Chapter 5 – atomic processes in astrophysical plasmas
01 Jan 1972-pp 293-397
About: The article was published on 1972-01-01. It has received 4 citations till now.
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TL;DR: In this article, a spectral line emitted by an atom or molecule (throughout the article, spectral lines seen in emission ere described; however, the theory applies equally well to the absorption of radiation in plasmas) has a finite width and there are three distinct physical processes which contribute to this broadening.
Abstract: A spectral line emitted by an atom or molecule (throughout the article, spectral lines seen in emission ere described; however, the theory applies equally well to the absorption of radiation in plasmas) has a finite width and there are three distinct physical processes which contribute to this broadening. The energy levels between which the transition takes place have a natural width, which arises from the reaction of the radiation field on matter. This means that an atom has a finite time in any level before sponteneou decay takes place. Also the emitting atom may be moving in the line of sight, which produces an apparent change in the line frequency through the Doppler effect. Finally, broadening of the spectral line is caused by the interaction of the emitting atom with surrounding particles in the plasma. Thus a detailed analysis of line shapes can yield much information on the density, temperature and motion of plasmas, whether they are high density plasma created in the laboratory, or the l...
21 citations
01 Jan 1989
TL;DR: In this paper, a survey of free-free transitions in electron-atom collisions occurring in a radiation field is presented, although the focus is on theoretical aspects, although also discuss the existing experimental evidence.
Abstract: We present a survey of free-free transitions (FFT) in electron-atom collisions occurring in a radiation field. Our emphasis is on theoretical aspects, although we also discuss the existing experimental evidence. The presentation is self-contained and rather detailed (we actually derive most of the basic formulas). The semiclassical treatment is adopted for the interaction of the particles with the radiation field. We envisage intensities ranging from the relatively weak ones of optical sources and CW lasers, to those of the superintense lasers now in operation, yielding values in excess of one atomic unit (3.5 · 1016 W/cm2). Therefore, we consider both the standard perturbation theory, applicable to the former case, and the nonperturbative methods of solution developed for the latter case, which are now in the center of attention. We handle all these by a unified method, based on Floquet theory. This applies to the case of a monochromatic field, the only one we shall discuss in detail.
5 citations
01 Jan 1986
TL;DR: In this paper, the authors pointed out the importance of excitation, ionization and recombination collisions in determining the state of the solar corona, and the nature of the emitted spectra.
Abstract: There has long been a close association between astrophysical studies and the development of atomic collision theory. This interaction took a major step forward when, some forty years ago, the real nature of the solar corona was first understood as a very hot tenuous plasma. The formulation of the so-called “coronal equilibrium” equations, (1948), pointed to the importance of excitation, ionization and recombination collisions in determining the state of the plasma, and the nature of the emitted spectra. In the earlier “local thermodynamic equilibrium” (LTE) model, the plasma state was determined only by its temperature and the laws of statistical mechanics.
1 citations
TL;DR: In this article, the orthogonalized plane wave (OPW) method was tested by the computation of photodetachment cross-sections and angular distributions for gaseous halide anions.
Abstract: s The orthogonalized plane wave (OPW) method of calculating electronic continuum wave- functions is tested by the computation of photodetachment cross-sections and angular distributions for gaseous halide anions. The results are compared to those obtained by a related augmented plane wave (APW) method involving the exact solution of a single-particle Schriidinger equation containing a piecewise Coulombic potential energy. These comparisons, as well as other involving experimental and theoretical cross-sections from the literature, indicate that OPW cross-sections are, at best, only semi-quantitatively reliable for describing photodetachment even at low photon energies, and that OPW cross-sections should be calculated using the dipole length operator rather than the dipole velocity operator. La mtthode des ondes planes orthogonalistes (OPW) pour calculer des fonctions d'onde du spectre continu a Ct6 test6 par un calcul des sections efficaces et des distributions angulaires assocites a des processus de photodttachement pour des anions halogtno'ides. Les rksultats sont comparts a ceux obtenus par la mtthode des ondes planes augmenttes (APW), qui comporte la solution exacte d'une tquation de Schrodinger B une particule, avec un potentiel coulombien par morceaux. Ces com- paraisons ainsi que d'autres bastes sur des sections efficaces exptrimentales et thtoriques de la litttrature indiquent que les sections efficaces OPW ne sont guerre dignes de confiance m&me pour des tnergies photoniques basses, et qu'elles doivent &re calculies avec I'optrateur dipolaire de longueur plutdt qu'avec celui de vitesse. Die OPW-Methode (orthogonalisierte ebene Wellen) fur die Berechnung von elektronischen Kontinuumfunktionen wird rnit einer Berechnung der mit Photoauslosevorgangen zusammenhangenden Querschnitte und Winkelverteilungen fur Halogenidanionen getestet. Die Ergebnisse werden mit denen verglichen, die durch eine APW-Methode erhalten werden. In dieser Methode wird eine exakte Lasung einer Schrodingergleichung fur ein Teilchen rnit einem stuckweise coulornbischen Potential erhalten. Diese Vergleichungen als auch andere, basierend auf experirnen- telle und theoretische Literaturquerschnitte, deuten an dass die OPW-Querschnitte nicht einmal fur geringe Photonenenergien zuverlassig sind, und dass sie mit dem Dipollange-operator eher als mit dem Dipolgeschwindigkeitsoperator berechnet werden sollen.
1 citations
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1,170 citations
TL;DR: In this article, an approximate formula is proposed to compute the cross-section for excitation by electron impact for the op, where the crosssection is defined as the number of electron impacts.
Abstract: An approximate formula is proposed to compute the cross-section for excitation by electron impact for the op
996 citations
TL;DR: In this paper, it is shown that for the simpler systems (i.e., systems with a single electron outside closed shells) the method gives remarkably accurate results; indeed, it appears superior to the normal rather laborious procedure involving the computation of the necessary wave functions, in each individual case, by solution of the appropriate Hartree or Fock differential equation.
Abstract: It is shown that in calculating transition integrals it is permissible to neglect the departure of the potential of an atom or ion from its asymptotic Coulomb form. This enables a general analytical expression for the transition integral to be derived. Tables are compiled from which the absolute strengths of large numbers of spectral lines can at once be obtained if the term values of the upper and lower levels are known. s-p , p-d and d-f transitions are all treated. Comparison with experimental data shows that for the simpler systems (i. e. systems with a single electron outside closed shells) the method gives remarkably accurate results; indeed, it appears superior to the normal rather laborious procedure involving the computation of the necessary wave functions, in each individual case, by solution of the appropriate Hartree or Fock differential equation. The method (in its most elementary form) may not be so satisfactory for complex systems (i. e. systems with unclosed shells) owing to difficulties associated with the identification of certain energy parameters. However, the rather scanty comparison data available suggest that even for such systems it yields useful (and in some cases precise) information on the line strengths. Incidentally, in the course of the work the accuracy of a few wave functions based on the self consistent field approximation (including exchange) was tested by using them to evaluate line strengths from both the dipole moment and the dipole velocity formulae. Appreciable defects were revealed.
942 citations
TL;DR: In this article, a generalized theory of collision broadening is developed, adequate for predicting line breadths in the microwave and infra-red regions, although it is limited to a classical picture of the relative motion of the colliding molecules as a whole.
Abstract: In this paper a generalized theory of collision broadening is developed, adequate for predicting line breadths in the microwave and infra-red regions. This theory differs from previous ones in taking into account transitions among quantum states caused by collisions, although it is limited to a classical picture of the relative motion of the colliding molecules as a whole. Formulas are derived for computing approximate line broadening collision cross-sections from known intermolecular interactions, and the results of the theory are successfully compared with experiments on self-broadening in the ammonia inversion spectrum and the vibrational band spectra of HCl and HCN. Some cases of foreign gas broadening in the microwave region are examined, but it is concluded that in general the Van der Waals interaction, commonly assumed to be the force important in causing foreign gas broadening, is not adequate to cause the observed broadenings. The more complicated types of forces which become important at short range would have to be taken into account to give good agreement with experiment in these cases.
894 citations