1,816 research outputs found
An ab initio and dynamics study of the photodissociation of nitric acid HNO3
We investigated the photodissociation of HNO3 within the first (300 nm) and the third (200 nm) absorption band. The relevant S1 and S3 potential energy surfaces were calculated by taking into account the N-O single bond and N=O “double” bond distances. The striking feature of the dynamical analysis is a bifurcation of the wave packet on the S3 surface which explains the branching into the two
reaction pathways with the products OH+NO2 and O+HONO found in experiments. Dissociation on the S1 surface is predicted to proceed along a single channel leading to OH+NO2, both in their electronic ground states.
Corresponding author
Fibronectin-Cleaving Activity in Bronchial Secretions of Patients with Cystic Fibrosis
In cystic fibrosis, colonization of the airways with Pseudomonas aeruginosa follows colonization with Staphylococcus aureus and is related to accelerated deterioration of pulmonary function. Because P. aeruginosa adheres better to cell surfaces devoid of fibronectin, we searched for fibronectin-cleaving activity in bronchial secretions and saliva from 24 patients with cystic fibrosis who were followed up for 4.5 y and from two control groups. Proteolytic activity against 125I-labeled fibronectin wascontinuously present in cystic fibrosis bronchial secretions; significantly higher fibronectin-cleaving activity was found in older vs. younger patients, in patients with advanced disease stages determined by a five-stage scoring system, and in those colonized with P. aeruginosa. The fibronectin-cleaving activity was due to neutrophil elastase and cathepsin G. Cystic fibrosis bronchial secretions had proteolytic activity against surface fibronectin of airway mucosal cells. Thus fibronectin-cleaving activity of bronchial secretions rather than of saliva may favor P. aeruginosa colonization of the upper respiratory tract in individuals with cystic fibrosi
Cosmogenic rare gases and 10-Be in a cross section of Knyahinya
The concentrations of cosmogenic nuclides were studied as a function of shielding on samples from a cross section of the 293 kg main fragment of the L5 chondrite Knyahinya. The stone broke into two nearly symmetrical parts upon its fall in 1866. The planar cross section has diameters between 40 and 55 cm. He, Ne, and Ar were measured on about 20 samples by mass spectrometry and the 10-Be activities on aliquots of 10 selected samples were determined by AMS. The 10-Be data are presented and the abundances of spallogenic nuclides are compared with the model calculations reported by Reedy for spherical L chondrites. The 10-Be production rates in Knyahinya are shown versus the shielding parameter 22-Ne/21-Ne
Levels of Free Granulocyte Elastase in Bronchial Secretions from Patients with Cystic Fibrosis: Effect of Antimicrobial Treatment Against Pseudomonas aeruginosa
Large amounts of free granulocyte elastase (GE), an enzyme capable of mediating airway damage, have been found in bronchial secretions of patients with cystic fibrosis who are infected with Pseudomonas aeruginosa. This finding indicates an imbalance between GE and its antiproteases, α1-proteinase inhibitor (α1-PI) and bronchial mucosal inhibitor (BMI), in the airways of these individuals. The effect of intravenous antimicrobial treatment against P. aeruginosa on activity and concentration of GE, BMI, and α1-PI was evaluated in 30 treatment courses of 20 patients with cystic fibrosis. Although sputum volume and level of immunoreactive GE decreased and concentrations of α1-PI and BMI increased significantly (P < .05), a high level of free GE persisted. No active α1-PI and BMI were detectable after treatment. High levels of GE correlated with a poor pulmonary condition (rs = .98, P < .001). In vitro, elastolytic activity of bronchial secretions from patients with cystic fibrosis was significantly inhibited by eglin C and an oxidation-resistant variant of α1-PI, both compounds currently produced by recombinant DNA technolog
First-Principles Calculations of Hyperfine Interactions in La_2CuO_4
We present the results of first-principles cluster calculations of the
electronic structure of La_2CuO_4. Several clusters containing up to nine
copper atoms embedded in a background potential were investigated.
Spin-polarized calculations were performed both at the Hartree-Fock level and
with density functional methods with generalized gradient corrections to the
local density approximation. The distinct results for the electronic structure
obtained with these two methods are discussed. The dependence of the
electric-field gradients at the Cu and the O sites on the cluster size is
studied and the results are compared to experiments. The magnetic hyperfine
coupling parameters are carefully examined. Special attention is given to a
quantitative determination of on-site and transferred hyperfine fields. We
provide a detailed analysis that compares the hyperfine fields obtained for
various cluster sizes with results from additional calculations of spin states
with different multiplicities. From this we conclude that hyperfine couplings
are mainly transferred from nearest neighbor Cu^{2+} ions and that
contributions from further distant neighbors are marginal. The mechanisms
giving rise to transfer of spin density are worked out. Assuming conventional
values for the spin-orbit coupling, the total calculated hyperfine interaction
parameters are compared to informations from experiments.Comment: 23 pages, 9 figure
Dimensionality Control of Electronic Phase Transitions in Nickel-Oxide Superlattices
The competition between collective quantum phases in materials with strongly
correlated electrons depends sensitively on the dimensionality of the electron
system, which is difficult to control by standard solid-state chemistry. We
have fabricated superlattices of the paramagnetic metal LaNiO3 and the wide-gap
insulator LaAlO3 with atomically precise layer sequences. Using optical
ellipsometry and low-energy muon spin rotation, superlattices with LaNiO3 as
thin as two unit cells are shown to undergo a sequence of collective
metalinsulator and antiferromagnetic transitions as a function of decreasing
temperature, whereas samples with thicker LaNiO3 layers remain metallic and
paramagnetic at all temperatures. Metal-oxide superlattices thus allow control
of the dimensionality and collective phase behavior of correlated-electron
systems
Collision Dynamics and Solvation of Water Molecules in a Liquid Methanol Film
Environmental molecular beam experiments are used to examine water
interactions with liquid methanol films at temperatures from 170 K to 190 K. We
find that water molecules with 0.32 eV incident kinetic energy are efficiently
trapped by the liquid methanol. The scattering process is characterized by an
efficient loss of energy to surface modes with a minor component of the
incident beam that is inelastically scattered. Thermal desorption of water
molecules has a well characterized Arrhenius form with an activation energy of
0.47{\pm}0.11 eV and pre-exponential factor of 4.6 {\times} 10^(15{\pm}3)
s^(-1). We also observe a temperature dependent incorporation of incident water
into the methanol layer. The implication for fundamental studies and
environmental applications is that even an alcohol as simple as methanol can
exhibit complex and temperature dependent surfactant behavior.Comment: 8 pages, 5 figure
Analysis of neural crest-derived clones reveals novel aspects of facial development
Cranial neural crest cells populate the future facial region and produce ectomesenchyme-derived tissues, such as cartilage, bone, dermis, smooth muscle, adipocytes, and many others. However, the contribution of individual neural crest cells to certain facial locations and the general spatial clonal organization of the ectomesenchyme have not been determined. We investigated how neural crest cells give rise to clonally organized ectomesenchyme and how this early ectomesenchyme behaves during the developmental processes that shape the face. Using a combination of mouse and zebrafish models, we analyzed individual migration, cell crowd movement, oriented cell division, clonal spatial overlapping, and multilineage differentiation. The early face appears to be built from multiple spatially defined overlapping ectomesenchymal clones. During early face development, these clones remain oligopotent and generate various tissues in a given location. By combining clonal analysis, computer simulations, mouse mutants, and live imaging, we show that facial shaping results from an array of local cellular activities in the ectomesenchyme. These activities mostly involve oriented divisions and crowd movements of cells during morphogenetic events. Cellular behavior that can be recognized as individual cell migration is very limited and short-ranged and likely results from cellular mixing due to the proliferation activity of the tissue. These cellular mechanisms resemble the strategy behind limb bud morphogenesis, suggesting the possibility of common principles and deep homology between facial and limb outgrowth
Aperiodic dynamical decoupling sequences in presence of pulse errors
Dynamical decoupling (DD) is a promising tool for preserving the quantum
states of qubits. However, small imperfections in the control pulses can
seriously affect the fidelity of decoupling, and qualitatively change the
evolution of the controlled system at long times. Using both analytical and
numerical tools, we theoretically investigate the effect of the pulse errors
accumulation for two aperiodic DD sequences, the Uhrig's DD UDD) protocol [G.
S. Uhrig, Phys. Rev. Lett. {\bf 98}, 100504 (2007)], and the Quadratic DD (QDD)
protocol [J. R. West, B. H. Fong and D. A. Lidar, Phys. Rev. Lett {\bf 104},
130501 (2010)]. We consider the implementation of these sequences using the
electron spins of phosphorus donors in silicon, where DD sequences are applied
to suppress dephasing of the donor spins. The dependence of the decoupling
fidelity on different initial states of the spins is the focus of our study. We
investigate in detail the initial drop in the DD fidelity, and its long-term
saturation. We also demonstrate that by applying the control pulses along
different directions, the performance of QDD protocols can be noticeably
improved, and explain the reason of such an improvement. Our results can be
useful for future implementations of the aperiodic decoupling protocols, and
for better understanding of the impact of errors on quantum control of spins.Comment: updated reference
Steady state of atoms in a resonant field with elliptical polarization
We present a complete set of analytical and invariant expressions for the
steady-state density matrix of atoms in a resonant radiation field with
arbitrary intensity and polarization. The field drives the closed dipole
transition with arbitrary values of the angular momenta and of
the ground and excited state. The steady-state density matrix is expressed in
terms of spherical harmonics of a complex direction given by the field
polarization vector. The generalization to the case of broad-band radiation is
given. We indicate various applications of these results.Comment: revtex, 26 pages, including 3 eps figures; PRA accepted for
publication;v2 three typos are fixe
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