Stefano Nannarone

The BEAR Beamline at Elettra

The BEAR (Bending Magnet for Emission Absorption and Reflectivity) beamline is installed at the right exit of the 8.1 bending magnet at ELETTRA. The beamline — in operation since January 2003 — delivers linear and circularly polarized radiation in the 5 – 1600 eV energy range. The experimental station is composed of a UHV chamber for reflectivity, absorption, fluorescence and angle resolved photoemission measurements and a UHV chamber for in‐situ sample preparation.

Deep ultraviolet plasmon resonance in aluminum nanoparticle arrays.

Small aluminum nanoparticles have the potential to exhibit localized surface plasmon resonances in the deep ultraviolet region of the electromagnetic spectrum, however technical and scientific challenges make it difficult to attain this limit. We report the fabrication of arrays of Al/Al2O3 core/shell nanoparticles with a metallic-core diameter between 12 and 25 nm that display sharp plasmonic resonances at very high energies, up to 5.8 eV (down to λ = 215 nm). The arrays were fabricated by means of a straightforward self-organization approach. The experimental spectra were compared with theoretical calculations that allow the correlation of each feature to the corresponding plasmon modes.

The UHV Experimental Chamber For Optical Measurements (Reflectivity and Absorption) and Angle Resolved Photoemission of the BEAR Beamline at ELETTRA

The experimental station of the BEAR (Bending magnet for Emission, Absorption and Reflectivity) beamline at ELETTRA (Trieste, Italy) is an UHV chamber conceived to fully exploit the spectroscopic possibilities offered by the light spot produced by the beamline. Spectroscopies include reflectivity (θ‐2θ and diffuse), optical absorption, fluorescence and angle resolved photoemission. The chamber can be rotated around the beam axis to select the s (TE) or p (TM) incidence conditions and/or the position of the ellipse of polarization with respect to the sample. Photon detectors (e.g. photodiodes) and electron detector (hemispherical analyzer — 1° angular resolution, 20 meV energy resolution) cover about completely the full 2π solid angle above the sample surface in any light incidence condition.

UPS, XPS, and NEXAFS Study of Self-Assembly of Standing 1,4-Benzenedimethanethiol SAMs on Gold

We report a study of the self-assembly of 1,4-benzenedimethanethiol monolayers on gold formed in n-hexane solution held at 60 °C for 30 min and in dark conditions. The valence band characteristics, the thickness of the layer, and the orientation of the molecules were analyzed at a synchrotron using high resolution photoelectron spectroscopy and near edge X-ray adsorption spectroscopy. These measurements unambiguously attest the formation of a single layer with molecules arranged in the upright position and presenting a free -SH group at the outer interface. Near edge X-ray absorption fine structure (NEXAFS) measurements suggest that the molecular axis is oriented at 24° with respect to the surface normal. In addition, valence band features could be successfully associated to specific molecular orbital contributions thanks to the comparison with theoretically calculated density of states projected on the different molecular units.

Gratings in a conical diffraction mounting for an extreme-ultraviolet time-delay-compensated monochromator

The conical diffraction mounting in which the direction of incident light belongs to a plane parallel to the direction of the grooves has the unique property of maintaining high diffraction efficiency, even in the extreme-ultraviolet (EUV) region. This property is useful for designing high-throughput time-delay-compensated monochromators for the spectral selection of ultrashort EUV pulses as the high-order harmonics generated by the interaction between an ultrashort laser pulse and a gas jet. The time compensation allows one to exploit the femtosecond scale duration of the harmonics both to have high intensity and to reach an unprecedented temporal resolution for pump and probe experiments. Because two gratings have to be used for time compensation, the high diffraction efficiency becomes an essential requirement, which can be fulfilled by the conical diffraction mounting. Measurements recently accomplished at the Bending Magnet for Emission Absorption and Reflectivity (BEAR) beam line (ELETTRA Synchrotron, Trieste, Italy) for three gratings in the 10-90 nm region are reported here that show a peak efficiency of as much as 0.7 in the first order. A model computing the electromagnetic propagation and the grating efficiency, implemented and tested with the experimental data, permits the study and design of rather complex systems operating in the conical mounting. Basic physical principles and mathematical aspects of the model are discussed here.

Monochromator for the synchrotron radiation beamline X-MOSS at ELETTRA

The optical configuration of the monochromator for the new beamline X-MOSS at the ELETTRA synchrotron ring is described. The requirements for this instrument are to collect a 3 mrad X 2 mrad aperture beam produced by a bending magnet in the 3 - 1400 eV energy range; the energy resolution has to be 3000 or better over the whole range, with a focused beam of the order of 10 - 50 micrometer. The designed monochromator, presently under construction, is a slitless four grazing incidence optical elements: the first element is a one-meter paraboloidal mirror in sagittal focusing, then there is a plane mirror-plane grating dispersion system and finally a second one-meter paraboloidal mirror, also used in sagittal focusing. The latter focuses the radiation on the monochromator exit slit. This monochromator design is not limited by a defined working curve: in this way it is possible to select the preferred operational parameters, to optimize either the flux or the resolution or the high order rejection. The monochromatic beam is finally sent on the sample under examination by an ellipsoidal refocusing mirror.

Determination of optical constants of scandium films in the 20-1000 eV range

The transmittance of thin films of Sc deposited by evaporation in ultrahigh vacuum conditions has been investigated in the 20-1000 eV spectral range. Transmittance measurements were performed in situ on Sc layers that were deposited over grids coated with a C support film. Transmittance measurements were used to obtain the extinction coefficient of Sc films at each individual photon energy investigated. These data, along with the data available in the literature for the rest of the spectrum, were used to obtain the refractive index of Sc by means of the Kramers-Krönig analysis. Sum-rule tests indicated an acceptable consistency of the data.

Triode electron bombardment evaporation source for ultrahigh vacuum thin film deposition

A new electron bombardment evaporation source for ultrahigh vacuum (UHV) thin film deposition is presented. It is based on an original electrodes configuration (modeled on that of a vacuum triode) featuring, besides anode and filament, a biased grid acting as a power modulator. The presence of a polarized grid helps to minimize space charge problems, to improve electron focusing and to fine control the dissipation on the evaporating material, governing the sublimation rate. The electrodes geometry was optimized by studying electron trajectories and electrical field distribution in the electrodes zone with ray-tracing analysis. Mechanical solutions and electrodes geometry were designed to ensure low outgassing and UHV compatibility, as well as to facilitate maintenance and source cleaning. The evaporator is water cooled and a mechanically operated shutter is present. The working principles and the technical details are presented together with operating data and evaporation performances.

Electronic structure of Pr0.67Ca0.33MnO3 near the Fermi level studied by ultraviolet photoelectron and x-ray absorption spectroscopy

We have investigated the temperature-dependent changes in the near-

Coordination of boron and phosphorous in borophosphosilicate glasses

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