[en] VUV photoemission spectra of the valence bands of the high-T/sub c/ superconductor YBa₂Cu₃O/sub 9-δ/ δ = 2.1, have been measured using the UV undulator (beam line U5U). Photon energies h nu = 45.0 eV and 52.0 eV lie within the first harmonic flux output from the undulator, while h nu = 105.0 lies within the second harmonic. the spectra at h nu = 21.2 eV and 40.8 were taken using a He resonance lamp

[en] The effect of water on the high-temperature superconductor YBa₂Cu₃O₇ has been studied. Photoemission measurements show that water modifies valence-band states, reduces the number of inequivalent Ba sites and induces O 1s corelevel components at 534 and 531 eV. The 534-eV core-level component is coincident with that of cryosorbed ice and the enhanced 531-eV component is due to a new water-induced bonding environment in the superconductor;this is most probably a hydroxyl species. Changes in the Ba core-level appearance suggest that water may be preferentially modifying YBa₂CuO₇ in a way that affects the Ba atomic environment. The effect of water on superconductivity and its possible implications for bulk superconducting transport in ceramics is discussed. Previous reports suggesting oxygen dimerization below T/sub c/, as well as other reported changes in the spectrum, are improbable in view of these present results

[en] A change in the electronic structure of ammonia when Cs is added is identified using combined soft x-ray absorption and photoemission measurements. Depositing Cs into an NH₃ film retained at 80 K causes the formation of new valence band features at -4.8 or -5.5 eV binding energy and a reduction in the intensity of the N K-edge feature (related to Rydberg transitions involving the unoccupied 4a₁ and 2e NH₃ orbitals). A shift of the Cs 4d core level -2.8 eV to a greater binding energy suggests the transfer of charge from Cs to NH_3. When ammonia is absorbed on a cesium film, similar changes are seen in the NH₃ valence band photoemission and near-edge features

[en] The interaction of solid molecular oxygen with Li, Cs, K, La, Ag, Cu, and Ba has been studied at 35 K or below using photoemission. A feature near 535 eV in the O 1s core-level spectra was observed when Li, Cs, K, and La were deposited on solid oxygen. This feature was identified with one electron being donated to an oxygen molecule, i.e., the superoxide species, which as far as we know has not been previously reported for La and Li. A feature at about 531.5--533 eV was identified as a peroxide species where two electrons were donated to an oxygen molecule. Finally, features at about 528--530.5 eV were identified as oxide phases where the molecular oxygen was dissociated into atomic O with formal oxidation state of -2. These identifications are crucial in the determinations of the exotic features in the x-ray photoelectron spectroscopy (XPS) O 1s spectra of the high T_c superconductors

[en] Valence-band photoemission measurements have been made both above and below the T/sub c/ of Ba₂YaCu₃O/sub 9-//sub δ/ where δ = 2.1. We find similarities to recent calculations of the electronic properties. However, the experimental binding energies are between 1.0 and 1.4 eV further from the Fermi level than the calculation. We suggest that this difference may affect attempts to predict T/sub c/ from tight-binding modeling of the electron-phonon mechanism

[en] Photoemission is used to examine valence electronic structure of Yb deposited on condensed ammonia or xenon films. The deposition of Yb onto solid ammonia produces a Yb phase whose two 4f peaks lie at a higher binding energy than Yb metal and also produces a feature near the Fermi level. The peak near E_f is not seen when Yb is isolated in xenon. From the appearance of the Yb 4f levels and the existence of a peak near E_f, it is concluded that the Yb is divalent and that (1) the peak near E_f is a shakedown satellite derived from the 4f states or (2) the peak near E_f is due to the hybridization of the Yb 4f states with other Yb or ligand states

[en] The work function changes for ammonia absorbed on Au, Ta and Yb have been measured at 80 K. With a layer of Cs deposited on the top of solid ammonia the work function became lower than that of Cs. By co-deposition of M (M = Cs, Na, Yb) with ammonia, a minimum work function of 0.9 (±0.1) eV was obtained which is independent on the feature of the metals as well as the substrate. The theoretical solution of 'large polaron' was used to calculate the frequency of the polarization field and the coupling constant

[en] Photoemission experiments were performed to investigate the properties of Na on solid ammonia. At low coverages of Na on solid NH₃ a nonmetallic feature of the Na 2p core level is seen at 33 eV binding energy, which is 2.4 eV deeper than that of the metallic Na 2p core level. This higher-binding-energy features is thought to be due to solvated Na atoms in the NH₃ matrix. At higher coverages it appears that a metallic phase of the Na atoms occurs, along with the appearance of a plasmon loss feature. It was found that the metal-nonmetal transition is associated with these two Na core-level features, coexisting at intermediate Na coverages. This regime is consistent with a phase-separation regime where metallic clusters coexist with a dilute phase which has Na solvated in the NH₃. Another fascinating feature of this Na--solid-NH₃ system is a work-function value as low as 0.9 eV when Na and NH₃ are codeposited at 80 K

[en] Superconducting Tl-Ca-Ba-Cu-O thin films with T_c onsets of 115 K and T_c (R=0) of 95 K have been prepared by reactive magnetron sputtering using Tl, Cu and Ca/Ba metal targets. It was found that proper thallium content a is crucial for obtaining a high transition temperature. Wet oxygen and sealed gold tube with additional thallium compounds were used to reduce the loss of thallium during annealing. X-ray diffraction spectra show that films with the sharpest transition at 95 K are predominantly c-axis oriented. XANES also shows a preferred c-axis orientation for the superconducting films while for a nonsuperconducting film the near edge structure suggests greater disorder. X-ray microprobe fluorescence measurements indicate that these films are close to the 2122 stoichiometry. Scanning electron microscopy on these films is also presented

[en] X-ray photoemission and synchrotron-radiation photoemission were used to study the reaction of metals and molecular oxygen at low temperatures. Combined core-level and valence-band measurements permit the identification of various oxygen species. When Li, Cs, K, and La are deposited on solid oxygen, an O 1s feature near 535 eV is observed and attributed to the superoxide species. Features ranging from 531.5 to 533 eV are also observed and identified as the peroxide species. In contrast to the case of Li and of most other metal oxides, for Cs or K ''oxides'' an O 1s feature is seen at 528 eV and attributed to the O^2- species in a metallic matrix