R Gotter

Electronic states of CuPc chains on the Au(110) surface

The electronic properties of Cu-phthalocyanine (CuPc) molecules flat lying along the channels of the Au(110) reconstructed surface have been investigated by means of ultraviolet and x-ray photoelectron spectroscopy. The ordered chains give rise to a highly ordered single-layer structure with a (5x3) symmetry. Although from the core-level analysis not any significant charge transfer between the molecules and the underlying Au surface is observed, the valence band photoemission data bring to light CuPc-induced features localized at the interface. In particular, energy versus momentum dispersion of an interface state reveals a bandwidth of about 90 meV along the enlarged Au channels, where the CuPc chains are formed, with a defined fivefold symmetry well fitting the CuPc intermolecular distance.

The ALOISA end station at Elettra: a novel multicoincidence spectrometer for angle resolved APECS

Abstract Coincidence measurements have been extensively performed in atomic and molecular physics since early 1970s. To apply this methodology to solids and surfaces has been a major target since early days, but the long average time needed to complete a coincidence experiment has hampered its attainment. In particular the coincidence technique has not been yet applied in an angle resolved way such for studying the momentum correlation in the ejection of electron pairs from solid surfaces. The experimental chamber at the ALOISA beamline at Elettra, by means of a set of seven homemade electron analyzers, is the first apparatus able to perform Angle Resolved – Auger Photoelectron Coincidence Spectroscopy (AR-APECS) from solid surfaces. In the typical setup ten different pairs of coincident electrons can be measured simultaneously, so reducing the acquisition time by one order of magnitude.

A Procedure for the Isolation of Asbestos Bodies from Lung Tissue by Exploiting their Magnetic Properties: A New Approach to Asbestos Body Study

The role of asbestos bodies (and associated proteinacious coating) in asbestos associated diseases is not well understood. Currently employed methods of isolation of these bodies employ harsh chemicals that lead to destruction of their proteinacious coating. In this work a method was developed that enabled the purification of whole, integral, unmodified asbestos bodies (AB) by exploiting their magnetic properties. Albumin and ferritin were found to be the major proteins associated with AB isolated from lung tissue of mesothelioma patients. Magnetically isolated AB were shown to be cytotoxic and to activate free radical production from inflammatory cells at a higher extent than that induced by bodies obtained by chemical digestion. The finding that hypochlorite-treated AB induce DNA damage, while AB obtained by the method described in this article failed to do so, together with the differential behavior of these bodies toward inflammatory cells, suggests that native asbestos bodies should be used to investigate the pathogenetic role of these structures.

Electron–electron coincidence spectroscopies at surfaces

Abstract In the past 20 years, a steadily increasing number of electron–electron coincidence experiments on atoms and molecules have contributed to a deeper understanding of electron–electron correlation effects. In more recent years this technique has been extended to the study of solid surfaces. This class of one photon IN two electrons OUT experiments will be discussed with an emphasis on grazing incidence geometry, that is expected to be particularly suited for studying surfaces. The crucial question of which is the dominant mechanism that leads to ejection of pairs of electron from the surface will be addressed. It will be shown that, depending on the kinematics chosen, the correlated behaviour of the pairs of electrons detected might be singled out from independent particle one.

First results from the new optical configuration for a synchrotron radiation monochromator applied to the ALOISA beamline

The new beamline ALOISA is now working at the Elettra synchrotron facility. It is equipped with a monochromator of new design to cover the 200 eV - 8 KeV energy range: the monochromator has two interchangeable dispersive systems, one for the low energies up to 2 KeV using reflection gratings and another with crystals for the higher energies. The monochromatic beam is finally re-focused from the fixed exit slit to inside the experimental chamber by a toroidal mirror. In this paper the results of the first resolution measurements are reported for the low energy section of the monochromator: a resolving power higher than 5000, with peaks of 10000 in the 400 - 500 eV region, is obtained while using all the optical elements at full illumination.

Study of the isotropic contribution to the analysis of photoelectron diffraction experiments at the ALOISA beamline

Abstract The angular distribution of the intensity in photoemission experiments is affected by electron diffraction patterns and by a smoothly varying contribution originated by both intrumental details and physical properties of the samples. The peculiar variety of scattering configurations available at the ALOISA beamline experimental station in Trieste stimulated the developement of an analytical description for the smooth angular dependence sustaining the diffraction features. We present here the basic formulae and their application to experimental data taken on the Fe/Cu 3 Au(001) system in order to highlight the role of the various parameters included in the distribution function. A specific model for the surface illumination has been developed, and the overlayer thickness and surface roughness have been considered.

Evidence for the collapse of short-range magnetic order in CoO at the Néel temperature

Auger-photoelectron coincidence spectroscopy is used for investigating the electronic properties of a CoO thin film above and below the magnetic transition temperature (TN). By using the dichroic effect in angle-resolved measurements, we identify and assign well-defined high-spin and low-spin structures in spite of the otherwise featureless Auger singles spectra, typically found for open-band systems. The disappearance of the dichroism for temperatures just above TN indicates a collapse of the surface short-range magnetic order, presumably due to a strongly reduced exchange field in the surface compared to that in the bulk.

EPITAXY OF ULTRATHIN CoO FILMS STUDIED BY XPD AND GIXRD

CoO layers have been grown by exposing to oxygen the (001) body-centered-tetragonal (bct) surface of a Co ultrathin film epitaxially grown on Fe(001). Different oxide thicknesses in the 2–15 ML range have been investigated by means of synchrotron-radiation-based techniques. X-ray photoelectron spectroscopy has been used to check the formation of the oxide films; X ray photoelectron diffraction has given information concerning the symmetry of their unit cell; grazing incidence X-ray diffraction has allowed to evaluate precisely their in-plane lattice constant. The films show a CoO(001) rocksalt structure, rotated by 45° with respect to the bct Co substrate, with the [100] direction parallel to the substrate [110] direction. Their in-plane lattice constant increases as a function of thickness, to release the in-plane strain due to the 3% mismatch between the bulk CoO phase and the underlying substrate.

PHOTOELECTRON DIFFRACTION STUDY OF THE(3×3)-Sn/Ge(111)STRUCTURE

The photoemission spectra of the Sn 4d electrons from the (3×3)-Sn/Ge(111) surface present two components which are attributed to inequivalent Sn atoms in T4 bonding sites. This structure has been explored by photoelectron diffraction experiments performed at the ALOISA beamline of the Elettra storage ring in Trieste (Italy). The modulation of the intensities of the two Sn components, caused by the backscattering of the underneath Ge atoms, has been measured as a function of the emission angle at fixed kinetic energies and vice versa. The bond angle between Sn and its nearest neighbor atoms in the first Ge layer (Sn–Ge1) has been measured by taking polar scans along the main symmetry directions and it was found almost equivalent for the two components. The corresponding bond lengths are also quite similar, as obtained by studying the dependence on the photoelectron kinetic energy with the photon polarization and the collection direction parallel to the Sn–Ge1 bond orientation (bond emission). A clear difference between the two bonding sites is observed when studying the energy dependence at normal emission, where the sensitivity to the Sn height above the Ge atom in the second layer is enhanced. The (3×3)-Sn/Ge(111) is thus characterized by a structure where the Sn atom and its three nearest neighbor Ge atoms form a rather rigid unit that presents a strong vertical distortion with respect to the underneath atom of the second Ge layer.

Insight on Hole-Hole Interaction and Magnetic Order from Dichroic Auger-Photoelectron Coincidence Spectra

The absence of sharp structures in the Auger line shapes of partially filled bands has severely limited the use of electron spectroscopy in magnetic crystals and other correlated materials. By a novel interplay of experimental and theoretical techniques we achieve a combined understanding of the photoelectron, Auger, and Auger-photoelectron coincidence spectra (APECS) of the antiferromagnetic CoO. A recently discovered dichroic effect in angle resolved (DEAR) APECS reveals a complex pattern in the Auger line shape, which is here explained in detail, labeling the final states by their total spin. Since the dichroic effect exists in the antiferromagnetic state but vanishes at the Néel temperature, the DEAR-APECS technique detects the phase transition from its local effects, thus providing a unique tool to observe and understand magnetic correlations where the usual methods are not applicable.