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 $K$ 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 “Comment on ‘Electron-induced bond breaking at low energies in HCOOH and glycine: The role of very short-lived σ anion states’ ”

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 σ*(OH) orbital. We argue that their objections do not hold up to scrutiny

Reply to “Comment on ‘Electron-induced bond breaking at low energies in HCOOH and glycine: The role of very short-lived σ anion states’ ”

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 σ*(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 electron–CF\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 CF3Cl− 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

Scale Invariance in Road Networks

We study the topological and geographic structure of the national road networks of the United States, England and Denmark. By transforming these networks into their dual representation, where roads are vertices and an edge connects two vertices if the corresponding roads ever intersect, we show that they exhibit both topological and geographic scale invariance. That is, we show that for sufficiently large geographic areas, the dual degree distribution follows a power law with exponent 2.2 < alpha < 2.4, and that journeys, regardless of their length, have a largely identical structure. To explain these properties, we introduce and analyze a simple fractal model of road placement that reproduces the observed structure, and suggests a testable connection between the scaling exponent alpha and the fractal dimensions governing the placement of roads and intersections.Comment: 6 pages, 10 figures; revision incorporates more rigorous statistical analyses; matches journal versio