oalib
Search Results: 1 - 10 of 100 matches for " "
All listed articles are free for downloading (OA Articles)
Page 1 /100
Display every page Item
Resonance Averaged Photoionization Cross Sections for Astrophysical Models  [PDF]
M. A. Bautista,P. Romano,A. K. Pradhan
Physics , 1997, DOI: 10.1086/313132
Abstract: We present ground state photoionization cross sections of atoms and ions averaged over resonance structures for photoionization modeling of astrophysical sources. The detailed cross sections calculated in the close-coupling approximation using the R-matrix method, with resonances delineated at thousands of energies, are taken from the Opacity Project database TOPbase and the Iron Project, including new data for the low ionization stages of iron Fe I--V. The resonance-averaged cross sections are obtained by convolving the detailed cross sections with a Gaussian distribution over the autoionizing resonances. This procedure is expected to minimize errors in the derived ionization rates that could result from small uncertainties in computed positions of resonances, while preserving the overall resonant contribution to the cross sections in the important near threshold regions. The detailed photoionization cross sections at low photon energies are complemented by new relativistic distorted-wave calculations for Z<= 12, and from central-field calculations for Z>12 at high energies, including inner-shell ionization. The effective cross sections are then represented by a small number of points that can be readily interpolated linearly for practical applications; a Fortran subroutine and data are available. The present numerically averaged cross sections are compared with analytic fits that do not accurately represent the effective cross sections in regions dominated by resonances.
Importance of an Astrophysical Perspective for Textbook Relativity  [PDF]
Eric G. Blackman
Physics , 1997, DOI: 10.1088/0143-0807/19/2/014
Abstract: The importance of a teaching a clear definition of the ``observer'' in special relativity is highlighted using a simple astrophysical example from the exciting current research area of ``Gamma-Ray Burst'' astrophysics. The example shows that a source moving relativistically toward a single observer at rest exhibits a time ``contraction'' rather than a ``dilation'' because the light travel time between the source and observer decreases with time. Astrophysical applications of special relativity complement idealized examples with real applications and very effectively exemplify the role of a finite light travel time.
Double Differential Cross-Sections for Electron Impact Ionization of Atoms and Molecules  [PDF]
Mevlut Dogan,Melike Ulu,Zehra Nur Ozer,Murat Yavuz,Gulin Bozkurt
Journal of Spectroscopy , 2013, DOI: 10.1155/2013/192917
Abstract: The single ionizing collision between an incident electron and an atom/molecule ends up two kinds of outgoing electrons called scattered and ejected electrons. As features of electron impact ionization, these two types of electrons are indistinguishable. Double differential cross-sections (DDCS) can be obtained by measuring the energy and angular distributions of one of the two outgoing electrons with an electron analyzer. We used He, Ar, H2, and CH4 targets in order to understand the ionization mechanisms of atomic and molecular systems. We measured differential cross-sections (DCS) and double differential cross-sections at 250?eV electron impact energy. The elastic DCSs were measured for He, Ar, H2, and CH4, whereas the inelastic DCSs of He were obtained for 21P excitation level for 200?eV impact electron energy. 1. Introduction Generally, atomic and molecular physics lead to discoveries about the structure of matter at the atomic or molecular level and explain natural laws. These goals can be achieved with collision methods. Applications of the results from collision physics are of most importance to atmospheric science, laser refinement, and meteorological phenomena. In recent years, an intensive effort of experimental and theoretical work has been devoted to the study of ionization differential cross-sections of atoms and molecules through electron impact. The ionization of rare gas atoms, particularly, the cross-sections obtained with ground state ionization, is considered as benchmark data. Doubly differential cross-sections (DDCS) of ionization, as a function of ejected energy, , and the angle of the ionized electron, , contain valuable information about both the collision dynamics and the internal structure of atomic or molecular systems. This paper is divided into four main parts. First, the theoretical and experimental studies of elastic DCS and DDCS for Helium, Argon, and Hydrogen molecules and Methane molecules are reviewed. Second, the experimental apparatus and signal processing are described in detail. Then, the results of the elastic DCS and DDCS measurements for He, Ar, H2, and CH4 at 200?eV electron impact energy are presented and discussed. Finally, general conclusions are drawn from the experimental results. 2. Review Absolute elastic differential cross-sections for electron scattering from helium were measured for electron energies from 100 to 200?eV by Kurepa and Vuskovic [1], from threshold to 400?eV by Shyn [2], and from threshold to 200?eV by Trajmar et al. [3] and Fon et al. [4]. Normalized DCS below 100?eV have been measured
Elastic and rotational excitation cross sections for electron scattering by polyatomic molecules
Machado, L. E.;Brescansin, L. M.;Lee, M.-T.;
Brazilian Journal of Physics , 2002, DOI: 10.1590/S0103-97332002000400023
Abstract: in this work we present a theoretical study on electron scattering by both polar and nonpolar polyatomic molecules in the low-energy range. more specifically, we report calculated elastic and rotationally inelastic differential cross sections for electron scattering by ch4, h2o, and h2s in the (2.14-30)-ev range. exact static-exchange plus model correlation-polarization potentials are used to represent the electron-molecule interaction. the schwinger variational iterative method is used to solve the scattering equations. in addition, the adiabatic-nuclei-rotation approximation is applied to calculate rotational cross sections. the comparison of our calculated results with experimental and other theoretical data available in the literature is encouraging.
Elastic and rotational excitation cross sections for electron scattering by polyatomic molecules  [cached]
Machado L. E.,Brescansin L. M.,Lee M.-T.
Brazilian Journal of Physics , 2002,
Abstract: In this work we present a theoretical study on electron scattering by both polar and nonpolar polyatomic molecules in the low-energy range. More specifically, we report calculated elastic and rotationally inelastic differential cross sections for electron scattering by CH4, H2O, and H2S in the (2.14-30)-eV range. Exact static-exchange plus model correlation-polarization potentials are used to represent the electron-molecule interaction. The Schwinger variational iterative method is used to solve the scattering equations. In addition, the adiabatic-nuclei-rotation approximation is applied to calculate rotational cross sections. The comparison of our calculated results with experimental and other theoretical data available in the literature is encouraging.
Numerical computation of isotropic Compton scattering  [PDF]
R. Belmont
Physics , 2009, DOI: 10.1051/0004-6361/20079113
Abstract: Compton scattering is involved in many astrophysical situations. It is well known and has been studied in detail for the past fifty years. Exact formulae for the different cross sections are often complex, and essentially asymptotic expressions have been used in the past. Numerical capabilities have now developed to a point where they enable the direct use of exact formulae in sophisticated codes that deal with all kinds of interactions in plasmas. Although the numerical computation of the Compton cross section is simple in principle, its practical evaluation is often prone to accuracy issues. These can be severe in some astrophysical situations but are often not addressed properly. In this paper we investigate numerical issues related to the computation of the Compton scattering contribution to the time evolution of interacting photon and particle populations. An exact form of the isotropic Compton cross section free of numerical cancellations is derived. Its accuracy is investigated and compared to other formulae. Then, several methods to solve the kinetic equations using this cross section are studied. The regimes where existing cross sections can be evaluated numerically are given. We find that the cross section derived here allows for accurate and fast numerical evaluation for any photon and electron energy. The most efficient way to solve the kinetic equations is a method combining a direct integration of the cross section over the photon and particle distributions and a Fokker-Planck approximation. Expressions describing this combination are given.
Quantum Computation with Vibrationally Excited Molecules  [PDF]
Carmen M. Tesch,Regina de Vivie-Riedle
Physics , 2002, DOI: 10.1103/PhysRevLett.89.157901
Abstract: A new physical implementation for quantum computation is proposed. The vibrational modes of molecules are used to encode qubit systems. Global quantum logic gates are realized using shaped femtosecond laser pulses which are calculated applying optimal control theory. The scaling of the system is favourable, sources for decoherence can be eliminated. A complete set of one and two quantum gates is presented for a specific molecule. Detailed analysis regarding experimental realization shows that the structural resolution of today's pulse shapers is easiliy sufficient for pulse formation.
Development of an Astrophysical Specific Language for Big Data Computation  [PDF]
Nicolas Kamennoff,Sébastien Foucaud,Sébastien Reybier
Physics , 2012,
Abstract: Astronomy is entering in a new era of Extreme Intensive Data Computation and we have identified three major issues the new generation of projects have to face: Resource optimization, Heterogeneous Software Ecosystem and Data Transfer. We propose in this article a middleware solution offering a very modular and maintainable system for data analysis. As computations must be designed and described by specialists in astronomy, we aim at defining a friendly specific programming language to enable coding of astrophysical problems abstracted from any computer science specific issues. This way we expect substantial benefits in computing capabilities in data analysis. As a first development using our solution, we propose a cross-matching service for the Taiwan Extragalactic Astronomical Data Center.
Electron-scattering total cross sections for triatomic molecules: NO2 and H2O
Czeslaw SZMYTKOWSKI,Pawel MOZEJKO
Optica Applicata , 2006,
Abstract: Absolute total cross sections (TCSs) for electron scattering on nitrogen dioxide (NO2) molecules and on water-vapour (H2O) were measured at energies ranging from 3 to 370 eV and 0.5 to 370 eV, respectively. Measurements were carried out using an electron spectrometer with an improved angular and energy resolution. The presented experimental TCS results are at intermediate energies compared with our total cross section estimations based on calculations of elastic and ionization cross sections.
Quantum Computation and Bell-state Measurement with Double-Dot Molecules  [PDF]
Hui Zhang,Guo-Ping Guo,Tao Tu,Guang-Can Guo
Physics , 2007, DOI: 10.1103/PhysRevA.76.012335
Abstract: We propose a quantum computation architecture of double-dot molecules, where the qubit is encoded in the molecule two-electron spin states. By arranging the two dots inside each molecule perpendicular to the qubit scaling line, the interactions between neighboring qubits are largely simplified and the scaling to multi-qubit system becomes straightforward. As an Ising-model effective interaction can be expediently switched on and off between any two neighboring molecules by adjusting the potential offset between the two dots, universal two-qubit gates can be implemented without requiring time-dependent control of the tunnel coupling between the dots. A Bell-state measurement scheme for qubit encoded in double-dot singlet and triplet states is also proposed for quantum molecules arranged in this way.
Page 1 /100
Display every page Item


Home
Copyright © 2008-2017 Open Access Library. All rights reserved.