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This work presents a study of the 1p0 excited states of He that can be reached by absorption of a single photon carrying an energy close to the double ionization threshold (DIT) (79eV). Above the DIT, these states are the double continuum states, below, they are the double excited states. these two types of states are tightly coupled to the single continuum states with or without excitation of the residual ion He+, owing to their degeneracy in energy. in a one-photon process ,theses states can only be formed owing to the elecronic correlations in the system which must be well described to obtain quantitatively good results. our study is part of a line of work which at a unified description of all these doubly excited, ionized-excited, and double continuum states.We use the Hyperspherical R-Matrix with Semiclassical Outgoing Waves (HRM-SOW) method, initially dedicated to double photoionization studies. We extend it ot extract information on the single continuum. This extension allows us to compute cross sections of single photoionization with or without excitaion up to n=50 for an excess energy of 100meV just above the double ionization threshold. A deep insight into his process is given by a partial wave analysis. The results obtained shed ligh on the key role of angular and radial correlations. The numerous data we obtain on double and single ionization allow us to establish relation between these two processes. We show that single ionization with an infinite excitation of the residual ion merges into double photoionization when the excess energy is redistributed between the two electrons. it appears that this relation is valid not only for low but also for high photon energy. Since the HRM-SOW method can produce the integrated cross section for double photoionization with a hig accuracy in the low energy domain, we check the Wannier threshold law. The parameters extracted support strongly this law and are in good agreement with experimental results.We complete this work with the study of doubly excited autoionizing states. The latter manifest themselves as Fano-like resonances in the ionization cross section below the double ionization threshold. We compute partial integrated as well as differential cross sections which allow us to identify a large number of resonances up to the fifh single ionization threshold. Some of them are observed for the first time., Nous étudions dans ce mémoire de thèse les états diexcités 1po de l'atome d'hélium, situés au voisinage de son seuil de double ionisation (79 eV), atteints par absorption d'un photon. Ces états diexcités sont soit les états du double continuum situés au-dessus du seuil de double ionisation soit les états doublements excités situés en dessous. Ces deux types d'états diexcités sont dégénérés aux états du continuum simple avec ou sans excitation de l'ion résiduel He+ et sont donc fortement couplés à ces derniers. Dans un processus de photoabsorption à un photon, ces états ne peuvent être accessibles que grâce aux corrélations électroniques du système. Le travail que nous avons réalisé s'inscrit dans une démarche qui vise à étudier tous ces états par une seule et même méthode contrairement aux travaux antérieurs où les méthodes se spécialisaient dans le traitement d'un seul type d'état.Nous utilisons pour cela la méthode HRM-SOW (Hyperspherical R-Matrix with Semiclassical Outgoing Waves), initialement dédiée à l'étude du double continuum de He.Dans ce travail, elle est étendue à la description du continuum simple avec ou sans excitation. Ainsi, nous avons pu calculer les sections efficaces de photoionisation simple avec ou sans excitation jusqu'à n=50 de He à 100meV seulement au dessous de son seuil de double ionisation. Une analyse détaillée en ondes partielles a permis de mettre en évidence le rôle des corrélation angulaires et radiales dans les états du simple continuum de He. Nous avons ensuite établi une relation de continuité entre la photoionisation double et la photoionisation simple avec une excitation infinie de l'ion résiduel He+. la méthode HRM-SOW permet également de calculer la section efficace intégrée de double photoionisation avec une grande précision y compris à très basse énergie. Nous l'avons donc appliquée pour différentes énergies de photon au voisinage du seuil afin de tester la loi de seuil de Wannier. Les résultats obtenus sont en accord d'une part avec les prédictions théoriques de Wannier, d'autre part avec les résultats expérimentaux. La méthode HRM-SOW apparaît donc comme une méthode complète au dessus du seuil de double ionisation. Nous avons poursuivi ce travail en amorçant l'étude des états doublement excités autoionisants. Ceux ci se manifestent comme des résonances dans les sections efficaces d'ionisation sous le seuil de double ionisation. Nos calculs de sections efficaces partielles intégrées et différentielles nous ont permis d'identifier ce type d'états dans la région allant jusqu'au cinquième d'ionisation simple. Certains ont été observés pour la première fois.

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We have modelled an experiment performed at the LULI facility (Ecole Polytechnique, Palaiseau, France). This experiment was devoted to the measurement of the temporal coherence of the transient Ni-like silver X-ray laser at the wavelength of 13.9 nm. In population kinetics studies of saturated lasers, it is necessary to account for the interaction between the X-ray laser electric field and the lasing ions. To this end, we have used the Maxwell–Bloch formalism in the paraxial approximation. The Zeeman sublevels (JM) associated with the lower lasing level (J = 1) are not identically affected by the X-ray laser field. As a result, their populations are different. However, elastic collisions between free electrons and lasing ions have the opposite effect: they tend to restore equilibrium between the sublevel populations. Therefore, elastic collision rates obtained in the distorted wave approximation have been included in the rate equations. Refraction of the X-ray beam, due to electron density gradients, is taken into account by using a ray-trace code which works as a post-processor of the hydro-code EHYBRID. We have checked that the Voigt profile is a good approximation for lasing lines in Ni-like ions. This allowed us to implement a subroutine calculating the Voigt profile in the Maxwell–Bloch code. Whilst the FWHM of the spontaneous emission profile is 12 mA, the amplified X-ray line shows a smaller width of ∼3 mA. This is known as the gain narrowing effect. We notice the saturation of the line-width for a propagation length of 2–3 mm. Comparison with experiment is discussed.

An overview of the recent experimental and theoretical works on photoinduced cascades of Auger and fluorescence transitions in atoms is presented. We concentrate on the angular correlation and polarization phenomena in such cascades and how they are related to the alignment and orientation of the core-excited or core-ionized atomic states produced in the photoabsorption. During the decay, the alignment and orientation are then transferred to the energetically lower atomic states, causing an anisotropy and polarization of the subsequent Auger or fluorescence emission. Special attention is paid to the practically important case of overlapping resonances and intermediate states. Coherent excitation of the resonances and the coherence transfer in the decay strongly influence the angular and polarization characteristics of the decay products. The studies on the angular correlations and polarization of the emitted particles provide detailed spectroscopic and dynamic information about the transitions involved. In some cases, moreover, a so-called ‘complete’ experiment, i.e. the experimental determination of the transition amplitudes including their relative phases, becomes possible. © 2007 Elsevier B.V. All rights reserved. PACS: 32.80.Hd; 32.80.Fb

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The Coulomb explosion of the hydrogen molecule, after absorption of a 76 eV photon, has been studied by momentum imaging the two electrons and the two protons. Absolute fully differential cross sections of high statistical quality are obtained. A subset of the overall data, namely, equal electron-energy sharing, is used to investigate the effects of molecular orientation on the photoelectron angular distribution. Departures from the first-order heliumlike model are evident in detection geometries where electronelectron correlation is ‘‘frozen.’’ During the last decade experimental techniques have been steadily improving, enabling studies of the correlated electron pair dynamics produced in photodouble ionization (PDI). PDI of the simplest two-electron molecule, H2 ,i s significantly more complex than the well-studied case of helium and introduces new physical effects. First, there is no unique double ionization threshold; the threshold energy depends on internuclear separation as the upper repulsive potential curve is purely Coulombic. Second, the ground state electronic configuration is inevitably more complex as a result of the two-center nuclear potential. Third, PDI in H2 is followed by a so-called ‘‘Coulomb explosion’’ as the two protons rapidly separate in opposite directions. Since the photo-fragmentation process is rapid compared to molecular rotation, the relative momentum of the two escaping protons defines the molecular alignment at the instant of double ionization. Energy- and angleresolved detection of all four particles —with a welldefined light polarization state —completely defines the PDI dynamics and allows one to study fully differential cross sections (FDCS) within the molecular frame using such ‘‘fixed-in-space’’ molecules. These measurements provide the most stringent tests for theory and the greatest possible physical insight into this prototypical 4-body process.

Extreme ultraviolet radiation, also referred to as soft X-rays or XUV, offers very special optical properties. The X-UV refractive index of matter is such that normal reflection cannot take place on polished surfaces whereas beam transmission through one micrometer of almost all materials reduces to zero. Therefore, it has long been a difficult task to imagine and to implement devices designed for complex optics experiments in this wavelength range. Thanks to new sources of coherent radiation - XUV-lasers and High Order Harmonics - the use of XUV radiation, for interferometry, holography, diffractive optics, non-linear radiation-matter interaction, time-resolved study of fast and ultrafast phenomena and many other applications, including medical sciences, is ubiquitous.

Dielectronic satellite lines due to 1s(2)n'1'-1snpn'1' (n = 3,4) transitions in Li-like Si (Si XII) occur at 5.818 and 5.565 angstrom, on the long wavelength side of the He-like Si (Si XIII) 1s(2)-1s3p and 1s(2)-1 s4p lines at 5.681 and 5.384 angstrom, respectively. They have been extensively observed with the RESIK crystal spectrometer on the Russian spacecraft CORONAS-F. As with corresponding satellites 1s(2)nl-1s2pnl on the long-wavelength side of the Si XIII 1s(2)-1s2p resonance line, there is an inverse temperature dependence of the intensity ratio of the satellites to the He-like ion lines (I-sat/I-He). New atomic data are used to calculate the Si XII satellite line intensities and thus the I-sat/I-He ratio. RESIK observations of the ratio in solar flares, together with temperatures from the ratio of the two GOES X-ray channels, are compared with theoretical variation of the ratio with temperature. The good agreement indicates this to be a valuable temperature diagnostic for solar flares and laboratory plasmas such as tokamaks. There are implications for similar satellites in Fe line spectra which are observed with broad-band resolution by the RHESSI solar flare mission. (C) 2005 COSPAR. Published by Elsevier Ltd. All rights reserved.

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The study of the emission of two electrons from an atom by absorption of a single energetic photon has become of much current interest because it provides the most detailed information on the interaction of the electrons between themselves. Its investigation in the simplest two-electron system, the He atom, in the last ten years has challenged experimentalists and theorists alike. By using selected examples from electron–electron and electron–recoil ion coincidence experiments the main achievements in the field are reviewed. It is shown that the dynamics of the electron pair is strongly constrained by its own symmetry and the Coulomb repulsion. The further aspects brought in by the experiments in diatomic molecules (H2, HD and D2) as well as in the heavier rare gases are also illustrated. Future perspectives which involve other processes which result in an electron pair in the continuum are considered.

Longitudinal coherence length in x-ray lasers depends strongly on the shape of the amplified line. We have modeled an experiment performed at the Lawrence Livermor e National Laboratory. The experiment was devoted to the study of the temporal (longitudinal) coherence of the transient x-ray laser at 14.7 nm in Ni-like palladium (4d-4 p transition). Only electron (collisional) and Doppler broadening play a role in the line profile of the 0-1 4 d-4 p transition. This allows us to use the Voigt shape in conditions where the amplifier, i.e., the plasma produced by the interaction of a high intensity laser with a slab target, is neither stationary nor homogeneous. Our calculations use a ray trace code which is constructed as a post-processor of the hydro-atomic code EHYBRID. In the saturation regime, there is need to account properly for the in teraction between the x-ray laser field and the lasing ions. This is done in the framework of the Maxwell-Bloch formalism. The FWHM of the spontaneous emission profile is a 28 mA, while the width of the amplified x-ray line a 4 mA. Comparison with experiment is discussed. Keywords: x-ray laser, XUV sources

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