Angelo Giglia

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.

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.

Tuning the Electronic Structure of Graphene by Molecular Dopants: Impact of the Substrate

A combination of ultraviolet and X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, and first principle calculations was used to study the electronic structure at the interface between the strong molecular acceptor 1,3,4,5,7,8-hexafluorotetracyano-naphthoquinodimethane (F6TCNNQ) and a graphene layer supported on either a quartz or a copper substrate. We find evidence for fundamentally different charge redistribution mechanisms in the two ternary systems, as a consequence of the insulating versus metallic character of the substrates. While electron transfer occurs exclusively from graphene to F6TCNNQ on the quartz support (p-doping of graphene), the Cu substrate electron reservoir induces an additional electron density flow to graphene decorated with the acceptor monolayer. Remarkably, graphene on Cu is n-doped and remains n-doped upon F6TCNNQ deposition. On both substrates, the work function of graphene increases substantially with a F6TCNNQ monolayer atop, the effect being more pronounced (∼1.3 eV) on Cu compared to quartz (∼1.0 eV) because of the larger electrostatic potential drop associated with the long-distance graphene-mediated Cu-F6TCNNQ electron transfer. We thus provide a means to realize high work function surfaces for both p- and n-type doped graphene.

Coordination of boron and phosphorous in borophosphosilicate glasses

We report boron K-edge x-ray absorption near-edge structure measurements on borophosphosilicate glasses and density functional theory calculations aimed at determining the most stable local geometries for boron and phosphorous. We demonstrate that phosphorous induces a modification in the local structure of boron, from a trigonal to a tetrahedral geometry. The microscopic mechanisms involved are discussed.

Transmittance and optical constants of Ce films in the 6–1200eV spectral range

The optical constants of Ce films were obtained in the 6–1200eV range from transmittance measurements obtained at room temperature. Thin films of Ce were deposited by evaporation in ultrahigh vacuum conditions and their transmittance was measured in situ. Ce films were deposited onto grid-supported, thin C films. Transmittance measurements of various film thicknesses were used to obtain the extinction coefficient k of Ce films at each individual photon energy investigated. The refractive index n of Ce was calculated with the Kramers–Kronig analysis using the current k data, which were extended toward smaller and larger energies with available data from literature and extrapolations. Ce has a low-absorption band right below O2,3 edge, with lowest absorption at 16.1eV. This makes Ce a promising material for the development of new filters and multilayer coatings below Ce O2,3 edge, in which few developments have been performed due to the lack of low-absorption materials. A good consistency of the data was ev...

Optical constants of Yb films in the 23-1700 eV range

The optical constants of Yb films have been determined in the 23-1700 eV spectral range from transmittance measurements performed in situ on Yb films deposited by evaporation in ultrahigh vacuum conditions. Yb films were deposited over grids coated with a thin carbon film. Transmittance measurements were used to obtain the extinction coefficient of Yb films at each individual photon energy investigated. The energy range investigated encompasses Yb edges from M(4,5) to O(2,3). The current results, along with data in the literature, show that Yb has an interesting low-absorption band in the approximately 12-24 eV range, which may be useful for the development of transmittance filters and multilayer coatings. The current data along with literature data and extrapolations were used to obtain n, the real part of the complex refractive index, using a Kramers-Krönig analysis. The application of the sum rules showed a good consistency of the results.

Reflectance measurements and optical constants in the extreme ultraviolet-vacuum ultraviolet regions for SiC with a different C/Si ratio

Reflectance versus incidence angle measurements have been performed from 5 to 152 nm on samples of SiC with a different C/Si ratio deposited with rf magnetron sputtering. The optical constants of the material at different wavelengths have been determined by using a curve-fitting technique of reflectance values versus incidence angle. Complementary measurements of the incident beam polarization, film thickness, surface roughness, and stoichiometry were performed to complete the analysis of the samples.