Table of contents

A generalized discrete self-trapping (DST) equation, which allows adjustment of the degree of nonlinearity is investigated both classically and quantum mechanically. Such higher degrees of nonlinearity arise in the boson representation of spin systems. With two freedoms, our system is closely related to the Feynman top. The Hamiltonian structure and classical motion are discussed in detail. Previous descriptions of the classical problem are shown to be special cases of the general formalism presented here.

The measured cross sections for K⁰_s and Λ⁰ (Σ⁰) production in A reactions at 607 MeV/c incident momentum and the number of quark recombinations give the cross section σ(s) for strange particle production for target nuclei with A = 2, 4, 20 nucleons, respectively. The result favours the relation σ(s) ∝ A^1/3. Scattering of an antiquark off a virtual s-pair, or gluon Bremsstrahlung emitted by scattering of an incident antiquark passing through nuclear matter, are possible processes consistent with this relation. Models inconsistent with this relation may be wrong.

This contribution outlines some ongoing activities within a joint programme of experimental and theoretical studies of outer-shell excitation and charge transfer processes in atomic collisions. The main emphasis is presently on alignment and orientation phenomena. The aim is to reveal the shape and dynamics of the electronic charge cloud as it develops in time along the trajectory, and to understand these phenomena in terms of the underlying, basic mechanisms responsible for collisional excitation and transfer.

A semiclassical theory of electron-positron pair production in heavy ion collisions is considered. The importance of higher order processes with excitation of the combined electron cloud is emphasized. The possible role of short range residual interactions in concentrating the transition strength in certain narrow peaks is discussed. Suggestions for further experimental tests are put forward.

We review some aspects of barrier models that have been developed during the last years to describe highly charged ion-atom collisions in the low-energy regime. These models are the products of a fruitful mixing of experiment and theory and provide us with a general, albeit still only rough, picture of many-body processes still to be described in detail by more elaborate physical theories.

Cross sections for single-electron capture from the K-shell of carbon by completely stripped projectiles are computed at incident energies ranging from 150 keV to 20 MeV. The CBI approximation of Belkić and co-workers is employed within an independent particle model and the Roothaan-Hartree-Fock screening for the target. The results are found to be in good agreement with the experimental data.

Our work on analytically continued scattering theory based on the Schrödinger equation is reviewed. We give a brief description of how resonances, here defined as partial wave S-matrix poles, can be calculated as complex eigenvalues to the complex scaled Schrödinger equation. A Mittag-Leffler type expansion is then introduced and it is shown how one can partition a scattering cross section into contributions from isolated S-matrix poles and a background. Computationally this method has proven to be considerably faster than conventional methods. A new, faster and more accurate integration method is used. Examples of detailed previous work as well as current research are given.

The semiclassical trajectory method for atomic processes induced by charged projectiles is reviewed. The perturbative SCA formalism is sketched with emphasis on the first-order time-dependent approach. Selected results from computations on asymmetric collision systems are presented. The kinematic scaling law for ionization and pair-production phenomena is treated and commented upon. Applications of this scaling law in high-energy atomic collision physics are discussed. Stress is put on its possible consequences for the interpretation of the anomalous positron production in very heavy ion-atom collisions.

We have calculated oscillator strengths in the length and velocity formulation for a large number of transitions in the Aluminum isoelectronic sequence from Si II through K VII, using the Multiconfiguration Optimized Potential Model. The results have been used to determine the lifetimes of 14 low-lying excited terms along the sequence. Comparison is made with experiment and with other theory where results are available. The agreement between our values and other theoretical results is generally good, although deviations do occur near level crossings. Some significant discrepancies between theory and experiment persist concerning lifetimes for S IV.

We review our work on the use of animated computer graphics for examination of numerical calculations in the theory of ion-atom collisions. The basic tool is given the name hypergram. Geometrical aspects of the wavefunction development during the collision are visualized. A simple model for alignment of excited p-states, originating from the graphics studies, is presented.

Prompt and delayed EUV spectra of foil-excited Ag have been recorded at ion energies of 19 and 25 MeV. The recent identifications of intercombination transitions in Ga- and Ge-like Ag ions are corroborated by spectra recorded with higher resolution than before and by lifetime measurements. Lifetime results for the resonance transitions of Cu-like ions and for the intercombination transitions in Zn- to Ge-like ions are compared with theoretical predictions.

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We have constructed, and performed the first experiments with, a new facility for studies of charge exchange in very slow collisions between highly charged ions and neutral atoms. State-selective angular differential cross sections for single-electron capture have been measured for laboratory collision energies down to 4.75q eV, where q is the charge state of the projectile. A very slow beam, with an energy spread of ~0.2q eV, is collimated to a maximum angular divergence of ±0.5° before it interacts with a gas-jet target. The post-collisional energy-, charge-, and angular-distributions are then analysed by means of a cylindrical energy analyzer and a two-dimensional position-sensitive detector. The relative energy resolution is measured to be 1.6% and the angular acceptance is ±9°. In the first experiment, we determined the dominating electron transfer mechanisms for single-electron capture, transfer ionization, and true double capture in 0.5-20 eV/amu Ar⁴⁺-Ar collisions by means of comparisons with multichannel Landau-Zener angular differential cross sections.

The cross section for two-electron capture by Mg²⁺ in a single collision with a Mg atom has been measured at impact energies ranging from 0.90 to 650 keV. The cross section exhibits a remarkable energy dependence which presumably reflects important details of the molecular dynamics of the collision.

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An ECR (Electron Cyclotron Resonance) ion source has recently been installed in Uppsala at The Svedberg Laboratory. The ion source will be used both as an injector for a K = 200 synchrocyclotron and to provide ion beams for atomic physics experiments. So far three different research programs in atomic physic are planned and a laboratory in connection to the source is presently under construction. After a short description of the ion source and the atomic physics laboratory, the three programs • Charge transfer mechanisms in slow collisions (J Nordgren) • Optical spectroscopy and laser excitation of doubly and multiply ionized atoms (0 Vogel) • Sputtering of organic solids with slow multi-charged ions (B Sundquist) are briefly presented.

To study the stability of a finite amplitude circularly polarized electromagnetic wave in a plasma with pressure anisotropy we make use of a generalized version of the Chew-Goldberger-Low equations. The dispersion relation is derived. Special attention is focused on the MHD-limit.

New expressions for particle and heat fluxes have been derived by solving the reduced Fokker-Planck equation for strongly inhomogeneous laser produced plasmas. We have systematically included the electrostatic potential alongwith the density gradient effect. The maximum heat flow expression is modified in the steep gradient limit. For typical parameters z = 5, T_H = 2T_c, n_H = n_c and λ₉₀/L_n = 0.4, the free streaming heat flux value corresponds to a flux limiting factor f~0.045 in contrast with 0.06 of our previous model [11]. However, in the gentle gradient limit the classical Spitzer-Harm heat flux expression is recovered.

The Pople and Karasz model is used to study how a system will "freeze" in a metastable state with two long range order parameters. For this purpose, first the equilibrium behaviour of the model is studied by the cluster variation method. Beside the stable and metastable solutions of the order parameters, we also found the unstable solutions which play an important role to explain how to get a system frozen-in in the metastable state. The phenomenon of frozen-in in the metastable state is shown by two different methods: (1) Displaying the free energy surfaces in the form of the contour mapping. (2) Solving the dynamic equations, which were constructed by introducing the detailed balancing condition directly, by means of the flow diagram.

Computer simulations of defocus diffraction contrast and lattice fringe images enable details of electron micrographs of small lattice defects to be interpreted if the electron optical imaging process is included in addition to the interaction between electron beam and crystal. Multi-slice calculations of high-resolution electron microscope images are applied to randomly disturbed silicon structures to evaluate the possibilities and limitations of local structure interpretation. An extension of the iteration procedure is proposed to apply the optical potential concept to eigenvalue system of inelastic-scattered electrons. All three examples given are possibilities of extending the applicability of approximations inherent in theoretical descriptions of electron beam-matter interaction.

An analysis of augmented plane wave results obtained for the cubic perovskite compounds BaBiO₃, KBiO₃, and BaPbO₃ of coherent potential approximation results for the alloys Ba_1-xK_xBiO₃ and BaPb_1-yBi_yO₃ is made by the alternating lattice method. We show that the electronic structure near the Fermi level of the pure compounds and their alloys can be well understood by the alternating nature of the Bi(or Pb)-O sublattices in a tight-binding scheme involving a small set of orbitals.

Selfconsistent Augmented Plane Wave (APW) calculations have been performed on transition metal disilicides TMSi₂ (with TM = Fe, Co, Ni and Y) in the high symmetry fluorite structure. For each compound a set of tight binding parameters is deduced from a Slater and Koster fit on the APW bands structures. The basis used is orthogonal and includes d states for transition metal and sp states for silicon, interactions are taken up to third neighbours. We show that these tight binding parameters are very accurate as the DOS of CoSi₂ calculated through a decimation technique is in good agreement with the APW one. CoSi₂ set of parameters is then compared to data deduced from Harrison scheme and Papaconstantopoulos handbook.

This paper describes the use of the spindependent part of the nuclear scattering length, (b⁺ – b⁻), in neutron diffraction experiments with polarized nuclei. It is shown, that such experiments are feasible by using the "brute force" technique to polarize the nuclei. Neutron scattering from polarized nuclei is obviously of great interest in the following two fields: the study of nuclear magnetism and the use of contrast variation in structure determination. Experimental results from these fields will demonstrate the potential of such studies: in the field of nuclear magnetism we will discuss the determination of the magnetic structure of the nuclear moments in Cu, the determination of nuclear temperatures in HoVO₄ and Cu, nuclear spin-lattice relaxation in Cu and the onset of ordering of the nuclear spins in HoVO₄. Studies on CoPt-alloys demonstrate, that contrast variation by means of nuclear polarization is a very promising way to improve structural investigations.