137 research outputs found
Angular correlation between photoelectrons and Auger electrons within scattering theory
International audienceIn this paper we present a single-particle scattering approach for the angular correlation between a photoelectron and the subsequent Auger electron from atomic targets. This method is proposed as an alternative approach with respect to the usual density matrix formalism, since it is more convenient for extension to the solid state case. Such an extension is required by the great progress made in the field of coincidence spectroscopy in condensed matter systems. We derived a tensor expression for the cross section and an equivalent expression in terms of convenient angular functions has been treated for the case of linearly polarized light. Numerical calculations are performed for the L3M2,3M2,3 transition in argon, in the single configuration Dirac-Fock scheme. Results are compared with experimental data for different final angular momentum states of the doubly charged ion and for different kinematical conditions
Free electron laser-driven ultrafast rearrangement of the electronic structure in Ti
High-energy density extreme ultraviolet radiation delivered by the FERMI seeded free-electron laser has been used to create an exotic nonequilibrium state of matter in a titanium sample characterized by a highly excited electron subsystem at temperatures in excess of 10âeV and a cold solid-density ion lattice. The obtained transient state has been investigated through ultrafast absorption spectroscopy across the Ti M2,3-edge revealing a drastic rearrangement of the sample electronic structure around the Fermi level occurring on a time scale of about 100 fs
Tracking interfacial changes of graphene/Ge(110) during in-vacuum annealing
Graphene quality indicators obtained by Raman spectroscopy have been
correlated to the structural changes of the graphene/Germanium interface as a
function of in-vacuum thermal annealing. Specifically, it is found that
graphene becomes markedly defected at 650 {\deg}C. By combining scanning
tunneling microscopy, x-Ray Photoelectron Spectroscopy and Near Edge x-ray
Absorption Fine Structure Spectroscopy, we conclude that these defects are due
to the release of H_{2} gas trapped at the graphene/Germanium interface. The
H_{2} gas was produced following the transition from the as-grown
hydrogen-termination of the Ge(110) surface to the emergence of surface
reconstructions in the substrate. Interestingly, a complete self-healing
process is observed in graphene upon annealing to 800 {\deg}C. The subtle
interplay revealed between the microscopic changes occurring at the
graphene/Germanium interface and graphene's defect density is valuable for
advancing graphene growth, controlled 2D-3D heterogeneous materials interfacing
and integrated fabrication technology on semiconductors
Polarization Control in Integrated Graphene-Silicon Quantum Photonics Waveguides
We numerically investigated the use of graphene nanoribbons placed on top of silicon-on-insulator (SOI) strip waveguides for light polarization control in silicon photonic-integrated waveguides. We found that two factors mainly affected the polarization control: the graphene chemical potential and the geometrical parameters of the waveguide, such as the waveguide and nanoribbon widths and distance. We show that the graphene chemical potential influences both TE and TM polarizations almost in the same way, while the waveguide width tapering enables both TE-pass and TM-pass polarizing functionalities. Overall, by increasing the oxide spacer thickness between the silicon waveguide and the top graphene layer, the device insertion losses can be reduced, while preserving a high polarization extinction ratio
Tunability experiments at the FERMI@Elettra free-electron laser
FERMI@Elettra is a free electron-laser (FEL)-based user facility that, after two years of commissioning, started preliminary users' dedicated runs in 2011. At variance with other FEL user facilities, FERMI@Elettra has been designed to deliver improved spectral stability and longitudinal coherence. The adopted scheme, which uses an external laser to initiate the FEL process, has been demonstrated to be capable of generating FEL pulses close to the Fourier transform limit. We report on the first instance of FEL wavelength tuning, both in a narrow and in a large spectral range (fine- and coarse-tuning). We also report on two different experiments that have been performed exploiting such FEL tuning. We used fine-tuning to scan across the 1sâ4p resonance in He atoms, at â23.74 eV (52.2 nm), detecting both UVâvisible fluorescence (4pâ2s, 400 nm) and EUV fluorescence (4pâ1s, 52.2 nm). We used coarse-tuning to scan the M4,5 absorption edge of Ge (âŒ29.5 eV) in the wavelength region 30â60 nm, measured in transmission geometry with a thermopile positioned on the rear side of a Ge thin foil
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