501 research outputs found
Elastic contact to a coated half-space - Effective elastic modulus and real penetration
A new approach to the contact to coated elastic materials is presented. A
relatively simple numerical algorithm based on an exact integral formulation of
the elastic contact of an axisymmetric indenter to a coated substrate is
detailed. It provides contact force and penetration as a function of the
contact radius. Computations were carried out for substrate to layer moduli
ratios ranging from 0.01 to 100 and various indenter shapes. Computed
equivalent moduli showed good agreement with the Gao model for mismatch ratios
ranging from 0.5 to 2. Beyond this range, substantial effects of inhomogeneous
strain istribution are evidenced. An empirical function is proposed to fit the
equivalent modulus. More importantly, if the indenter is not flat-ended, the
simple relation between contact radius and penetration valid for homogeneous
substrates breaks down. If neglected, this phenomenon leads to significant
errors in the evaluation of the contact radius in depth-sensing indentation on
coated substrates with large elastic modulus mismatch
Enhancement of the Benjamin-Feir instability with dissipation
It is shown that there is an overlooked mechanism whereby some kinds of
dissipation can enhance the Benjamin-Feir instability of water waves. This
observation is new, and although it is counterintuitive, it is due to the fact
that the Benjamin-Feir instability involves the collision of modes with
opposite energy sign (relative to the carrier wave), and it is the negative
energy perturbations which are enhanced.Comment: 15 pages, 2 figures To download more papers, go to
http://www.cmla.ens-cachan.fr/~dias. Physics of Fluids (2007) to appea
Photodetachment of H near a partial reflecting surface
Theoretical and interpretative study on the subject of photodetachment of
H near a partial reflecting surface is presented, and the absorption
effect of the surface is investigated on the total and differential cross
sections using a theoretical imaging method. To understand the absorption
effect, a reflection parameter is introduced as a multiplicative factor to
the outgoing detached-electron wave of H propagating toward the wall. The
reflection parameter measures, how much electron wave would reflect from the
surface; K=0 corresponds to no reflection and K=1 corresponds to the total
reflection.Comment: 8 pages, 4 figure
Positronium collisions with rare-gas atoms
We calculate elastic scattering of positronium (Ps) by the Xe atom using the
recently developed pseudopotential method [I. I. Fabrikant and G. F. Gribakin,
Phys. Rev. A 90, 052717 (2014)] and review general features of Ps scattering
from heavier rare-gas atoms: Ar, Kr, and Xe. The total scattering cross section
is dominated by two contributions: elastic scattering and Ps ionization
(breakup). To calculate the Ps ionization cross sections we use the
binary-encounter method for Ps collisions with an atomic target. Our results
for the ionization cross section agree well with previous calculations carried
out in the impulse approximation. Our total Ps-Xe cross section, when plotted
as a function of the projectile velocity, exhibits similarity with the
electron-Xe cross section for the collision velocities higher than 0.8 a.u.,
and agrees very well with the measurements at Ps velocities above 0.5 a.u.Comment: 7 pages, 7 figures, submitted to J. Phys.
On the Complexity of Local Search for Weighted Standard Set Problems
In this paper, we study the complexity of computing locally optimal solutions
for weighted versions of standard set problems such as SetCover, SetPacking,
and many more. For our investigation, we use the framework of PLS, as defined
in Johnson et al., [JPY88]. We show that for most of these problems, computing
a locally optimal solution is already PLS-complete for a simple neighborhood of
size one. For the local search versions of weighted SetPacking and SetCover, we
derive tight bounds for a simple neighborhood of size two. To the best of our
knowledge, these are one of the very few PLS results about local search for
weighted standard set problems
Reply to âComment on âElectron-induced bond breaking at low energies in HCOOH and glycine: The role of very short-lived Ï anion statesâ â
Rescigno et al. [Phys. Rev. A 80, 046701 (2009)] criticized our theoretical treatment of dissociative electron attachment in formic acid in which we show that this process can be explained by electron attachment into the temporary negative ion state formed by occupation of the Ï*(OH) orbital. We argue that their objections do not hold up to scrutiny
Reply to âComment on âElectron-induced bond breaking at low energies in HCOOH and glycine: The role of very short-lived Ï anion statesâ â
Rescigno et al. [Phys. Rev. A 80, 046701 (2009)] criticized our theoretical treatment of dissociative electron attachment in formic acid in which we show that this process can be explained by electron attachment into the temporary negative ion state formed by occupation of the Ï*(OH) orbital. We argue that their objections do not hold up to scrutiny
Nonlocal theory of dissociative electron attachment to H\u3csub\u3e2\u3c/sub\u3e and HF molecules
We develop a method for calculation of dissociative electron attachment cross sections based on the Fano- Feshbach projection-operator approach. The coupling between the diabatic state and electron continuum is calculated with the inclusion of orthogonality scattering and long-range electron-molecule interaction. The dynamics of nuclear motion in the non-local complex potential is treated by semiclassical theory. We apply the theory to the calculation of dissociative attachment to the H2 and HF molecules. Our results for attachment to ground-state hydrogen molecules and the near-threshold vibrational enhancement of H2 are essentially the same as previous non-local results. However, the shape of the energy dependence of the cross section for attachment to vibrationally excited states of H2 is slightly different. The calculated value for the total attachment cross section to the ground state of HF is consistent with the little experimental data available; more definite conclusions are difficult because of the approximate nature of the experimental results. The results for the vibrational enhancement are in very good agreement with experiment
Semiempirical \u3ci\u3eR\u3c/i\u3e-matrix theory of low energy electronâCF\u3csub\u3e3\u3c/sub\u3eCl inelastic scattering
We apply a semiempirical R-matrix theory to calculations of vibrational excitation and dissociative attachment in the CF3Cl molecule for electron energies below about 3 eV. We employ two sets of model parameters corresponding to two different forms of the CF3Clâ potential curve. We find that our present, ab initio calculated anion curve gives vibrational excitation and dissociative attachment cross sections in good agreement with experimental measurements. We also compare the results of our theory with those of a recently published classical theory
Near-threshold behavior of positronium-antiproton scattering
Using the convergent close-coupling theory we study the threshold behavior of cross sections for positronium (Ps) of energy E scattering on antiprotons. In the case of Ps(1s) elastic scattering, simple power laws are observed for all partial waves studied. The partial-wave summed cross section is nearly constant, and dominates the antihydrogen formation cross section at all considered energies, even though the latter is exothermic and behaves as 1/E1/2. For Ps(2s), oscillations spanning orders of magnitude on top of the 1/E behavior are found in the elastic and quasielastic cross sections. The antihydrogen formation is influenced by dipole-supported resonances below the threshold of inelastic processes. Resonance energies form a geometric progression relative to the threshold. The exothermic antihydrogen formation cross sections behave as 1/E at low energies, but are oscillation free. We demonstrate that all these rich features are reproduced by the threshold theory developed by Gailiti
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