Search results for "530"
showing 10 items of 1476 documents
First experimental results of a cryogenic stopping cell with short-lived, heavy uranium fragments produced at 1000 MeV/u
2013
A cryogenic stopping cell (CSC) has been commissioned with U-238 projectile fragments produced at 1000 MeV/u. The spatial isotopic separation in flight was performed with the FRS applying a monoenergetic degrader. For the first time, a stopping cell was operated with exotic nuclei at cryogenic temperatures (70 to 100K). A helium stopping gas density of up to 0.05mg/cm(3) was used, about two times higher than reached before for a stopping cell with RF ion repelling structures. An overall efficiency of up to 15%, a combined ion survival and extraction efficiency of about 50%, and extraction times of 24ms were achieved for heavy a-decaying uranium fragments. Mass spectrometry with a multiple-r…
Temperature Effects on the Phonon Spectrum in YBa2Cu3O7 Single Crystals and Thin Films
1989
We have performed a detailed investigation of the temperature dependence of the 385cm-1 phonon in single crystals and thin films of the YBa2Cu3O7 superconductor by means of Raman spectroscopy. In the single crystal the frequency of this phonon shows a downshift of about 5 cm-1 on passing the superconducting transition from above, which is referred to a strong electron-phonon interaction in the superconductor. The shift of the phonon in thin epitaxial films on MgO or SrTiO3 substrates is only about 2.5 cm-1. This difference may be a result of a structural transition in the single crystal which is possibly suppressed in the films because of the epitaxy. The electron-phonon interaction is also…
Hydrogen and helium films as model systems of wetting
1997
Optical experiments on the wetting properties of liquid 4He and molecular hydrogen are reviewed. Hydrogen films on noble metal surfaces serve as model systems for studying triple point wetting, a continuous transition between wetting and non-wetting. By means of optically excited surface plasmons, the adsorbed film thickness for temperatures around, and far below, the bulk melting temperature is measured, and the physical mechanisms responsible for the transition are elucidated. Possible applications for other experiments in pure and applied research are discussed. Thin films are droplets of liquid helium are studied on cesium surfaces, on which there is a first order wetting transition. Ou…
Substrate templating upon self-assembly of hydrogen-bonded molecular networks on an insulating surface.
2012
M olecular self-assembly on insulating surfaces, despite being highly relvant to many applications, generally suffers from the weak molecule–surface interactions present on dielectric surfaces, especially when benchmarked against metallic substrates. Therefore, to fully exploit the potential of molecular self-assembly, increasing the infl uence of the substrate constitutes an essential prerequisite. Upon deposition of terephthalic acid and trimesic acid onto the natural cleavage plane of calcite, extended hydrogen-bonded networks are formed, which wet the substrate. The observed structural complexity matches the variety realized on metal surfaces. A detailed analysis of the molecular struct…
Orientation of non-spherical protonated water clusters revealed by infrared absorption dichroism
2018
Infrared continuum bands that extend over a broad frequency range are a key spectral signature of protonated water clusters. They are observed for many membrane proteins that contain internal water molecules, but their microscopic mechanism has remained unclear. Here we compute infrared spectra for protonated and unprotonated water chains, discs, and droplets from ab initio molecular dynamics simulations. The continuum bands of the protonated clusters exhibit significant anisotropy for chains and discs, with increased absorption along the direction of maximal cluster extension. We show that the continuum band arises from the nuclei motion near the excess charge, with a long-ranged amplifica…
Aluminum-lithium clusters: First-principles simulation of geometries and electronic properties
2002
The geometries and electronic properties of small lithium-rich ${\mathrm{Al}}_{N}{\mathrm{Li}}_{5N}$ $(N=1--6,10)$ clusters are studied using first-principles simulations. Aluminum ions form a compact inner core configuration in the clusters that changes into a chainlike skeleton embedded in a lithium surrounding as the cluster size increases. This behavior restricts $s\ensuremath{-}p$ hybridization effects and causes separate s and p bands in the electronic energy spectrum. A significant charge transfer from Li ions and nearby Al ions strengthens ionic Al-Li bonds, while Al-Al bonds gain a more covalent nature. The evolution of some bulk properties of $B2$ and $B32$ phases of AlLi alloys i…
On-surface synthesis on a bulk insulator surface
2018
On-surface synthesis has rapidly emerged as a most promising approach to prepare functional molecular structures directly on a support surface. Compared to solution synthesis, performing chemical reactions on a surface offers several exciting new options: due to the absence of a solvent, reactions can be envisioned that are otherwise not feasible due to the insolubility of the reaction product. Perhaps even more important, the confinement to a two-dimensional surface might enable reaction pathways that are not accessible otherwise. Consequently, on-surface synthesis has attracted great attention in the last decade, with an impressive number of classical reactions transferred to a surface as…
Multi-scale characterization of porosity in Boom Clay (HADES-level, Mol, Belgium) using a combination of X-ray μ-CT, 2D BIB-SEM and FIB-SEM tomography
2015
Abstract The Oligocene age Boom Clay is a potential host material for radioactive waste disposal in Belgium. To better understand the physical basis of transport mechanisms of radionuclides, we aim to characterize the pore space and its connectivity at nm-scale in 3D. In the present study, X-ray μ-CT and FIB-SEM (focused ion beam scanning electron microscopy) tomography were combined, to investigate the 3D pore space of a Boom Clay sample from the Mol-1 borehole (depth corresponding to the level of the HADES-URF – ‘high activity disposal experimental site underground research facility’) at the Mol–Dessel research site for radioactive waste disposal (Belgium). BIB-SEM (broad ion beam scannin…
Excitation of different chromium centres by synchrotron radiation in MgO:Cr single crystals
2015
The excitation spectra for the emissions of chromium-containing centres have been measured at 10 K using synchrotron radiation of 4–32 eV in MgO single crystals with different content of Cr$^{3+}$ (5–850 ppm) and Ca$^{2+}$ impurity ions. Both virgin crystals and the samples preliminarily irradiated with x-rays at 295 K have been studied. The role of complex chromium centres containing two Cr$^{3+}$ and a cation vacancy (sometimes nearby a Ca$^{2+}$ ion) on the luminescence processes and the transformation/creation of structural defects has been analysed. Such anharmonic complex centres could serve as the seeds for the creation of 3D defects that facilitate the cracking and brittle destructi…
Isotope effects on the dynamics of a supercooled van der Waals liquid
2000
Deuteron magnetic resonance was used to study three differently isotope-labeled species of the fragile glass-forming liquid ortho-terphenyl. The calorimetric glass transition of the isotope deuterated only at the central phenyl ring is significantly lower than that of the perdeuterated one. It is shown that while the ortho-terphenyl molecule is not as rigid as previously often assumed, its overall reorientation geometry is independent of deuteration. The characteristic jump angles are found to increase with temperature, thus resolving an apparent discrepancy previously noted when comparing typical jump sizes from NMR with other data.