B. P. Tonner

Structure and growth mode of metastable fcc cobalt ultrathin films on Cu(001) as determined by angle-resolved x-ray photoemission scattering

Abstract The structure and growth mode of metastable fcc Co(001) films on Cu(001) substrates were determined by using angle-resolved X-ray photoemission scattering (ARXPS). The results show that at room temperature on Cu(001), cobalt grows with a small but measureable deviation from ideal layer-by-layer mode, with the formation of two-layer thick islands for average coverages below two monolayers (ML). When the coverage exceeds two monolayers, cobalt grows to a well-ordered fcc-(001) film via a layer-by-layer growth mode. The combination of low-energy electron diffraction (LEED) and ARXPS as the structural probes shows that high quality fcc films of Co of up to 20 ML thickness can be stabilized at room-temperature by epitaxial growth. Upon heating to 400° C, an unusual structure is formed in which substrate Cu atoms diffuse through the cobalt film to form a flat layer on top of an ordered fcc cobalt film.

XANES spectra of a variety of widely used organic polymers at the C K-edge

Abstract The total yield XANES spectra of 13 commonly used organic polymers have been obtained at the C K-edge using synchrotron radiation, for use as a reference source for studies on polymers. Thin films of polymers made by spin casting have been used as samples in order to minimize radiation damage and charging. Spectral features have been successfully assigned using the building block and bond length correlation concepts and by analogy with the assignment of small molecules with similar chemical structures.

Structural effects in single-crystal photoelectron, Auger-electron, and Kikuchi-electron angular diffraction patterns

Abstract The full-hemisphere diffraction patterns of primary photoelectrons, photoemission Auger electrons, and Kikuchi electrons are reported for single-crystal surfaces of Cu(100), Cu(111), Cu(110), Ir(111), and Ag(100), to test models for direct structure determinations from angle-dependent final-state diffraction patterns. Our measurements show a simple correlation between the low-index crystallographic directions of the substrate and local intensity maxima in the electron angular distributions. We find that the angular anisotropy can be qualitatively explained in all cases studied by final-state elastic forward scattering. The strong forward scattering features in photoemission diffraction patterns are used to measure bond angles and determine the structure of Cu ultrathin films on Ir(111). In addition, a radial image function based on a holographic Fourier-transform algorithm is evaluated for the determination of bond lengths from three-dimensional images reconstructed from the two-dimensional diffraction pattern.

MEPHISTO spectromicroscope reaches 20 nm lateral resolution

The recently described tests of the synchrotron imaging photoelectron spectromicroscope MEPHISTO (Microscope a Emission de PHotoelectrons par Illumination Synchrotronique de Type Onduleur) were complemented by further resolution improvements and tests, which brought the lateral resolution down to 20 nm. Images and line plot profiles demonstrate such performance.

A photoemission microscope with a hemispherical capacitor energy filter

Abstract A purely electrostatic photoemission electron microscope with a bandpass energy filter is described. The electron optics are of a hybrid design, incorporating a high-voltage and high-magnification objective and intermediate lens, coupled to a low-voltage hemispherical capacitor configured as an achromatic imaging bandpass filter. The electron optical system design is described, along with initial performance tests. The instrument is used for both imaging X-ray absorption spectroscopy and photoemission spectroscopy.

MEPHISTO: Performance tests of a novel synchrotron imaging photoelectron spectromicroscope

We discuss the scheme and test performances of this recently commissioned system in its final configuration. The tests show that the improvements in the electron optics system with respect to other instruments in the same class made it possible to reach lateral resolutions in the 50 nm range. They also demonstrate rather good spectromicroscopy and spectroscopy performances, reliability and flexibility of operation.

X‐ray absorption near edge structures of intermediate oxidation states of silicon in silicon oxides during thermal desorption

X‐ray absorption near edge structure (XANES) spectra are measured from silicon oxides using a photoemission microscope. Native oxide overlayers on Si(100) and Si(111) crystals are studied at various stages of the thermal desorption process. Because of the very high energy resolution inherent in the microscope, we are able to resolve for the first time the spin–orbit splitting of the core exciton structure in SiO2. The spectroscopic signature of this transition is unique, and it has enabled us to identify the core exciton XANES structures of two intermediate oxidation states of silicon (Si2+ and Si3+). We have found the Si2+ and Si3+ states existing at the Si–SiO2 interface. We have also found, for the first time, that Si2+ can exist independently on the silicon surface apart from any SiO2.X‐ray absorption near edge structure (XANES) spectra are measured from silicon oxides using a photoemission microscope. Native oxide overlayers on Si(100) and Si(111) crystals are studied at various stages of the thermal desorption process. Because of the very high energy resolution inherent in the microscope, we are able to resolve for the first time the spin–orbit splitting of the core exciton structure in SiO2. The spectroscopic signature of this transition is unique, and it has enabled us to identify the core exciton XANES structures of two intermediate oxidation states of silicon (Si2+ and Si3+). We have found the Si2+ and Si3+ states existing at the Si–SiO2 interface. We have also found, for the first time, that Si2+ can exist independently on the silicon surface apart from any SiO2.

Magnetic phases of ultrathin Fe films on fcc Co(001)

Abstract A sequence of three distinct magnetic phases of epitaxial ultrathin films of Fe grown on fcc Co(100) surfaces have been identified using low-energy electron diffraction (LEED) and X-ray magnetic circular dichroism (XMCD). The magnetic phases are correlated with changes in film structure that produce different surface reconstructions, which are similar to those found for epitaxial Fe ultrathin films grown on Cu(100) substrates. The room temperature magnetic phase diagram has been determined using X-ray dichroism magnetometry. For Fe coverages below 5 ML the films crystallize in an fct phase and are ferromagnetic. Between 5 and 11 ML the Fe grows in an fcc phase which is nonmagnetic at room temperature, but has a magnetic live monolayer at the Co Fe interface. The magnetic and compositional interfaces do not coincide. Above 11 ML the Fe films grow in the ferromagnetic bcc phase.

Epitaxy of fcc and bcc Co, Ni, and Cu studied by X-ray photoelectron diffraction

The structures of epitaxial transition-metal films with thicknesses of only a few monolayers were determined by X-ray photoemission diffraction (XPD) techniques. Ni ultrathin films were studied on two Cu single-crystal surfaces, and Cu/Ni multilayers (or “sandwich” structures) were also grown. On Cu(100), the initial Ni epitaxy is found to result in expansion of the Ni lattice constant to achieve lateral coherence with the substrate, along with a reduction in the Ni interplanar spacing perpendicular to the substrate, approximately maintaining a fixed Ni-Ni nearest-neighbor distance. This structure is a body-centered tetragonal distortion of the normal fcc(100) lattice. The behavior on Cu(111) is different, in that the epitaxial Ni films retain the c/a axis relationship of bulk Ni(111). Films of Co, Ni, and Cu, which are normally fcc or hcp crystals at room temperature, were grown on single-crystal Fe(100) substrates to induce bcc ordering. Both Co and Ni grow as bcc(100) films on iron, but the Cu films are disordered. This is in contrast to epitaxy of Cu on strained layer Fe(100) films, which do support the growth of bcc Cu(100). Modelling of ultrathin film XPD using multiple-scattering calculations was used for bond-length determinations with high accuracy, and for a determination of growth modes.

Microanalysis surface studies and photoemission properties of CsI photocathodes

We present recent results of the study of surface properties and quantum efficiency (QE) of CsI photocathodes prepared on various substrates. Microanalysis methods provide laterally resolved surface morphology and chemical composition of the photoemissive film. Integral measurements of the QE of CsI were done with a monochromator system and a RICH device. It was shown that CsI films deposited on large area Ni- or Ni-Au-coated printed circuit electrodes have a uniform crystalline structure and an average QE close to that reached on polished stainless steel. The films have a good stability in air over periods of Ih. On a microscopic scale of 3-30 mu m, the films exhibit nonuniform emission properties correlated with variations in the chemical composition.