145 research outputs found
Potential Energy Surface for H_2 Dissociation over Pd(100)
The potential energy surface (PES) of dissociative adsorption of H_2 on
Pd(100) is investigated using density functional theory and the full-potential
linear augmented plane wave (FP-LAPW) method. Several dissociation pathways are
identified which have a vanishing energy barrier. A pronounced dependence of
the potential energy on ``cartwheel'' rotations of the molecular axis is found.
The calculated PES shows no indication of the presence of a precursor state in
front of the surface. Both results indicate that steering effects determine the
observed decrease of the sticking coefficient at low energies of the H_2
molecules. We show that the topology of the PES is related to the dependence of
the covalent H(s)-Pd(d) interactions on the orientation of the H_2 molecule.Comment: RevTeX, 8 pages, 5 figures in uufiles forma
Episiotomy practices in France: epidemiology and risk factors in non-operative vaginal deliveries
Episiotomy use has decreased due to the lack of evidence on its protective effects from maternal
obstetric anal sphincter injuries. Indications for episiotomy vary considerably and there are a great
variety of factors associated with its use. The aim of this article is to describe the episiotomy rate
in France between 2013 and 2017 and the factors associated with its use in non-operative vaginal
deliveries. In this retrospective population-based cohort study, we included vaginal deliveries
performed in French hospitals (N= 584) and for which parity was coded. The variable of interest was
the rate of episiotomy, particularly for non-operative vaginal deliveries. Trends in the episiotomy rates
were studied using the Cochran-Armitage test. Hierarchical logistic regression was used to identify
variables associated with episiotomy according to maternal age and parity. Between 2013 and 2017,
French episiotomy rates fell from 21.6 to 14.3% for all vaginal deliveries (p< 0.01), and from 15.5
to 9.3% (p< 0.01) for all non-operative vaginal deliveries. Among non-operative vaginal deliveries,
epidural analgesia, non-reassuring fetal heart rate, meconium in the amniotic fluid, shoulder dystocia,
and newborn weight (≥4,000 g) were risk factors for episiotomy, both for nulliparous and multiparous
women. On the contrary, prematurity reduced the risk of its use. For nulliparous women, breech
presentation was also a risk factor for episiotomy, and for multiparous women, scarred uterus and
multiple pregnancies were risk factors. In France, despite a reduction in episiotomy use over the last
few years, the factors associated with episiotomy have not changed and are similar to the literature.
This suggests that the decrease in episiotomies in France is an overall tendency which is probably
related to improved care strategies that have been relayed by hospital teams and perinatal networks
Ab initio Quantum and ab initio Molecular Dynamics of the Dissociative Adsorption of Hydrogen on Pd(100)
The dissociative adsorption of hydrogen on Pd(100) has been studied by ab
initio quantum dynamics and ab initio molecular dynamics calculations. Treating
all hydrogen degrees of freedom as dynamical coordinates implies a high
dimensionality and requires statistical averages over thousands of
trajectories. An efficient and accurate treatment of such extensive statistics
is achieved in two steps: In a first step we evaluate the ab initio potential
energy surface (PES) and determine an analytical representation. Then, in an
independent second step dynamical calculations are performed on the analytical
representation of the PES. Thus the dissociation dynamics is investigated
without any crucial assumption except for the Born-Oppenheimer approximation
which is anyhow employed when density-functional theory calculations are
performed. The ab initio molecular dynamics is compared to detailed quantum
dynamical calculations on exactly the same ab initio PES. The occurence of
quantum oscillations in the sticking probability as a function of kinetic
energy is addressed. They turn out to be very sensitive to the symmetry of the
initial conditions. At low kinetic energies sticking is dominated by the
steering effect which is illustrated using classical trajectories. The steering
effects depends on the kinetic energy, but not on the mass of the molecules.
Zero-point effects lead to strong differences between quantum and classical
calculations of the sticking probability. The dependence of the sticking
probability on the angle of incidence is analysed; it is found to be in good
agreement with experimental data. The results show that the determination of
the potential energy surface combined with high-dimensional dynamical
calculations, in which all relevant degrees of freedon are taken into account,
leads to a detailed understanding of the dissociation dynamics of hydrogen at a
transition metal surface.Comment: 15 pages, 9 figures, subm. to Phys. Rev.
Spectroscopic ellipsometry study of the In1-x Gax Asy P1-y / InP Heterojunctions grown by metalorganic chemical-vapor deposition
The dielectric functions of InP, In0.53Ga0.47As, and In0.75Ga0.25As0.5P0.5 epitaxial layers have been measured using a polarization modulation spectroscopic ellipsometer in the 1.5 to 5.3 eV region. The oxide removal procedure has been carefully checked by comparing spectroscopic ellipsometry and x ray photoelectron spectroscopy measurements. These reference data have been used to investigate the structural nature of metalorganic chemical vapor deposition grown In0.53Ga0.47As/InP and In0.75Ga0.25As0.5P0.5/InP heterojunctions, currently used for photodiodes and laser diodes. The sharpness of the interfaces has been systematically compared for the two types of heterojunctions: In1 xGaxAsy/InP and InP/In1 xGaxAsyP1 y. The sharpest interface is obtained for InP growth on In0.75Ga0.25As0.5P0.5 where the interface region is estimated to be (10±10) Å thick. The importance of performing in situ SE measurements is emphasized
Simultaneous measurement of film and substrate optical parameters from multiple sample single wavelength ellipsometric data
A procedure has been developed for the accurate measurement of film and substrate optical
parameters from the multiple
sample single-wavelength ellipsometric data. The dimensional
reduction of the unknowns from
newly formulated ellipsometric functions, the root selection and
the thickness-dependent integer
deduction enhance the rapidity of finding solutions and the
convergence from a wide range of
initial guesses while avoiding undesirable solutions. An error
analysis carried out shows that the
procedure is very resistant to the propagation of angular errors
and allows the estimation of optimum film
thickness ranges under which the parameters can be
accurately found. The standard SiO2/Si
structure is particularly studied using the procedure that
is further illustrated with the experimental
data on Ni/BK7-glass structures. The SiO2 film
refractive index and thickness are thus shown to be accurately determined when sought along
with the substrate optical constants. Moreover, the film and substrate real indexes are not altered
in the presence of an interface layer between the film and the substrate while its existence is
indicated by a systematic lowering of the Si substrate extinction coefficient. The procedure can
be efficiently used in the continuous real-time optical characterization of films growing on
substrates
Effect of deposition temperature on surface acoustic wave velocity of aluminum nitride films determined by Brillouin spectroscopy
3 pages, 4 figures.-- PACS: 78.35.+c; 68.35.Iv;
78.66.FdBrillouin spectroscopy has been used to study the effect of the deposition temperature on the surface acoustic wave (SAW) propagation velocity of aluminum nitride (AlN) films. The results show a dependence of the SAW propagation velocity on the growth temperature of AlN films. The highest value of acoustic velocity was obtained for the film elaborated without heating. Structural characterization of the AlN films synthesized at various deposition temperatures was carried out by x-ray diffraction. These analyses pointed out that the deposition temperature influences the standard deviation of (002) AlN film preferred orientation. The growth temperature clearly influences the acoustical and crystalline properties of AlN thin films.This work was partially supported by the Spanish Ministery
of Education and Science (Project No. MAT-2003-
01880), the autonomous community of Madrid (Project No.
07N/0077/2002), and by a bilateral cooperation between
CNRS and CSIC within the frame of the Picasso program
common project Assouar/Jiménez (France) and Acción Integrada
2004FR0008 (Spain).Peer reviewe
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