Table of contents

The 20th annual conference of EGAS (European Group for Atomic Spectroscopy) was hosted by the Institut für Experimentalphysik, Technische Universität Graz, Austria, from 12–15 July 1988. About 250 spectroscopists from 25 countries attended the meeting. The program consisted of 13 survey lectures by invited speakers and of 195 contributed papers, presented as posters or orally. Eleven of the invited lectures have been prepared for publication by the authors and are presented in this issue of Physica Scripta. On the occasion of the 20th anniversary an additional contribution concerning the history of EGAS, written by Professor Dr P F A Klinkenberg (one of the founders of EGAS) is included in these proceedings.

Symbol of the meeting: Spectrum of the Balmer-line H_β in an electric field. The field strength increases along the z-axis up to a maximum of 900 kV/cm and then decreases again to zero. L Windholz, 1976

The history of EGAS is briefly reviewed. Conception and birth, affiliation with EPS, EGAS annual conferences and cooperation with the EPS Division on Atomic and Molecular Physics is discussed successively, after which a critical evaluation is presented.

Recent, partly unpublished results obtained with CW high resolution laser-atomic-beam spectroscopy on the 1snp bound Rydberg series of ^3,4He and on autoionising 5dnf series of Ba, excited field free as well as in the presence of an electric field, will be reviewed. In the He 1snp series isotope shift, hyperfine structure and linear Stark effect were investigated. Ba 5dnf series show extremely narrow linewidths enabling the first observation of hyperfine structure, isotope shift and resolved angular momentum manifolds in autoionising series.

Second harmonic generation and parametric emission in K and Na have been extensively studied. The experimental measurements for the s, p and d states of K (n = 7-34) and the 5s state of Na are compared with theoretical models of ionization d.c.-induced field and collisional l-mixing of states. Collisional energy transfer in the wings of Na 3p and between Na and Na₂ has been used to excite tunable stimulated emission in the range 800-850 nm.

The contemporary state of the theory of the structure and spectra of complex many-electron atoms and ions, the methods of theoretical determination of their energy spectra and the characteristics of electronic transitions are analysed. The problems of the accuracy of the data obtained, the optimization of the coupling scheme used to classify the experimentally measured energy spectra and some applications to plasma spectroscopy and astrophysics are discussed as well.

The use of orthogonal operators to fit atomic energy levels often requires the calculation of matrix elements of multi-electron operators defined by the irreducible representations (irreps) of Lie groups. One way to do this is to imagine the 2^{4/ +2} states of an atomic l shell as arising from the coupling in SO(2l + 1) of four inequivalent quasi-particles, each of which is labeled by the elementary spinor irrep (½ ½... ½) (of dimension 2^l) of SO(2l + 1). Two parity labels (specifying the oddness or evenness of the numbers of electrons with spins up and spins down) complete the classification scheme, which leads to N (electron number) and S (total spin) as good quantum numbers. A unique correspondence (to within a phase) is established with the classic states of Racah. The Coulomb operators e₂ and e₃ for f electrons are examined, and the selection rule ΔS = 0 for e₃ reappears as symmetry conditions on the 9-W symbols, where W is an irrep of SO(7). Work underway (with Shaozhong Li) is outlined. In keeping with a presentation to a general EGAS audience, these matters are described broadly and through examples.

After surveying measurements of fine structure (fs) intervals of excited helium atoms, I discuss some new experiments in more detail, where extensive use has been made of the strong polarizability of these atoms in external electric fields. Using rf electric fields, resonance transitions between the substates of 1s nl multiplets with l ⩾ 2 could be induced, and the first measurements of the fs intervals of the 1s4f configuration became possible. Using static electric fields of several kV/cm, singlet and triplet 1s nd states can be strongly mixed at various anticrossings. These anticrossings provide an opportunity for precision measurements of the 1s2s¹S-³S fs separation. In view of the dramatic improvements recently made in solving the non-relativistic Schrödinger equation of two-electron systems, the helium atom may evolve as a new testing ground for basic problems in atomic physics.

One of the problems encountered in thermonuclear fusion research deals with the presence of elements other than hydrogen isotopes in the plasmas created for these studies, and the effects of such impurities on plasma behaviour. As a result, atomic physics has played an important role in the investigation of plasma properties and behaviour ever since the early days of fusion research.

With the tokamak device as an example, we will review some characteristic features of the spectra arising in such plasmas, and how the emitted lines may be used for localized plasma diagnostics. Some specific examples of diagnostic applications will be given.

We review experiments carried out at Stony Brook during the past three years on (de-)excitation and ionization of highly-excited hydrogen atoms by microwave electric fields. These include single-frequency experiments in both the low- and high-scaled-frequency regime and two- frequency experiments in the low-scaled-frequency regime. We compare these experimental results with "3-d" atoms to the rapidly expanding body of calculations using both classical and quantal theory. We offer a brief comparison of laser- and microwave-ionization experiments with free atoms.

The Z-expansion method for calculation of atomic characteristics is discussed. The calculation is performed for different autoionization states: Two-electron (2lnl'); three-electron (is2lnl') are given, Z-dependences of atomic characteristics, influence of relativistic and radiative effects are considered. All results are obtained by different programs: MZ, MCHF, MCDF, SUPERSTRUCTURE. Possibilities of each of the programs are discussed. For the satellite spectra of H- and He-like ions, theoretical and experimental data are compared. For satellite spectra of Ne-like ions, analytical formulae are given for transition energies and for probabilities of radiative transitions and nonradiative decays as functions of the number of electrons in the systems being considered.

A short description is given of the principal methods by which Stark effect splittings in high static fields can be investigated. In a second part of the paper the present state of the experimental results on the Stark effects of Balmer lines is sketched taking into account the historical development of the "classical" studies. Finally investigations which have been carried out in this field at the Institute of Experimental Physics of Graz Technical University during recent years are briefly reviewed.

The properties of small clusters can be investigated by various methods. For very small clusters having 3 or 4 atoms, the spectroscopic techniques are suitable and results are presented on Na₃ for which the dynamical Jahn-Teller effect, the ground state structure and predissociated states have been observed. For larger clusters, various other methods can be used and some of them are detailed in the paper. General trends in the cluster properties are given for selected examples of atomic clusters studied in the literature. Rare gas clusters are dominated by the van der Waals bonding. Alkaline clusters are well interpreted by a very simple shell model. In divalent metal clusters, a transition from a van der Waals to metallic structure is expected and has been observed in mercury clusters. Finally, the more complicated case of semi-metal clusters is also discussed. Results are presented on antimony and bismuth clusters.

Lasers are finding many applications in medicine and biology. While most laser use focuses on the heat generation of the laser beam interacting with tissue, spectroscopic aspects play a more dominating part in a number of emerging applications. Tumour-seeking agents, such as hematoporphyrin derivative (HPD), in combination with laser radiation provide new possibilities for cancer tumour detection and treatment. The fluorescence emission from tissue irradiated with UV light can be used for tumour localization. Natural tissue fluorescence as well as fluorescence due to injected HPD can be utilized. Results from studies on animals and from clinical applications are reported. The use of fluorescence for identifying atherosclerotic plaque in human vessels is illustrated. Spectroscopic analysis of the laser-induced plasma obtained when a high-power, pulsed laser beam interacts with tissue is also discussed with applications to plaque removal and laser fragmentation of kidney- and gall-stones.