Table of contents

The possibility of optical target preparation has added new dimensions to atomic collision spectroscopy. By suitable choices of photon wavelength and polarization, the shape and dynamics of the target atom can be controlled and manipulated before the interaction with the projectile. Similarly, the shape and dynamics of the atomic states in the exit channel can be monitored by polarization analysis of the photons emitted in the subsequent optical decay. Finally, the target temperature may be strongly reduced by laser cooling and used for recoil ion momentum spectroscopy. A series of simple examples are presented in order to demonstrate the kind of insight into the interaction dynamics that may be gleaned from combined experimental and theoretical studies of this kind.

If an ensemble of atoms or molecules is excited by linearly polarized light, then usually one expects linearly polarized fluorescence form the ensemble. This reflects a symmetry of the excitation process, namely the fact that the electric field in a light wave and consequently the angular momentum distribution of atoms or molecules in an excited state can be characterized by a double headed arrow ⇐ ⇒ . An ensemble of particles in an excited state possesses a net alignment. In this paper some processes that for molecules'in an external field can break this symmetry are reviewed. As a result of symmetry breaking the ensemble of molecules acquires a net orientation of angular momentum, that can be characterized by a single headed arrow ⇒, and one can expect a circularly polarized fluorescence from this ensemble.

Laser spectroscopic access to short-lived nuclear states, ∼1 ms lifetime, irrespective of the chemical properties of the isotope, is now possible at the laser-IGISOL facility, JYFL, Jyväskylä. A newly commissioned ion beam cooler at the facility has increased the scope for future optical studies and a low energy spread (∼1 eV), point-like source of radioactive ions can be supplied independent of the method of sample production. The fully operational cooler will enable the trapping and optical study of a wealth of exotic nuclear systems for which no previous systematic access has been possible.

A large body of work to calculate nonrelativistic energies, relativistic corrections, and quantum electrodynamic (QED) energy shifts in helium is reviewed and compared with experiment. Of particular significance are measurements of the QED (Lamb) shift for the low-lying S-states of helium, where the accuracy rivals the best measurements in the corresponding He⁺ hydrogenic ion. Progress in a program to extract a value for the fine structure constant α from a comparison between theory and experiment for the fine structure splittings of the helium 2 ³P_J manifold of states is discussed. Extensions to three-electron systems such as lithium are briefly summarized.

Multiply-excited neutral atoms, or nearly neutral ions, with empty or only sparsely populated inner shells are termed "hollow atoms". Hollow atoms can be created in a variety of ion collision experiments. An alternative method of generation is to use photo-excitation. The first photogeneration of hollow atom triply-excited states in neutral lithium was reported in 1994. The production of such triply-excited states is completely dependent on electron–electron correlations and their study has proved a challenge to both experimentalists and theorists. Results of recent experiments on hollow atoms carried out at synchrotron facilities in Europe, the USA and Japan are summarised and comparisons made with various theoretical predictions.

Optical lattices are formed when cold atoms get trapped in the periodic array of optical potential wells created by a number of traveling light waves. Exact periodicity is guaranteed when the number of traveling waves exceeds the number of dimensions by no more than one. We present a few simple exactly solvable models for the transport of atoms over a lattice in the tight-binding limit. One model concerns the spreading of an atom by tunneling through the barriers and by momentum recoil induced by optical pumping. Another model gives results for the amplitude of Bloch oscillations, which occur when a uniform is added to the periodic potential. Finally the dissipative spreading of the quasimomentum due to optical pumping is modeled.

This paper reviews recent progress on microwave clocks using laser cooled neutral atoms. With an ultra-stable cryogenic sapphire oscillator as interrogation oscillator, a cesium fountain operates at the quantum projection noise limit. With 6 · 10⁵ detected atoms, the relative frequency stability δv/v is 4 · 10^-14τ^-1/2 where τ is the integration time in seconds. This stability is comparable to that of hydrogen masers. At τ = 2 · 10⁴ s, the measured stability reaches 6 · 10^-16. Equally important is the accuracy of the frequency standard since ¹³³Cs is the primary reference for the definition of the time unit, the second. The accuracy of our cesium fountain FO1 is presently 1 · 10^-15, currently the best reported value.

A ⁸⁷Rb fountain has also been constructed and the ⁸⁷Rb ground-state hyperfine energy has been compared to the Cs primary standard with a relative accuracy of 2.5 · 10^-15. Comparing the hyperfine energies of atoms with different atomic numbers Z, one can search for possible variations of the fine structure constant α = e²/ℏc with time. Measurements of the ratio v(⁸⁷Rb)/v(¹³³Cs) spread over an interval of 24 months indicate no change at a level of 3.110^-15/year, placing a new upper limit for 1/α(dα/dt). The second attractive feature of ⁸⁷Rb fountains is the smallness of the frequency shift induced by the mean field interaction between atoms. This shift is found to be at least ∼ 50 times below that of cesium.

Finally, the interest of the microgravity of space for cold atom experiments is outlined. A space mission, ACES, can ultra-stable clocks, is presented. ACES has been selected by the European Space Agency to fly on the International Space Station in 2004.

Light-assisted inelastic cold collisions are the main loss process for neutral atoms trapped and laser cooled in magneto-optical traps. Alkaline earth atoms do not have hyperfine structure, which is expected to make the comparison between theoretical predictions and experimental results much easier, compared to the case of alkali-metal atoms. We present here a numerical calculation for the loss rates in a weak laser field for cold magnesium atoms.

Recent experiments searching for violations of the spin-statistics connection and of the symmetrization postulate of quantum mechanics are reviewed. Spectroscopic tests on O₂ and CO₂ molecules are described in detail and it is shown that these should be considered as tests of the spin-statistics connection for the spin-0 oxygen nuclei. The possibility of new experiments to test the spin-statistics connection and the symmetrization postulate is discussed.

Hollow atoms are characterized by a large number of vacant inner shells and electrons occupying the outer shells. These "exotic" atomic species are formed in collision processes between highly-charged ions and several types of targets. The scope of the present contribution is to examine the decay properties of hollow atom states populated in collision with atoms, molecules, solid surfaces and clusters.

We discuss recent laboratory experiments and astronomical observations aimed at testing the possible space-time variability of fundamental physical constants, predicted by the modern theory. Specifically, we consider two of the dimensionless physical parameters which are important for atomic and molecular physics: the fine-structure constant α and the electron-to-proton mass ratio μ. We review the current status of such experiments and critically analyze recent claims of a detection of the variability of the fine-structure constant on the cosmological time scale. We stress that such a detection remains to be checked by future experiments and observations. The tightest of the firmly established upper limits, derived from analyses of quasar spectra formed at the early cosmological epoch (∼ 10¹⁰ yr ago), read: |/α| < 1.1 × 10^-14 yr^-1, |/μ| < 1.5 × 10^-14 yr^-1. These limits may be used as an effective tool for selection of theoretical models which predict space-time variations of physical constants.

Coherent population trapping (CPT) is treated in multi–level systems, excited by two– or four–frequency laser light. The dynamics of filling non–absorbing (dark) states is investigated. In cases where all transitions involved are excited (closed systems), the effect becomes dependent on the relative phase of the light waves. Further electromagnetically induced transparency (EIT) is described in three– and four–level systems and used for effective side band generation.

The origin of cosmic rays is a continuing problem in Astrophysics. The majority appear to be of Galactic origin and the manner of acceleration seems to involve ions from the ISM being accelerated by shocks from supernova remnants. Dust in the ISM may also play a part in the early stages. The paper gives a brief account of the origin problem and the likely acceleration mechanisms.

The 32^nd annual Conference (Europhysics Conference) of the European Group for Atomic Spectroscopy (EGAS) was held in Vilnius (the Republic of Lithuania) on July 4–7, 2000 – for the first time in a country of Eastern Europe. About 160 scientists from 31 different countries (even such as Japan, Mexico, Taiwan) have participated in it.

The Conference was opened by the President of the European Physical Society Sir Arnold Wolfendale. Fifteen invited speakers have presented comprehensive scientific talks on different modern aspects of contemporary physics, non-linear optics and spectroscopy, laser cooling, atom optics, collisions and inner shell physics, hollow atoms, clusters, atomic and molecular physics at surfaces, highly charged ions, instrumentation and applications of spectroscopy included.

232 abstracts of 15 invited talks as well as 32 original oral and l85 poster presentations (547 authors) were published in the book of abstracts of the Conference (416 pages).

During the Conference two EGAS Board meetings as well as the EGAS General Assembly were held. During the latter a few new EGAS Board members were elected.

Professor Bernd Craseman, the representative of the American Physical Society responsible for international relations, also participated in the Conference. He organized a special meeting with the presidents of the Physical Societies of the Baltic countries (Estonia, Latvia and Lithuania) to discuss the widening of the international co-operation between the physicists in the USA and the Baltic states as well as the ways and possibilities to preserve their scientific potential.

Five Lithuanian high tech companies have organized exhibitions of their production.

The Conference was financially supported by the East–West Task Force of the European Physical Society, the Ministry of Science and Education as well as the State Fund for Science and Studies of the Republic of Lithuania, and by a number of Lithuanian companies and firms (Joint Stock Company Ltd "ABB Technika", Joint Stock Company "Klaipëdos nafta", "EKSMA" (lasers, optics, electronics), etc.). The majority of local organizers were from the Department of the Theory of Atoms of the State Institute of Theoretical Physics and Astronomy. The Faculty of Physics of Vilnius University as well as the Faculty of Physics and Technology of Vilnius Pedagogical University have also contributed to the organisation of the conference. The assistance of Vilnius Planetarium and the Lithuanian Branch of the International Centre for Scientific Culture – World Laboratory must be acknowledged, too.

Thirteen invited speakers have presented their papers for publication in the present issue of "Physica Scripta".