Table of contents

The 25th Conference of the European Group for Atomic Spectroscopy (EGAS) was held in the premises of Caen University, France, from July 13 to July 16, 1993. It was organized by Laboratoire de Physique Atomique and by Centre Interdisciplinaire de Recherche avec les Ions Lourds. 264 Physicists, from 25 different countries, presented 216 contributions (36 oral and 180 posters), in addition to the 11 invited lectures. One of the scientific sessions was dedicated to the late Professor Bengt Edlén, who died in February 1993.

Nine of the invited lectures have been prepared for publication by the authors and are gathered in the present issue of Physica Scripta, together with Willy Persson's and Indrek Martinson's homage to the memory of Bengt Edlén and with Derek Stacey's review of the first 25 years of EGAS history.

The European Group for Atomic Spectroscopy celebrated its 25th anniversay in 1993 by holding its annual conference in Caen, its birthplace. The occasion was marked by a splendid display of fireworks in the town (on 14th July) and a talk which is reproduced below.

In this paper two groups of applications of ion traps will be discussed: those on high precision spectroscopy of single ions and on the investigation of phase transitions of a few ions in traps. Emphasis will be given on recent experiments in our laboratory.

The present review is devoted to the recent advances performed in alkaline-earth atoms by the selective laser preparation of autoionizing asymmetrical double Rydberg states which have, so far, not been observed in natural environments. Because the great amount of flexibility achieved by the sequential laser electron excitations, a wide choice of two-electron situations have been investigated and analyzed which exhibit spectral features due to long-range effects of the Coulomb electron-electron repulsion. To overcome the autoionization broadening of the lines, double Rydberg states with a non-core penetrating high-ℓ outer electron were produced by combining temporal laser excitation technique with the electric-field switching method. The study of the spectral correlation signatures in Nsnℓ double Rydberg states versus ℓ allow to understand their evolution from simple spectra (ℓ ⩾ 10) due to long-range dipole interaction to more complex data (ℓ ⩽ 7) induced by short-range multipole effects when two electrons start to influence more each other.

The heavy ion synchrotron/storage ring facility at GSI, SIS/ESR, provides intense beams of cooled, highly-charged ions up to naked uranium (U⁹²⁺). By electron capture during ion-atom collisions in the gas target of the ESR or by recombination at ion-electron encounters in the "electron cooler" excited states are populated. The detailed structure of very heavy one-, two-and three-electron ions is studied. The different mechanisms leading to the excited states are described, as well as the new experimental tools now available for a detailed spectroscopy of these interesting systems. Special emphasis is given to x-ray transitions to the groundstates in H- and He-like systems. For the heaviest species the groundstate Lambshift can now be probed on an accuracy level of better than 10% using solid-state x-ray detectors. Applying dispersive x-ray analysing techniques, this accuracy will certainly be improved in future. However, utilizing the dielectronic recombination resonances for a spectroscopy, the structure in Li-like heavy ions can already be probed now on the sub eV level.

High resolution measurements of projectile Auger electrons ejected in multiply charged ion-atom collisions are reviewed. Low incident energies are treated where the projectile velocity is small in comparison with the relevant electrons. Specific information is given about the atomic structure aspect that manifests itself in Auger line energies and transition rates. Emphasis is given on collision mechanisms involving double capture into multiply-charged projectiles. Particular attention is devoted to dielectronic processes that are created by the electron-electron interaction during the collision. Angular distributions of the Auger electrons are analyzed to gain information about the population of magnetic quantum numbers. Observed alignment phenomena are discussed in terms of the dielectronic processes.

Solar photospheric abundances, which are generally used as references for other stars, are now, on the whole, in very good agreement with the meteoritic abundances. This is not due to progress in solar spectroscopy but to better atomic data. We show that to go further into the analyses of solar and stellar spectra, improvements are needed in the number and the accuracy of atomic data.

We discuss some theoretical and experimental aspects of quantum non-demolition (QND) measurements in the optical domain. The efficiency of a QND measurement can be characterized using three criteria, which describe respectively the quality of the quantum measurement, the non-destruction of the signal, and the conditional variance of the output signal beam, given the output meter beam (quantum-state-preparation criterion). Quantitative limits can be defined with respect to these criteria, delimiting "classical" and "quantum" domains of operation. We describe the implementation of two experiments which fulfill these criteria, using either three-level atoms inside a doubly-resonant optical cavity, or semiconductors emitters and receivers.

Some applications of semiconductor diode lasers in atomic spectroscopy are discussed by describing different experiments performed with lasers emitting in the visible and in the near-infrared region. I illustrate the results obtained in the investigation of near-infrared transitions of atomic oxygen and of the visible intercombination line of strontium. I also describe how two offset-frequency-locked diode lasers can be used to excite velocity selective Raman transitions in Cs.

I discuss the spectral resolution, the accuracy of frequency measurements, and the detection sensitivity achievable with diode lasers.

A tutorial review is given on recent experiments performed with 1 ⩽ N ⩽ 110 atoms in a microscopic high-finesse optical cavity. The resonator has length 1 mm and finesse 80 000. The radiative properties of the strongly coupled atom-cavity system differ from those of the individual components. For example, spontaneous emission is reversible, the first excited state of the composite system splits into two levels and the intensity fluctuations of the light transmitted by the resonator are nonclassical. We observe photon-antibunching and sub-Poissonian photon statistics. In terms of practical applications, the system renders possible (i) the detection of single atoms with high sensitivity and (ii) optical bistability for a few atoms and photons only.

Recent progress in laser cooling and trapping of neutral atoms is paving the way for the development of more accurate caesium atomic clocks. This paper surveys the progress of ground-based fountain clocks, reporting in detail on some recent results obtained with the prototype clock under development at LPTF, and also discusses the prospects for satellite-based clocks, which, taking advantage of a reduced-gravity environment, may enable the full potential of laser cooling to be realized.