81 research outputs found
Transition-pathway models of atomic diffusion on fcc metal surfaces. I. Flat surfaces
Numerical calculation of minimum-energy paths and activation energy barriers for various atomic diffusion processes on fcc metal surfaces are presented. The computational method employed is the action-derived molecular dynamics that searches the approximate Newtonian trajectory on potential-energy surfaces. The minimization of a modified action, which facilitates the conservation of total energy and the control of kinetic energy, enables us to find efficiently the minimum-energy paths of complex microscopic processes. Diverse diffusion mechanisms on flat fcc substrates are investigated in this first part of the series. More complicated systems including surface steps are simulated in paper II.open2
Surfactant-like Effect and Dissolution of Ultrathin Fe Films on Ag(001)
The phase immiscibility and the excellent matching between Ag(001) and
Fe(001) unit cells (mismatch 0.8 %) make Fe/Ag growth attractive in the field
of low dimensionality magnetic systems. Intermixing could be drastically
limited at deposition temperatures as low as 140-150 K. The film structural
evolution induced by post-growth annealing presents many interesting aspects
involving activated atomic exchange processes and affecting magnetic
properties. Previous experiments, of He and low energy ion scattering on films
deposited at 150 K, indicated the formation of a segregated Ag layer upon
annealing at 550 K. Higher temperatures led to the embedding of Fe into the Ag
matrix. In those experiments, information on sub-surface layers was attained by
techniques mainly sensitive to the topmost layer. Here, systematic PED
measurements, providing chemical selectivity and structural information for a
depth of several layers, have been accompanied with a few XRD rod scans,
yielding a better sensitivity to the buried interface and to the film long
range order. The results of this paper allow a comparison with recent models
enlightening the dissolution paths of an ultra thin metal film into a different
metal, when both subsurface migration of the deposit and phase separation
between substrate and deposit are favoured. The occurrence of a surfactant-like
stage, in which a single layer of Ag covers the Fe film is demonstrated for
films of 4-6 ML heated at 500-550 K. Evidence of a stage characterized by the
formation of two Ag capping layers is also reported. As the annealing
temperature was increased beyond 700 K, the surface layers closely resembled
the structure of bare Ag(001) with the residual presence of subsurface Fe
aggregates.Comment: 4 pages, 3 figure
Photochemistry of the PAH pyrene in water ice: the case for ion-mediated solid-state astrochemistry
Context. Icy dust grains play an important role in the formation of complex
inter- and circumstellar molecules. Observational studies show that polycyclic
aromatic hydrocarbons (PAHs) are abundantly present in the ISM in the gas
phase. It is likely that these non-volatile species freeze out onto dust grains
as well and participate in the astrochemical solid-state network, but
experimental PAH ice studies are largely lacking. Methods. Near UV/VIS
spectroscopy is used to track the in situ VUV driven photochemistry of pyrene
containing ices at temperatures ranging from 10 to 125 K. Results. The main
photoproducts of VUV photolyzed pyrene ices are spectroscopically identified
and their band positions are listed for two host ices, \water and CO. Pyrene
ionisation is found to be most efficient in \water ices at low temperatures.
The reaction products, triplet pyrene and the 1-hydro-1-pyrenyl radical are
most efficiently formed in higher temperature water ices and in low temperature
CO ice. Formation routes and band strength information of the identified
species are discussed. Additionally, the oscillator strengths of Py, Py^+ and
PyH are derived and a quantitative kinetic analysis is performed by fitting a
chemical reaction network to the experimental data. Conclusions. Pyrene is
efficiently ionised in water ice at temperatures below 50 K. Hydrogenation
reactions dominate the chemistry in low temperature CO ice with trace amounts
of water. The results are put in an astrophysical context by determining the
importance of PAH ionisation in a molecular cloud. The photoprocessing of a
sample PAH in ice described in this manuscript indicates that PAH
photoprocessing in the solid state should also be taken into account in
astrochemical models.Comment: 11 pages, 8 figures, accepted for publication in A&
Ab initio study of step formation and self-diffusion on Ag(100)
Using the plane wave pseudopotential method we performed density functional
theory calculations on the stability of steps and self-diffusion processes on
Ag(100). Our calculated step formation energies show that the {111}-faceted
step is more stable than the {110}-faceted step. In accordance with
experimental observations we find that the equilibrium island shape should be
octagonal very close to a square with predominately {111}-faceted steps. For
the (100) surface of fcc metals atomic migration proceeds by a hopping or an
exchange process. For Ag(100) we find that adatoms diffuse across flat surfaces
preferentially by hopping. Adatoms approaching the close-packed {111}-faceted
step edges descend from the upper terrace to the lower level by an atomic
exchange with an energy barrier almost identical to the diffusion barrier on
flat surface regions. Thus, within our numerical accuracy (approx +- 0.05 eV)
there is no additional step-edge barrier to descent. This provides a natural
explanation for the experimental observations of the smooth two-dimensional
growth in homoepitaxy of Ag(100). Inspection of experimental results of other
fcc crystal surfaces indicates that our result holds quite generally.Comment: 10 pages, 9 figures. Submitted to Phys. Rev B (October 31, 1996
Stable isotope dilution analysis of N-acetylaspartic acid in CSF, blood, urine and amniotic fluid: Accurate postnatal diagnosis and the potential for prenatal diagnosis of canavan disease
A sensitive and selective analytical technique is described for the determination of N-acetylaspartic acid in body fluids using stable isotope dilution in combination with positive chemical ionization mass spectrometry with selected ion monitoring. Control mean and ranges have been established: in urine 19.5 and 6.6-35.4 μmol/mmol creat.; in plasma 0.44 and 0.17-0.81 μmol/L; in cerebrospinal fluid 1.51 and 0.25-2.83 μmol/L; and in amniotic fluid 1.27 and 0.30-2.55 μmol/L. In a patient with Canavan disease, N-acetylaspartic acid concentration was elevated 80-fold in urine and 20-fold in plasma compared to the control means. A subsequent pregnancy of the mother was monitored and the N-acetylaspartic acid concentration in the amniotic fluid was within the control range and a healthy child was born
Self-diffusion of adatoms, dimers, and vacancies on Cu(100)
We use ab initio static relaxation methods and semi-empirical
molecular-dynamics simulations to investigate the energetics and dynamics of
the diffusion of adatoms, dimers, and vacancies on Cu(100). It is found that
the dynamical energy barriers for diffusion are well approximated by the
static, 0 K barriers and that prefactors do not depend sensitively on the
species undergoing diffusion. The ab initio barriers are observed to be
significantly lower when calculated within the generalized-gradient
approximation (GGA) rather than in the local-density approximation (LDA). Our
calculations predict that surface diffusion should proceed primarily via the
diffusion of vacancies. Adatoms are found to migrate most easily via a jump
mechanism. This is the case, also, of dimers, even though the corresponding
barrier is slightly larger than it is for adatoms. We observe, further, that
dimers diffuse more readily than they can dissociate. Our results are discussed
in the context of recent submonolayer growth experiments of Cu(100).Comment: Submitted to the Physical Review B; 15 pages including postscript
figures; see also http://www.centrcn.umontreal.ca/~lewi
Rebalancing of mitochondrial homeostasis through an NAD+-SIRT1 pathway preserves intestinal barrier function in severe malnutrition.
BACKGROUND: The intestine of children with severe malnutrition (SM) shows structural and functional changes that are linked to increased infection and mortality. SM dysregulates the tryptophan-kynurenine pathway, which may impact processes such as SIRT1- and mTORC1-mediated autophagy and mitochondrial homeostasis. Using a mouse and organoid model of SM, we studied the repercussions of these dysregulations on malnutrition enteropathy and the protective capacity of maintaining autophagy activity and mitochondrial health. METHODS: SM was induced through feeding male weanling C57BL/6 mice a low protein diet (LPD) for 14-days. Mice were either treated with the NAD +-precursor, nicotinamide; an mTORC1-inhibitor, rapamycin; a SIRT1-activator, resveratrol; or SIRT1-inhibitor, EX-527. Malnutrition enteropathy was induced in enteric organoids through amino-acid deprivation. Features of and pathways to malnutrition enteropathy were examined, including paracellular permeability, nutrient absorption, and autophagic, mitochondrial, and reactive-oxygen-species (ROS) abnormalities. FINDINGS: LPD-feeding and ensuing low-tryptophan availability led to villus atrophy, nutrient malabsorption, and intestinal barrier dysfunction. In LPD-fed mice, nicotinamide-supplementation was linked to SIRT1-mediated activation of mitophagy, which reduced damaged mitochondria, and improved intestinal barrier function. Inhibition of mTORC1 reduced intestinal barrier dysfunction and nutrient malabsorption. Findings were validated and extended using an organoid model, demonstrating that resolution of mitochondrial ROS resolved barrier dysfunction. INTERPRETATION: Malnutrition enteropathy arises from a dysregulation of the SIRT1 and mTORC1 pathways, leading to disrupted autophagy, mitochondrial homeostasis, and ROS. Whether nicotinamide-supplementation in children with SM could ameliorate malnutrition enteropathy should be explored in clinical trials. FUNDING: This work was supported by the Bill and Melinda Gates Foundation, the Sickkids Research Institute, the Canadian Institutes of Health Research, and the University Medical Center Groningen
Prioritizing Emerging Zoonoses in The Netherlands
Background: To support the development of early warning and surveillance systems of emerging zoonoses, we present a general method to prioritize pathogens using a quantitative, stochastic multi-criteria model, parameterized for the Netherlands. Methodology/Principal Findings: A risk score was based on seven criteria, reflecting assessments of the epidemiology and impact of these pathogens on society. Criteria were weighed, based on the preferences of a panel of judges with a background in infectious disease control. Conclusions/Significance: Pathogens with the highest risk for the Netherlands included pathogens in the livestock reservoir with a high actual human disease burden (e.g. Campylobacter spp., Toxoplasma gondii, Coxiella burnetii) or a low current but higher historic burden (e.g. Mycobacterium bovis), rare zoonotic pathogens in domestic animals with severe disease manifestations in humans (e.g. BSE prion, Capnocytophaga canimorsus) as well as arthropod-borne and wildlife associated pathogens which may pose a severe risk in future (e.g. Japanese encephalitis virus and West-Nile virus). These agents are key targets for development of early warning and surveillance.Infrastructures, Systems and ServicesTechnology, Policy and Managemen
Breaking Traditions:An Isotopic Study on the Changing Funerary Practices in the Dutch Iron Age (800-12 bc)
Urnfields in the Dutch river area were replaced by cemeteries with a mixture of cremation and inhumation graves around the sixth century bc. This study provides the first biogeochemical evidence that the Iron Age communities were heterogeneous in terms of geological origins. The high percentage of non-locally born individuals (~48%) supports the hypothesis that the change in burial practice was the result of the influx of foreign people, who were being allowed to keep their own burial customs, whereas some of the local inhabitants adapted the burial rites of foreign cultures, leading to a heterogeneous burial rite for some centuries
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