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We discuss lower-than-predicted collisional-excitation cross sections for helium-like xenon measured at an Electron Beam Ion Trap facility. In a review paper (H. Chen and P. Beiersdorfer. Can. J. Phys. 86, 55 (2008)), the authors find a significant effect due to the Breit interaction between the free and the bound electrons in the excitation process of He-like xenon. The authors state that the agreement between the measured and calculated cross section values can only be found when the generalized Breit interaction is included in the calculations. We have performed new calculations with a multi-configuration Dirac-Fock code, as well as with the Penn State University suite of codes, and our conclusions are that the contribution of the Breit interaction is much lower than found in the calculations presented in the abovementioned article. In fact, our predictions are subsequently almost twice as large as the experimental values. We present these considerations in hopes of motivating new experimental investigations. Key words: atomic physics, electron-impact excitation, electron beam ion trap, multi-configuration Dirac-Fock, generalized Breit interaction. Nous discutons ici de mesures de sections efficaces d'excitation collisionnelle pour les ions heliumoides de xenon realisees aupres de l'installation Electron Beam Ion Trap. Ces sections mesurees se sont averees plus faibles que les predictions theoriques. Dans une publication recente (H. Chen et P. Beiersdorfer. Can. J. Phys. 86, 55 (2008)), les auteurs trouvent un effet significatif de l'interaction de Breit entre les electrons lies et libres dans le processus d'excitation du xenon heliumoide. Les auteurs soutiennent que l'accord entre les valeurs mesurees et calculees des sections efficaces n'est possible qu'en introduisant l'interaction de Breit generalisee dans les calculs. Nous avons effectue de nouveaux calculs avec un code multi-configurations Dirac-Fock ainsi qu'avec la suite de codes de l'universite Penn State et nos conclusions sont que la contribution de l'interaction de Breit est beaucoup plus faible que celle trouvee dans les calculs cites plus haut. En fait, nos predictions sont deux fois plus importantes que les valeurs experimentales. Nous les presentons dans l'espoir de motiver de nouvelles etudes experimentales. Mots-cles: physique atomique, excitation par impact electronique, piege ionique a faisceau electronique, Dirac-Fock multi-configurations, interaction de Breit generalisee., 1. Introduction A few years ago, in a very useful review paper entitled 'Electronimpact excitation cross-section measurements at Electron Beam Ion Traps (EBITs) from 1986 to 2006', Chen and Beiersdorfer [...]

We present a review of two methods used to model recent LCLS experimental results for the 3C/3D line intensity ratio of Fe XVII (S. Bernitt, et al. Nature, 492, 225 (2012)), the time-dependent collisional-radiative method and the density matrix approach. These are described and applied to a two-level atomic system excited by an X-ray free electron laser. A range of pulse parameters is explored and the effects on the predicted Fe XVII 3C and 3D line intensity ratio are calculated. To investigate the behavior of the predicted line intensity ratio, a particular pair of A-values for the 3C and 3D transitions was chosen (2.22 x [10.sup.13][s.sup.-1] and 6.02 x [10.sup.12] [s.sup.-1] for 3C and 3D, respectively), but our conclusions are independent of the precise values. We also reaffirm the conclusions from Oreshkina et al. (N.S. Oreshkina, et al. Phys. Rev. Lett. 113,143001 (2014); Ibid. J Phys. B, 49,094003 (2015)): the nonlinear effects in the density matrix are important and the reduction in the Fe XVII 3C/3D line intensity ratio is sensitive to the laser pulse parameters, namely, pulse duration, pulse intensity, and laser bandwidth. It is also shown that for both models the lowering of the 3C/3D line intensity ratio below the expected time-independent oscillator strength ratio has a significant contribution due to its emission from the plasma after the laser pulse has left the plasma volume. Laser intensities above ~1 x [10.sup.12] W/[cm.sup.2] are required for a reduction in the 3C/3D line intensity ratio below the expected time-independent oscillator strength ratio. Key words: Fe XVII, XFEL, LCLS, oscillator strength, non-equilibrium. Nous presentons une revue de deux methodes, la methode collision-radiation dependante du temps et l'approche de la matrice de densite, afin de modeliser les resultats experimentaux au LCLS du SLAC pour le rapport d'intensite de raies 3C/3D, ou plus explicitement [2[p.sup.5]3d ([sup.1][P.sub.1]) [right arrow] 2[p.sup.6] ([sup.1][S.sub.1]) / 2[p.sup.5]3d ([sup.3][D.sub.1]) [right arrow] 2[p.sup.6] (1S1)] du Fe XVII (S. Bernitt, et al. Nature, 492, 225 (2012)). [C.sub.e]s methodes sont decrites et appliquees a un systeme atomique a deux niveaux excite par un laser X a electrons libres. Nous explorons un domaine de parametres d'impulsion X et nous calculons les effets predits sur le rapport entre les intensites de raies 3C et 3D du Fe XVII. Afin de sonder le comportement des rapports d'intensite, nous choisissons une paire de valeurs pour l'absorbance A, 2,22 x [10.sup.-13] [s.sup.-1] et 6,02 x [10.sup.12] [s.sup.-1] pour 3C et 3D respectivement, mais nos resultats sont independants de ce choix. Nous reaffirmons les conclusions de Oreshkina et al. (N.S. Oreshkina, et al. Phys. Rev. Lett. 113,143001 (2014); Ibid. J. Phys. B, 49, 094003 (2015)): les effets non lineaires dans la matrice de densite sont importants et la reduction du rapport d'intensite 3C/3D du Fe XVII est sensible aux parametres de l'impulsion laser, nommement la duree de l'impulsion, son intensite et la largeur de bande du laser. Nous montrons aussi que pour les deux modeles, la diminution du rapport 3C/3D sous le rapport attendu des forces d'oscillateur independantes du temps recoit une contribution significative provenant du plasma apres que l'impulsion laser ait quitte le volume du plasma. Des intensites au dessus de 1 x [10.sup.12] W/[cm.sup.2] sont requises pour une reduction du rapport 3C/3D sous le rapport attendu du rapport des forces d'oscillateur independantes du temps. [Traduit par la Redaction] Mots-cles: Fe XVII, LXEL (laser X a electrons libres), LCLS, force d'oscillateur, hors d'equilibre., 1. Introduction Spectral emission from Fe XVII can be used as a valuable plasma diagnostic for both laboratory and astrophysical plasmas [1, 2]. The ratio of the 3C line intensity [...]

Nearly a century ago it was recognized (1) that radiation absorption by stellar matter controls the internal temperature profiles within stars. Laboratory opacity measurements, however, have never been performed at stellar interior conditions, introducing uncertainties in stellar models (2-5). A particular problem arose (2,3,6,8) when refined photosphere spectral analysis (9,10) led to reductions of 30-50 per cent in the inferred amounts of carbon, nitrogen and oxygen in the Sun. Standard solar models (11) using the revised element abundances disagree with helioseismic observations that determine the internal solar structure using acoustic oscillations. This could be resolved if the true mean opacity for the solar interior matter were roughly 15 per cent higher than predicted (2,3,6-8), because increased opacity compensates for the decreased element abundances. Iron accounts for a quarter of the total opacity (2,12) at the solar radiation/convection zone boundary. Here we report measurements of wavelength-resolved iron opacity at electron temperatures of 1.9-2.3 million kelvin and electron densities of (0.7-4.0) X [10.sup.22] per cubic centimetre, conditions very similar to those in the solar region that affects the discrepancy the most: the radiation/convection zone boundary. The measured wavelength-dependent opacity is 30-400 per cent higher than predicted. This represents roughly half the change in the mean opacity needed to resolve the solar discrepancy' even though iron is only one of many elements that contribute to opacity., In the region of the solar radiation/convection zone boundary (11), the electron temperature [T.sub.e] ≅ 2.1 X [10.sup.6] K, the electron density [n.sub.e] ≅ 9 X [10.sup.22] [cm.sup.-3], and the [...]

The population of magnetic sublevels in hydrogen-like uranium ions has been investigated in relativistic ion- atom collisions by observing the subsequent X-ray emission. Using the gas target at the experimental storage ring facility we observed the angular emission of Lyman-α radiation from hydrogen-like uranium ions. The alignment parameter for three different interaction energies was measured and found to agree well with theory. In addition, the use of different gas targets allowed for the electron-impact excitation process to be observed. PACS Nos: 34.10.+x, 34.80.Dp, 33.20.Rm, 32.70.Fw La population des sous-niveaux magnetiques des ions hydrogenoides d'uranium a ete etudiee dans les collisions relativistes ion-atome en observant remission subsequente de rayons-X. Utilisant la cible gazeuse d'un anneau de stockage, nous avons observe remission angulaire du rayonnement Lyman-α d'ions hydrogenoi'des d'uranium. Le parametre d'alignement pour les trois energies d'interaction a ete mesuree et a ete trouve en accord avec la theorie. En outre, l'utilisation de differents types de gaz comme cible a permis l'observation le processus d'excitation par impact electronique. [Traduit par la Redaction], 1. Introduction The polarization and anisotropic emission of atomic radiation is indicative of a preferred direction in space. Such directionality appears in environments with intense charged particle beams, such as [...]

We have obtained expressions for the creation, destruction, and transfer of atomic multipole moments by electron scattering under relativistic conditions. More specifically, we have obtained separate expressions for different-level processes (inelastic scattering) and for same-level processes (elastic and inelastic scattering). The cross sections for different-level processes are expressed in terms of inelastic magnetic sublevel cross sections, except for the coherence transfer cross section, which is expressed in terms of an angular integral of a product of inelastic magnetic sublevel amplitudes. The same-level cross sections are expressed in terms of the imaginary part of the elastic forward scattering amplitude and in terms of elastic scattering magnetic sublevel cross sections, except for the coherence transfer cross section, which is expressed in terms of the (complex) forward elastic scattering amplitudes and an angular integral of a product of elastic scattering magnetic sublevel amplitudes. If the collisional model supports the optical theorem, then the same-level cross sections can be rewritten in such a form that they are broken up into two parts: an elastic scattering part and an inelastic scattering part. In carrying out this work, we have used the density matrix formalism of Fano and Blum in combination with the electron scattering formalism of Gell-Mann and Goldberger. PACS Nos: 34.80Pa, 34.80 Bm, 34.80 Dp, 52.20 Hv Nous avons obtenu des expressions pour la creation, la destruction et le transfert de moments multipolaires atomiques par diffusion d'electrons relativistes. Plus precisement, nous avons obtenu des expressions differentes pour des mecanismes entre des niveaux differents (diffusion inelastique) et pour des mecanismes impliquant le meme niveau (diffusion elastique et inelastique). Les sections efficaces pour les mecanismes entre niveaux differents sont exprimees en fonction de sections efficaces inelastiques de sous-niveaux magnetiques, sauf pour la section efficace de transfert de coherence, qui est exprimee en fonction d' une integrale angulaire d' un produit d' amplitudes inelastiques de sous-niveaux magnetiques. Les sections efficaces impliquant le meme niveau sont donnees en fonction de la partie imaginaire d'une amplitude de diffusion avant elastique et en fonction de sections efficaces elastiques de sous-niveaux magnetiques, sauf pour la section efficace de transfert de coherence, qui est exprimee en fonction d'amplitudes (complexes) elastiques de diffusion avant et une integrale angulaire d'un produit d'amplitudes elastiques de sous-niveaux magnetiques. Si le modele de collisions supporte le theoreme optique, alors les sections efficaces impliquant le meme niveau peuvent etre reecrites sous une forme ou elles sont brisees en deux parties : une partie elastique et une partie inelastique. Dans le cours de ce travail, nous avons utilise le formalisme de matrices de densite de Fano et Blum avec le formalisme de diffusion d'electrons de Gell-Mann et Goldberger. [Traduit par la Redaction], 1. Introduction In [1] and [2], cross sections for the creation, destruction, and transfer of atomic multipole moments by electron scattering have been defined using the wave-packet propagation scheme along [...]

Recently, the contribution of the generalized Breit interaction to electron impact ionization was identified for the first time in a high‐Z system, namely, hydrogen‐like uranium. This study employed a measurement of the relative population of the j = 1/2 and j = 3/2 states of the L shell by projectile excitation in collision of U91+ with hydrogen and nitrogen targets. However, for a rigorous test of ion–atom collision theory, also the absolute excitation cross sections are of great importance. In the present work, we report on our efforts to extend the previous study to a determination of the absolute projectile excitation cross sections by normalization to the well‐known radiative electron capture process. [ABSTRACT FROM AUTHOR]