Ni sites. Because of this, CO is adsorbed at Cu sites at much lower CO exposures when H2 is first preadsorbed.

Studies of Surface Electronic Structure and Surface Chemistry Using Synchrotron Radiation

Photoemission studies for hv < 400 eV are used to illustrate some of the advantages of synchrotron radiation for studying the surface electronic structure and chemistry of solids. Studies of GaAs and Pt are used to illustrate the following advantages (all of which depend on a source of radiation with a continuously tunable photon energy): (1) both valence and core states can be examined, (2) photon energies can be chosen so that the electron escape length can be minimized, restricting the studies to the first several atomic or molecular layers of the solids, (3) since the escape depth minimum is rather shallow the matrix element for excitation from a given set of levels may be maximized consistent with probing the last few layers, (4) when two sets of levels are degenerate or near degenerate in energy, e.g. the solid valence levels and the orbits of an adsorbed gas, the photon energy dependence of the matrix element can be used to separate the photoemission from the two states, and (5) good energy resolution (often 0.2 eV or better) can be obtained. For GaAs, studies of the surface valence electronic structure (hv ≈ 21 eV) in conjunction with LEED results indicated that rearrangement of the atoms within the last unit cell of the surface plays an important role in determining the electronic structure associated with this last layer of the crystal. In fact, it appears that measurement of this electronic structure may prove to be one of the most sensitive ways of determining the details of the atomic rearrangement. Studies of the Ga and As 3d core shifts on chemisorption of oxygen indicate that electrons are transferred from the As to the oxygen. The next step in oxidation, the formation of true As and Ga oxides, has also been followed using the core shifts. CO adsorbed on metals has been studied extensively previously using photoemission methods; however, neither the CO 3σ levels nor the shake up structure has been clearly seen in the past due to background produced by the strong Pt 5d valence photoemission. Making use of the Cooper minimum in the 5d excitation cross section at hv = 150 eV, the CO 3σ and shake up structures have been clearly seen for the first time.

Angle‐resolved photoemission studies of oxide formation on Cu(100)

We performed ARPES studies of the oxidation of Cu(100). The results are in agreement with a model where oxygen chemisorption above the surface is followed by incorporation into the bulk and finally copper (I) oxide formation. The spectra obtained for Cu2O show little angular anisotropy. They are in good agreement with earlier angle‐integrated measurements of Evans using a polycrystalline sample and ARPES measurements of Lloyd, Quinn, and Richardson also using Cu(100). However, the latter authors appear to have misplaced the Fermi level in their spectra.

AbstractPhotoemission studies of clean and oxygen-covered Pt 6(111) × (100)

Abstract Employing the enhanced sensitivity obtained by using synchrotron radiation near the Cooper minimum for the 5d valence electrons, we have located the oxygen 2p and 2s levels for oxygen chemisorbed on a Pt 6(111) × (100) crystal. We find the oxygen 2p level located −6 eV with a FWHM of 3 eV and the 2s at −21.6 eV. A factor of four difference in saturation coverage is measured between temperatures of 300 and 120 K, but the position and width of the 2p level is independent of temperature. We observe also the 1b1 orbital of weakly adsorbed H2O molecules, which has pure O 2p parentage; from the intensity of this orbital, we are able to suggest why it is difficult to observe the oxygen 2p signal at low photon energies. In addition, we note a strong preferential attenuation in the Pt states near Ef for the adsorbed H2O in spite of the weak nature of the bond.