136 research outputs found
Low-frequency modes in the Raman spectrum of sp-sp2 nanostructured carbon
A novel form of amorphous carbon with sp-sp2 hybridization has been recently
produced by supersonic cluster beam deposition showing the presence in the film
of both polyynic and cumulenic species [L. Ravagnan et al. Phys. Rev. Lett. 98,
216103 (2007)]. Here we present a in situ Raman characterization of the low
frequency vibrational region (400-800 cm-1) of sp-sp2 films at different
temperatures. We report the presence of two peaks at 450 cm-1 and 720 cm-1. The
lower frequency peak shows an evolution with the variation of the sp content
and it can be attributed, with the support of density functional theory (DFT)
simulations, to bending modes of sp linear structures. The peak at 720 cm-1
does not vary with the sp content and it can be attributed to a feature in the
vibrational density of states activated by the disorder of the sp2 phase.Comment: 15 pages, 5 figures, 1 tabl
Poly(methyl methacrylate) - Palladium clusters nanocomposite formation by supersonic cluster beam deposition: a method for microstructured metallization of polymer surfaces
Nanocomposite films were fabricated by supersonic cluster beam deposition
(SCBD) of palladium clusters on Poly(methyl methacrylate) (PMMA) surfaces. The
evolution of the electrical conductance with cluster coverage and microscopy
analysis show that Pd cluster are implanted in the polymer and form a
continuous layer extending for several tens of nanometers beneath the polymer
surface. This allows the deposition, using stencil masks, of cluster-assembled
Pd microstructures on PMMA showing a remarkably high adhesion compared to
metallic films obtained by thermal evaporation. These results suggest that SCBD
is a promising tool for the fabrication of metallic microstructures on flexible
polymeric substrates.Comment: 11 pages, 3 figure
core level spectroscopy of free titanium clusters in supersonic beams
Synchrotron radiation x-ray absorption spectroscopy (XAS) is one of the most powerful techniques to interrogate the local electronic structure and chemical status of bulk and nanostructured systems. The application of this technique to the study of size effects in free clusters of transition metal atoms would advance substantially fundamental knowledge of nano-objects and the tailoring of their magnetic and catalytic properties. To date core level spectroscopy of free transition metal clusters has been out of reach due to the lack of a cluster source able to produce clusters in the gas phase with a density suitable for synchrotron radiation sources. Here we demonstrate the XAS characterization of free titanium clusters in a supersonic molecular beam. We use a high-intensity cluster beam source coupled to a synchrotron beamline to investigate the size dependence of core level excitation of Tin clusters in the mass range 1
The first search for bosonic super-WIMPs with masses up to 1 MeV/c with GERDA
We present the first search for bosonic super-WIMPs as keV-scale dark matter
candidates performed with the GERDA experiment. GERDA is a neutrinoless
double-beta decay experiment which operates high-purity germanium detectors
enriched in Ge in an ultra-low background environment at the Laboratori
Nazionali del Gran Sasso (LNGS) of INFN in Italy. Searches were performed for
pseudoscalar and vector particles in the mass region from 60 keV/c to 1
MeV/c. No evidence for a dark matter signal was observed, and the most
stringent constraints on the couplings of super-WIMPs with masses above 120
keV/c have been set. As an example, at a mass of 150 keV/c the most
stringent direct limits on the dimensionless couplings of axion-like particles
and dark photons to electrons of and
at 90% credible interval,
respectively, were obtained.Comment: 6 pages, 3 figures, submitted to Physical Review Letters, added list
of authors, updated ref. [21
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Diffraction imaging of light induced dynamics in xenon-doped helium nanodroplets
We explore the light induced dynamics in superfluid helium nanodroplets with wide-angle scattering in a pump–probe measurement scheme. The droplets are doped with xenon atoms to facilitate the ignition of a nanoplasma through irradiation with near-infrared laser pulses. After a variable time delay of up to 800 ps, we image the subsequent dynamics using intense extreme ultraviolet pulses from the FERMI free-electron laser. The recorded scattering images exhibit complex intensity fluctuations that are categorized based on their characteristic features. Systematic simulations of wide-angle diffraction patterns are performed, which can qualitatively explain the observed features by employing model shapes with both randomly distributed as well as structured, symmetric distortions. This points to a connection between the dynamics and the positions of the dopants in the droplets. In particular, the structured fluctuations might be governed by an underlying array of quantized vortices in the superfluid droplet as has been observed in previous small-angle diffraction experiments. Our results provide a basis for further investigations of dopant–droplet interactions and associated heating mechanisms
Ultrafast resonant interatomic coulombic decay induced by quantum fluid dynamics
Interatomic processes play a crucial role in weakly bound complexes exposed to ionizing radiation; therefore, gaining a thorough understanding of their efficiency is of fundamental importance. Here, we directly measure the timescale of interatomic Coulombic decay (ICD) in resonantly excited helium nanodroplets using a high-resolution, tunable, extreme ultraviolet free-electron laser. Over an extensive range of droplet sizes and laser intensities, we discover the decay to be surprisingly fast, with decay times as short as 400 fs, nearly independent of the density of the excited states. Using a combination of time- dependent density functional theory and ab initio quantum chemistry calculations, we elucidate the mechanisms of this ultrafast decay process, where pairs of excited helium atoms in one droplet strongly attract each other and form merging void bubbles, which drastically accelerates ICD
MariX, an advanced MHz-class repetition rate X-ray source for linear regime time-resolved spectroscopy and photon scattering
The need of a fs-scale pulsed, high repetition rate, X-ray source for time-resolved fine analysis of matter
(spectroscopy and photon scattering) in the linear response regime is addressed by the conceptual design of a
facility called MariX (Multi-disciplinary Advanced Research Infrastructure for the generation and application of
X-rays) outperforming current X-ray sources for the declared scope. MariX is based on the original design of a
two-pass two-way superconducting linear electron accelerator, equipped with an arc compressor, to be operated
in CW mode (1 MHz). MariX provides FEL emission in the range 0.2–8 keV with 108 photons per pulse ideally
suited for photoelectric effect and inelastic X-ray scattering experiments. The accelerator complex includes an
early stage that supports an advanced inverse Compton source of very high-flux hard X-rays of energies up
to 180 keV that is well adapted for large area radiological imaging, realizing a broad science programme and
serving a multidisciplinary user community, covering fundamental science of matter and application to life
sciences, including health at preclinical and clinical level
Calibration of the Gerda experiment
The GERmanium Detector Array (Gerda) collaboration searched for neutrinoless double-β decay in 76Ge with an array of about 40 high-purity isotopically-enriched germanium detectors. The experimental signature of the decay is a monoenergetic signal at Qββ= 2039.061 (7) keV in the measured summed energy spectrum of the two emitted electrons. Both the energy reconstruction and resolution of the germanium detectors are crucial to separate a potential signal from various backgrounds, such as neutrino-accompanied double-β decays allowed by the Standard Model. The energy resolution and stability were determined and monitored as a function of time using data from regular 228Th calibrations. In this work, we describe the calibration process and associated data analysis of the full Gerda dataset, tailored to preserve the excellent resolution of the individual germanium detectors when combining data over several years
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