111 research outputs found
Scattering of rare-gas atoms at a metal surface: evidence of anticorrugation of the helium-atom potential-energy surface and the surface electron density
Recent measurements of the scattering of He and Ne atoms at Rh(110) suggest
that these two rare-gas atoms measure a qualitatively different surface
corrugation: While Ne atom scattering seemingly reflects the electron-density
undulation of the substrate surface, the scattering potential of He atoms
appears to be anticorrugated. An understanding of this perplexing result is
lacking. In this paper we present density functional theory calculations of the
interaction potentials of He and Ne with Rh(110). We find that, and explain
why, the nature of the interaction of the two probe particles is qualitatively
different, which implies that the topographies of their scattering potentials
are indeed anticorrugated.Comment: RevTeX, 4 pages, 10 figure
First enantioseparation and circular dichroism spectra of Au38 clusters protected by achiral ligands
Bestowing chirality to metals is central in fields such as heterogeneous catalysis and modern optics. Although the bulk phase of metals is symmetric, their surfaces can become chiral through adsorption of molecules. Interestingly, even achiral molecules can lead to locally chiral, though globally racemic, surfaces. A similar situation can be obtained for metal particles or clusters. Here we report the first separation of the enantiomers of a gold cluster protected by achiral thiolates, Au38(SCH2CH2Ph)24, achieved by chiral high-performance liquid chromatography. The chirality of the nanocluster arises from the chiral arrangement of the thiolates on its surface, forming 'staple motifs'. The enantiomers show mirror-image circular dichroism responses and large anisotropy factors of up to 4×10−3. Comparison with reported circular dichroism spectra of other Au38 clusters reveals that the influence of the ligand on the chiroptical properties is minor
Measurement of global polarization of {\Lambda} hyperons in few-GeV heavy-ion collisions
The global polarization of {\Lambda} hyperons along the total orbital angular
momentum of a relativistic heavy-ion collision is presented based on the high
statistics data samples collected in Au+Au collisions at \sqrt{s_{NN}} = 2.4
GeV and Ag+Ag at 2.55 GeV with the High-Acceptance Di-Electron Spectrometer
(HADES) at GSI, Darmstadt. This is the first measurement below the strangeness
production threshold in nucleon-nucleon collisions. Results are reported as a
function of the collision centrality as well as a function of the hyperon
transverse momentum (p_T) and rapidity (y_{CM}) for the range of centrality
0--40%. We observe a strong centrality dependence of the polarization with an
increasing signal towards peripheral collisions. For mid-central (20--40%)
collisions the polarization magnitudes are (%) = 6.0 \pm 1.3
(stat.) \pm 2.0 (syst.) for Au+Au and (%) = 4.6 \pm 0.4 (stat.)
\pm 0.5 (syst.) for Ag+Ag, which are the largest values observed so far. This
observation thus provides a continuation of the increasing trend previously
observed by STAR and contrasts expectations from recent theoretical
calculations predicting a maximum in the region of collision energies about 3
GeV. The observed polarization is of a similar magnitude as predicted by 3D
fluid dynamics and the UrQMD plus thermal vorticity model and significantly
above results from the AMPT model.Comment: 8 pages, 4 figure
Inclusive ee production in collisions of pions with protons and nuclei in the second resonance region of baryons
Inclusive ee production has been studied with HADES in + p,
+ C and reactions, using the GSI pion beam at
= 1.49 GeV. Invariant mass and transverse momentum
distributions have been measured and reveal contributions from Dalitz decays of
, mesons and baryon resonances. The transverse momentum
distributions are very sensitive to the underlying kinematics of the various
processes. The baryon contribution exhibits a deviation up to a factor seven
from the QED reference expected for the dielectron decay of a hypothetical
point-like baryon with the production cross section constrained from the
inverse n p reaction. The enhancement is attributed
to a strong four-momentum squared dependence of the time-like electromagnetic
transition form factors as suggested by Vector Meson Dominance (VMD). Two
versions of the VMD, that differ in the photon-baryon coupling, have been
applied in simulations and compared to data. VMD1 (or two-component VMD)
assumes a coupling via the meson and a direct coupling of the photon,
while in VMD2 (or strict VMD) the coupling is only mediated via the
meson. The VMD2 model, frequently used in transport calculations for dilepton
decays, is found to overestimate the measured dielectron yields, while a good
description of the data can be obtained with the VMD1 model assuming no phase
difference between the two amplitudes. Similar descriptions have also been
obtained using a time-like baryon transition form factor model where the pion
cloud plays the major role.Comment: (HADES collaboration
Impact of the Coulomb field on charged-pion spectra in few-GeV heavy-ion collisions
In nuclear collisions the incident protons generate a Coulomb field which acts on produced charged particles. The impact of these interactions on charged-pion transverse-mass and rapidity spectra, as well as on pion–pion momentum correlations is investigated in Au + Au collisions at = 2.4 GeV. We show that the low-m region (m < 0.2 GeV / c) can be well described with a Coulomb-modified Boltzmann distribution that also takes changes of the Coulomb field during the expansion of the fireball into account. The observed centrality dependence of the fitted mean Coulomb potential energy deviates strongly from a scaling, indicating that, next to the fireball, the non-interacting charged spectators have to be taken into account. For the most central collisions, the Coulomb modifications of the HBT source radii are found to be consistent with the potential extracted from the single-pion transverse-mass distributions. This finding suggests that the region of homogeneity obtained from two-pion correlations coincides with the region in which the pions freeze-out. Using the inferred mean-square radius of the charge distribution at freeze-out, we have deduced a baryon density, in fair agreement with values obtained from statistical hadronization model fits to the particle yields
Homochirality in biomineral suprastructures induced by assembly of single-enantiomer amino acids from a nonracemic mixture
© 2019, The Author(s). Since Pasteur first successfully separated right-handed and left-handed tartrate crystals in 1848, the understanding of how homochirality is achieved from enantiomeric mixtures has long been incomplete. Here, we report on a chirality dominance effect where organized, three-dimensional homochiral suprastructures of the biomineral calcium carbonate (vaterite) can be induced from a mixed nonracemic amino acid system. Right-handed (counterclockwise) homochiral vaterite helicoids are induced when the amino acid l-Asp is in the majority, whereas left-handed (clockwise) homochiral morphology is induced when d-Asp is in the majority. Unexpectedly, the Asp that incorporates into the homochiral vaterite helicoids maintains the same enantiomer ratio as that of the initial growth solution, thus showing chirality transfer without chirality amplification. Changes in the degree of chirality of the vaterite helicoids are postulated to result from the extent of majority enantiomer assembly on the mineral surface. These mechanistic insights potentially have major implications for high-level advanced materials synthesis
Gender differences in social support and leisure-time physical activity
OBJECTIVE To identify gender differences in social support dimensions’ effect on adults’ leisure-time physical activity maintenance, type, and time. METHODS Longitudinal study of 1,278 non-faculty public employees at a university in Rio de Janeiro, RJ, Southeastern Brazil. Physical activity was evaluated using a dichotomous question with a two-week reference period, and further questions concerning leisure-time physical activity type (individual or group) and time spent on the activity. Social support was measured with the Medical Outcomes Study Social Support Scale. For the analysis, logistic regression models were adjusted separately by gender. RESULTS A multinomial logistic regression showed an association between material support and individual activities among women (OR = 2.76; 95%CI 1.2;6.5). Affective support was associated with time spent on leisure-time physical activity only among men (OR = 1.80; 95%CI 1.1;3.2). CONCLUSIONS All dimensions of social support that were examined influenced either the type of, or the time spent on, leisure-time physical activity. In some social support dimensions, the associations detected varied by gender. Future studies should attempt to elucidate the mechanisms involved in these gender differences
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