Valerie P. Myerscough

Bio: Valerie P. Myerscough is an academic researcher from Queen Mary University of London. The author has an hindex of 1, co-authored 1 publications receiving 4 citations.

The width of spectral lines

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...

Free-Free Transitions of Electron-Atom Systems in Intense Radiation Fields

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.

Electron-ion and ion-ion collisions in astrophysics

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.

A Comparison of Orthogonalized Plane Wave and Augmented Plane Wave Methods for Calculating Photodetachment Cross-Sections

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.