Table of contents

The 5th International Colloquium on Atomic Spectra and Oscillator Strengths for Astrophysical and Laboratory Plasmas was held in Meudon, France, from August 28 to 31 1995. It was the fifth in a series started by the Atomic Spectroscopic Group at the University of Lund, Sweden, in 1983. Then followed the meetings in Toledo, USA, Amsterdam, The Nether- lands and Gaithersburg, USA, with a three year period. The original title of the series ended with "... for Astrophysics and Fusion Research" and became more general with the 4th colloquium in Gaithersburg.

The purpose of the present meeting was, in line with tradition, to bring together "producers" and "users" of atomic data so as to ensure optimal coordination. Atomic physicists who study the structure of atoms and their radiative and collisional properties were invited to explain the development of their work, emphasizing the possibilities of producing precise transition wavelengths and relative line intensities. Astrophysicists and laboratory plasma physicists were invited to review their present research interests and the context in which atomic data are needed.

The number of participants was about 70 for the first three meetings, then exploded to 170 at Gaithersburg. About 140 participants, coming from 13 countries, attended the colloquium in Meudon. This large gathering was partly due to a number of participants from Eastern Europe larger than in the past, and it certainly showed a steady interest for interdisciplinary exchanges between different communities of scientists.

This volume includes all the invited papers given at the conference and, in the appendix, practical information on access to some databases. All invited speakers presented their talks aiming at good communication between scientists from different backgrounds. A separate bound volume containing extended abstracts of the poster papers has been published by the Publications de l'Observatoire de Paris, (Meudon 1996), under the responsibility of the same editors.

The major trends already apparent during the preceding meetings have been confirmed. The quality of astrophysical observations is being continuously improved In particular, the Hubble Space Telescope launched between the third and the fourth colloquia continues to bring high-resolution data while the Solar Heliospheric Observatory (SOHO) is at the beginning of its adventure and the Far Ultraviolet Spectroscopic Explorer (FUSE) is expected. Astrophysicists interested in solar or stellar atmospheres are facing new diagnostic problems concerning line broadening and deviation from local thermodynamic equilibrium (LTE). Studies of laboratory plasma like X-ray lasers and fusion plasma are still demanding more atomic data, for instance for problems related to a tokamak divertor in the future International Thermonuclear Experimental Reactor (ITER). The need for atomic data in the interpretation of the new observations constitutes a stimulating challenge for atomic physicists. Theoreticians not only continue to improve the description of atomic structure including relativistic and nuclear hyperhe effects, but also carry out methodological developments, such as global and statistical approaches. They are able, using up-to-date computational techniques, to produce large amounts of data and are organising these data in user-friendly computerised databases. Results of systematic calculations (such as the Opacity Project) now are being exploited for modelling. A new tool for plasma diagnostics is proposed via the polarisation of X-ray emission. On the experimental side, novelties of the last decade like the Electron Beam Ion Trap and the VW Fourier transform spectrometer are fully operational. Emission measurements and the combination of atomic beam and laser techniques proved to be complementary and fruitful for lifetime determinations. Efforts are made by atomic physicists to estimate the precision of measured or calculated data released.

Laboratory spectroscopy and astronomy have a long common history. Indeed, we are reminded that a century ago, terrestrial and solar helium were identified with one another through spectral lines, while recent studies of nebulae and the interstellar medium provide nice examples of the continuing interplay between spectroscopy and astrophysical models. During the Amsterdam meeting, some astrophysicists deplored that the so-called "traditional term analysis" was disappearing. They should be reassured about the near future owing to three invited papers and a great number of posters on this type of work.

We are grateful to the members of the Scientific Committee who helped us to set up the scientific programme and to all the referees who reviewed the manuscripts published in this volume. We thank the Observatoire de Paris-Meudon, the Centre National de la Recherche Scientifique and the Centre National d'Etudes Spatiales for financial support. We also thank the Ministère de l'Education Nationale, de l'Enseignement Supérieur, de la Recherche et de l'Insertion Professionnelle, the National Aeronautics and Space Administration and the Laboratoire Aimé Cotton for providing travel grants, Further support from SUN Microsystems, Alcatel and Crédit Lyonnais is gratefully acknowledged. The colloquium greatly benefited from the infrastructure of the Observatoire de Paris-Meudon and the cooperation of its staff, especially members of the Département Atomes, Molécules en Astrophysique (DAMAp).

The 6th International Colloquium on Atomic Spectra and Oscillator Strengths for Astrophysical and Laboratory Plasmas is scheduled to be held in Victoria, Canada in 1998.

A term analysis of a complex spectrum is like a giant jigsaw puzzle with pieces of different sizes, shapes, colours, etc., and the result is a picture of the atomic structure of an atom or ion having several valence electrons. This knowledge of the atomic structure tests the theoretical model of complex atoms and theoretical calculations of various atomic parameters. These atomic parameters are fundamental ingredients in, e.g. abundance analyses of astrophysical spectra. We discuss the atomic structure of transition group elements based on term analyses of their spectra. We give examples from Fe II and Zr II. The strong coupling to high-resolution stellar spectroscopy, in particular in the satellite ultraviolet and near-infrared wavelength regions, is emphasized. Missing atomic data for transition elements is still the main reason for missing opacity in stellar spectra, calculated from stellar model atmospheres.

The current program at NIST to observe spectral lines of atoms and atomic ions and determine atomic energy levels is reviewed. Results for the doubly ionized atoms Zr III, Hg III and Dy III, which are astrophysically important, are summarized in some detail, as are recent results for the Li-like atoms Li I and Fe XXIV. Progress on installation of a very high resolution Fourier transform spectrometer at NIST is reported.

Discussed in this article are two classes of configurations for which atomic calculations performed with a Cowan Code package fail to give sufficiently reliable results.

The general situation with the accuracy of the theoretical predictions of the structure of the 3p⁵3d^N+1 configurations is outlined. Results of semiempirical calculations of energy levels in Fe VII and Fe VIII are presented. It is shown that deviations from experimental values of energy levels are in the range of several thousand wavenumbers. New identifications are made for the 3p⁵3d² levels in Fe VIII.

A comparison of experimental spectra of the 4d⁹-4d⁸4f transitions in Cd IV-Sb VII with the calculated ones is made. It is shown that calculated relative transition probabilities disagree with an order of magnitude with the relative intensities of the lines of this transition array in experimental spectra of Cd IV-Sn VI ions.

The useful range of Fourier transform spectrometry (FTS) has been extended into the vacuum ultraviolet (VUV) region as far as about 1400 Å. The advantages offered by FTS over other methods for the acquisition of the high quality laboratory data now required to exploit observations from satellite-borne instruments are discussed. The efficiency of FTS is compared with that of grating spectrometry to show how an effective short wavelength limit of the former is reached, and possibilities for extending this are mentioned. Some examples of VUV FTS data of astrophysical significance are given.

The beam-foil source can produce spectra of almost every ion and allows easy measurement of atomic lifetimes of excited levels. Beam-foil spectra have some unique characteristics that made them valuable tool for the term analysis of highly charged ions. As an example, we will describe the work undertaken at Liège on Ne IV and Ne V. Beam-foil is still the only general method for measuring the lifetime of atomic levels in highly charged ions. However, the accuracy of the classical beam-foil method is limited by the non-selective broadband excitation in the foil that populates almost all excitation levels up to very high energy and corrupts by cascading phenomena the normal single-exponential decay curves. We will describe some techniques that have been invented to obviate this infamous "cascading effect", especially the beam-foil–laser spectroscopy method recently developed in our laboratory. We will present an overview of the beam-foil spectroscopy research performed at the University of Liège together with other recent trends in the field of fast-beam spectroscopy.

Emission spectroscopy as one of the standard methods to measure atomic transition probabilities is discussed with its advantages, its restrictions and its pitfalls. The measurement of branching fractions combined with lifetime data obtained with modern laser techniques is discussed as the prevailing measuring procedure which guarantees large sets of reliable f-values. The trend to shorter wavelengths in astrophysics and in fusion research will also in the future keep emission spectroscopy attractive.

The results of a new series of precision lifetime measurements on alkali atoms by means of beam–gas–laser spectroscopy are reported and discussed. The lifetimes of the first excited states in lithium, sodium, potassium and rubidium were measured with uncertainties ranging from ±0.14% to ±0.25%. The results are discussed in comparison with other recent experimental and theoretical work. The longstanding discrepancy between ab initio line strength calculations and the measurements of Gaupp et al. [Phys. Rev. A26, 3351 (1982)] for the resonance transitions in lithium and sodium is resolved by the new measurements in favour of the theoretical calculations. Finally the role of the beam divergence in lifetime measurements on fast particle beams is discussed and a method to correct for this effect is presented. The importance of the beam divergence effect is illustrated by additional recent lifetime measurements on helium and on lithium.

Relativistic theory of few-electron heavy ions is outlined. Ions with increasing number of electrons are studied to show the interplay of many-body and relativistic effects. A complete calculation of two-electron and of the ground configuration of titanium-like ions is reported. The influence of pair creation and correlation on the relativistic M1 transition (1s2s³S₁ → 1s^{2 1} S₀) in heliumlike ions is studied. Hyperfine quenching of the J = 4 level of the ground configuration of titanium-like ions is shown to give information on the nuclear electric quadrupole moment.

The aims, methods and some of the results of the Iron Project are described.

The progress achieved over the last few years in variational Multiconfiguration Hartree-Fock and Configuration Interaction calculations is reviewed. The high level of accuracy which can be reached in the ab initio calculation of atomic properties of light systems is illustrated through the results of calculations of isotope shifts, hyperfine structures and transition probabilities. The active space method and associated restricted active space methods allowing systematic calculations from which the accuracy of the studied property can be estimated will be discussed. We exemplify quality checks through results in small systems for which spectroscopic accuracy is often required. For transition probabilities, recent progress derived from new algorithm developments allowing independent optimization of the states is emphasized.

Complete sets of orthogonal operators are used to calculate eigenvalues and eigenvector compositions in complex spectra. The latter are used to transform the LS-transition matrix into realistic intermediate coupling transition probabilities. Calculated transition probabilities for some close lying levels in Ni V and Fe III illustrate the power of the complete orthogonal operator approach.

The studies of hyperfine structure (HFS) of free atoms is shortly summarized. A hyperfine structure many-body parametrization method is described. The utility of this method as a source of information on atomic structure and for prediction of hfs of the fine structure levels, still undetermined experimentally, has been demonstrated on the examples of Co, Ti and Ta atoms.

The model of Unresolved Transition Arrays (UTA) has been proposed some years ago for modelling a transition array, without resorting to any large energy-matrix diagonalization. For each UTA, the low-order moments of the distribution of the line energies weighted by their strengths are computed exactly, in the form of compact formulae. Then, this distribution is represented in the spectrum by one Gaussian, or one skewed Gaussian, whose axis, variance and asymmetry are obtained from the calculated moments. This model is well suited to the case of emission spectra of highly-charged atomic ions, where the low configurations generally do not overlap, and where the recorded lines often coalesce into broad bands. Recently, some progress in the global approach has been made for the cases (i) where configuration-interaction effects are important and (ii) where the lines are resolved. It is now possible to propose simulations of a transition array where the total strength is corrected for the interaction with a large number of configurations, and/or where the lines are obtained by a Monte Carlo method. In such simulations, the low-order moments of the weighted and unweighted distributions of line wavelengths and amplitudes are correct, and the number of lines and the total strength in small wavelength ranges are in good agreement with a line-by-line calculation. Thus, it is possible to introduce thousands of transition arrays in the interpretation of laser-produced plasma spectra, or in the calculation of astrophysical opacities.

In recent years, the quality and resolution of observational data, notably from the Hubble Space Telescope, has so improved that the accuracy of atomic data is frequently the limiting factor in progress. It is therefore imperative not merely that atomic data of improved accuracy are determined, but also that a realistic assessment of that accuracy is provided.

Calculations of oscillator strengths are necessarily approximate. We discuss various indicators of accuracy, how semi-empirical corrections can be made to improve ab initio results, and how this data can be used to assess the level of accuracy of calculations for specific transitions. By way of illustration, these ideas are applied to set estimates of accuracy of a recent calculation of the lifetime of the 2s2p³⁵S₂^o level of N II.

The accurate quantitative analysis of high resolution ultraviolet spectra of ultra-sharp-lined, early-type, chemically peculiar stars, obtained with the Goddard High Resolution Spectrograph on the Hubble Space Telescope, has been made possible by the synergism between state-of-the-art astrophysics and state-of-the-art atomic spectroscopy. We illustrate this with several examples, including the discovery of spin-forbidden "parasite" transitions of Fe II, the first observations of hyperfine components and isotopic shifts of stellar lines of thallium, and measurements of the photospheric abundances of a large number of normally trace elements.

An overview of the SOHO mission is given, including brief descriptions of the major spectroscopic instruments CDS and SUMER. The range of ions whose spectra will be observed is discussed in the context of available fundamental atomic data, principally energy levels, transition probabilities and electron excitation rate coefficients. The use of ADAS (Atomic Data and Analysis Structure) to store the fundamental data and to produce the derived data necessary to interpret the solar spectral intensities is described.

The far ultraviolet spectral window (910-1200 Å) has long been recognized for its unique importance for studying many astrophysical problems. For the first time since the Copernicus mission in the 1970s new flight instruments are producing significant advances in this wavelength domain. This paper reviews plans for the Far Ultraviolet Spectroscopic Explorer to be launched in 1998 for a three-year mission. Some highlights from the recent flights of the Hopkins Ultraviolet Telescope, the Orbiting Retrievable Far and Extreme Ultraviolet Spectrograph, and Interstellar Medium Absorption Profile Spectrograph are also summarized. The paper includes several lists of far ultraviolet transitions where accurate atomic data are essential for some important scientific problems with FUSE and other missions.

Archived monochromatic opacities, obtained in the course of the work of the Opacity Project, are used to obtain data required for the calculation of radiative accelerations. These data are being made generally available through the Centre de Donnees de Strasbourg (CDS).

The new atomic data are used in an exploratory study of the abundances of Manganese in HgMn stars. In most such stars observations show enhanced atmospheric Mn abundances, which result from processes of diffusion. We define χ to be the factor by which abundances are enhanced (or depleted). Due to saturation effects, the radiative accelerations g_rad(χ) decrease as χ increases and may eventually reach values smaller that the gravitational acceleration g_grav: χ_stat is defined to be the value of χ for which the diffusion velocity is zero. Using results from ultra-violet observations one obtains abundances χ_obs always a good deal smaller than χ_stat. It follows that there must be outflow at the stellar surfaces.

Solutions are obtained of the equations for time-dependent diffusion. The initial conditions are taken to be χ = 1 (unmodified abundances) at time t = 0. With such abundances the flux F = nv for Mn is found to be very large at depths with temperatures in the vicinity of log (T) ≃ 5.3, the region giving the "Z-bump" in Rosseland mean opacities. As time advances, those large fluxes move to the outer layers and give atmospheric abundances in agreement with the maximum values which are observed. At later times the atmospheric abundances decrease.

The past developments and some of the conditions for a renewal of deep (optical) spectroscopy of gaseous nebulae are reviewed.

We briefly review the state-of-the-art in non-LTE modeling of hot star atmospheres. During the last decade, the development of a new class of radiation transfer methods, based on the Accelerated Lambda Iteration method, have brought considerable progress in this field. Severe limitations of the size of model atoms used in non-LTE model atmospheres have been removed, so that realistic model atoms can now be considered. In recent years, the problem of non-LTE line blanketing, which aims to include the influence of thousands to millions of lines on the atmospheric structure, has been attacked, and first non-LTE line-blanketed models have been calculated. We illustrate the importance of non-LTE line blanketing with the example of a hot subdwarf, BD +75° 325.

As a consequence of these developments, there is a need for extensive radiative and collisional atomic databases to build non-LTE models. Recent projects, like the Opacity Project, provide a substantial quantity of the necessary data. We discuss the current needs of atomic data for the purpose of building reliable non-LTE line-blanketed model atmospheres. A critical evaluation of the quality of such extensive sets of atomic data is essential. Finally, we present a new, IDL-based, graphic tool intended to facilitate the manipulation of large amounts of data needed to build the realistic model atoms that we use.

The composition of solar and meteoritic matter serves as a primary reference for cosmochemical studies. Element abundances in the Sun - and in cool stars in general - are usually determined by spectroscopy of a stellar atmosphere whose properties differ from most laboratory sources: (i) there are strong gradients of temperature and pressure, (ii) the main constituents are neutral hydrogen and helium, while the fractional abundance of electrons is only ≈10⁻⁴, (iii) the plasma is immersed in an intense, anisotropic radiation field, and is highly turbulent.

The present contribution discusses meteoritic and solar abundances and their suitability as cosmic reference data. Also addressed are mayor diagnostic problems encountered in the analysis of the photospheric line spectrum of cool stars - in particular line broadening and deviations from LTE in neutral iron. This paper is an update of a review [1] given at the 1987 Meudon symposium on high-S/N stellar spectroscopy.

Most interstellar species have a large fraction of their electronic transitions at far ultraviolet wavelengths. Observations at these wavelengths reveal spectra rich in absorption lines seen against the continuum of a background source, such as a hot star in our Galaxy, a supernova in a nearby galaxy, or even a bright nucleus in an active galaxy. Most of the observations continue to be made with space-borne instruments, but recent work includes measurements of extragalactic material at large redshifts obtained at high resolution with large ground-based telescopes (e.g., the Keck Telescope). The combination of precise experimental oscillator strengths, large-scale computations, and astronomical spectra with high signal-to-noise ratios are yielding a set of self-consistent f-values that span a range in strength in excess of 100 for more and more species. The large range is important for studies involving the different environments probed by the various background sources. This review highlights recent work on the atomic species. Si II, S I, and Fe II, and on the molecules, CO and C₂.

This paper reviews how atomic physics data are provided and needed in hot dense plasmas produced by the interaction of high-power lasers with solid targets. These plasma are bright x-ray sources localised in space and time and their x-ray emission is studied mainly in the 0.1-3 keV spectral range by emission and absorption spectroscopy and imaging. Detailed atomic models are required to analyse the experiments and radiative hydrocode simulations need the description of atomic physics in a very wide range of plasma parameters. In this paper, emission spectroscopy is illustrated by the study of the collision of two aluminium plasmas. The example of probing an x-ray laser medium shows the important parameters for absorption spectroscopy. Opacity measurements of x-ray heated samples done by different laboratories mainly in Al, Ge and Fe samples give detailed and Rosseland mean opacities in the XUV range.

This paper provides an updated review of the capabilities of the Electron Beam Ion Trap (EBIT) as an advanced light source for atomic spectroscopy. Recent developments extending the spectral range to over five orders of magnitude in wavelength are highlighted. Distinguishing features that make EBIT a powerful tool both for simplifying spectral analysis and for making precise and accurate measurements are discussed. Various limitations of the machine are also discussed, with the intent to provide an introductory guide for scientists who may be planning EBIT spectroscopy experiments for the first time.

Minimizing the consequences of the interaction of confined magnetic plasmas with the walls of the vacuum chamber has long been recognized as crucial for fusion research. Recently, more emphasis has been placed on solving this problem by producing high density, low temperature conditions in the plasma near the surface. In a divertor tokamak such conditions lead to significant reduction in the flow of particles, momentum and energy to the target structure. With this change in emphasis, a variety of new atomic and molecular processes are reevaluated as to their importance in the divertor plasma.

X-ray lines are emitted by very hot plasmas such as fusion plasmas (magnetic or inertial confinement) and astrophysical plasmas (solar and stellar flares, galaxies, etc.). The excitation processes of the atomic levels are often anisotropic and the emission lines can be expected to be polarized. The measurements of line polarization could give valuable information on the electron energy and angular distribution. The atomic data required to calculate the polarization being very sensitive to the quality of the theoretical method used, it is important to be able to check their accuracy by comparison with experiments. Recently the development of EBIT sources has allowed it to be done. In these devices, the ions being excited by an electron beam, the x-ray emission is often polarized.

New theoretical results concerning the Stark broadened profiles of the Paschen lines of hydrogen and He⁺ ion are presented. Effects due to the ionic motions are taken into account. Analytical expressions are proposed for the halfwidths at low densities. Comparisons with available experimental data are performed.

During the course of critical compilations of atomic oscillator strengths at the National Institute of Standards and Technology over the last 30 years, we have developed and refined a general scheme to assess the accuracy of the pertinent experimental and theoretical data. To arrive at the accuracy ratings, we have utilized general assessments of the various approaches, the attention the authors give to the critical factors of their method, the authors' uncertainty estimates, extensive data comparisons, and fits into systematic trends. Some examples will be shown which indicate that accuracy estimates in the literature are often too optimistic.

A list is given of references concerning calculated and measured probabilities for forbidden transitions in atoms and ions. The selection is restricted to electric quadrupole (E2), magnetic dipole (M1), magnetic quadrupole (M2) and magnetic octupole (M3) transitions. In particular, spin-forbidden, hyperfine-structure induced and multiphoton transitions are not considered here. Some comments are included on important aspects of recent work in the field. The period covered goes from 1989 to mid-1995. This paper is thus a complement to our previous bibliography on the subject for the period 1968-1989 [Biémont, E. and Zeippen, C. J., J. de Physique IV, C1, 209 (1991)].

The main results from the international Opacity Project are described, emphasizing its atomic database service, TOPbase, installed at the Centre de Données Astronomiques de Strasbourg, France, since January 1993. A critical evaluation of the numerical approach used in the Project to compute atomic data (energy levels, gf-values and photoionization cross sections) for astrophysical applications is also given, in particular in relation to recent developments to treat bound-bound transitions in intermediate-coupling as applied to the intercombination lines of the Be isoelectronic sequence.