Table of contents

For a number of years, there has been a major effort to calculate electron-impact excitation data for every ion stage of iron embodied by the ongoing efforts of the IRON project by Hummer et al (1993 Astron. Astrophys.279 298). Due to the complexity of the targets, calculations for the lower stages of ionization have been limited to either intermediate-coupling calculations within the ground configurations or LS-coupling calculations of the ground and excited configurations. However, accurate excitation data between individual levels within both the ground and excited configurations of the low charge-state ions are urgently required for applications to both astrophysical and laboratory plasmas. Here we report on the results of the first intermediate-coupling R-matrix calculation of electron-impact excitation for Fe⁴⁺ for which the close-coupling (CC) expansion includes not only those levels of the 3d⁴ ground configuration, but also the levels of the 3d³4s, 3d³4p, 3d³4d and 3d²4s² excited configurations. With 359 levels in the CC expansion and over 2400 scattering channels for many of the JΠ partial waves, this represents the largest electron–ion scattering calculation to date and it was performed on massively parallel computers using a recently developed set of relativistic parallel R-matrix programs.

The formation mechanisms of K_α emission from hot low-density plasmas are analysed on the basis of accurate measurements of the He-like tokamak spectra carried out over a wide range of plasma conditions. The analysis is supported by the improved calculations of the relevant atomic data. The main focus was made on a study of recombination processes responsible for ionization balance and atomic levels population. In distinction to astrophysical plasma, the simulation of fusion spectra in the coronal approximation showed conflicting results. The derived density of Li-like ions is highly overestimated, while the density of H-like ions appears to be close to the corresponding coronal distribution. The complex influence of charge-exchange recombination (CXR) on the intensity of He-like lines is shown to be a main reason for such confusing interpretation of the measurements. A simple analytical consideration is proposed to explain the behaviour of H-like ions in ohmically heated plasmas. The results of the systematic study of the ionization balance for such plasmas based on the spectra measured at the tokamak TEXTOR have been used to provide a quantitative description of the influence of the background of neutral particles and anomalous transport on the intensity of the spectral lines. The possibility of separating the influence of CXR and impurity transport from the analysis of the spectra is also shown.

In preparation of a laser ion source, we have investigated multi-step laser ionization via Rydberg and autoionizing states for atomic Ni and Ge using a mass separator with an ion beam energy of 20 keV. For both elements resonant three-step excitation schemes suitable for modern Ti:sapphire laser systems were developed. Rydberg series in the range of principal quantum numbers 20 ⩽ n ⩽ 80 were localized, assigned and quantum numbers were allocated to the individual resonances. Ionization potentials (IP) were extracted from fits of the individual series and quantum defects of individual levels were analysed for confirmation of series assignment. For Ni the ionization potential could be extracted with significantly increased precision compared to literature with a value of E_IP (Ni) = 61 619.77(14) cm⁻¹. Also, at least one notable autoionizing state above the first IP was discovered for both elements, and the different ionization schemes via Rydberg or autoionizing states were compared with respect to line shape, ionization efficiency and selectivity.

We have studied the temporal effects on ultra-short pulses during their propagation in air below the collapse threshold where spontaneous self-channelling is observed. A rich dynamics in the spatio-temporal properties of the pulse has been obtained both in the theory and experiment. We have shown that different parts of the pulse do not have the same temporal properties because they do not propagate in the same way. Experimental measurements confirm the dependence of the temporal pulse width with the propagation as well as with the transversal coordinate predicted by the theoretical calculations. In particular, this study has lead us to identify the conditions for which an important reduction of the temporal width as well as a clear pedestal cleaning can be achieved.

We describe the construction of a Lebedev discrete variable representation (DVR) (Dickinson and Certain 1968 J. Chem. Phys.49 4209, Lill, Parker and Light 1982 Chem. Phys. Lett.89 483, Light, Hamilton and Lill 1985 J. Chem. Phys.82 1400). This DVR is based upon a new algorithm for constructing a generalized DVR in one or more dimensions. The novel part of this algorithm is a contraction of the underlying functional basis that is based upon the given points and weights. The contraction allows us to define a set of basis functions and a set of points, of equal number, for which the transformation between the 'point basis' and the 'function basis' is neither singular nor close to it. We present both a strictly orthogonal and a discretely orthogonal version of the Lebedev DVR, which are based on the octahedrally symmetric Lebedev quadratures for the sphere. The discretely orthogonal DVR is a classic generalized DVR (Light, Hamilton and Lill 1985 J. Chem. Phys.82 1400) and the strictly orthogonal version is similar to a diagonalization DVR. These DVRs perform well in test calculations. The algorithm is general and should be applicable to nonseparable bases in many dimensions.

We study the Bogoliubov spectrum of an elongated Fermi superfluid confined in a one-dimensional superfluid along the Bose–Einstein-condensate (BEC)–Bardeen–Cooper–Schrieffer (BCS) crossover. We derive analytic expressions for the effective mass and the Bogoliubov excitation spectrum of the axial quasiparticles along the crossover based on the hydrodynamic theory. Our investigation reveals interesting signatures of BEC–BCS crossover in an optical lattice which deserve experimental investigation.

Different compression schemes for the production of sub-10 fs laser pulses are tested using double ionization of H₂. Compression schemes differ in the laser beam guiding modes which are necessary in order to get a spatially-homogeneous spectral broadening through self-phase modulation in argon. Guiding may take place in a hollow fibre waveguide or may be self-induced in a filament. We tested four set-ups: a single hollow fibre, two hollow fibres, a hollow fibre then a filament, and two filaments. The shortest pulses around 8 fs were recorded with the set-ups involving one or two filaments. Double ionization of H₂ and the associated proton spectra give results in good agreement with the spectral and interferometric autocorrelation measurements. The two-filament set-up gives the shortest pulses. In addition, the H₂ experiment reveals that the spectral phase induced with two filaments cannot be fully compensated with second-order chirped mirrors as for the other tested set-ups.

A new method to prepare dark squeezed states of the centre-of-mass motion of a trapped ion that interacts with a travelling wave light field is proposed. This method is based on modulation of the frequency of the travelling wave field. In addition, an experiment configuration about its preparation is proposed and discussed.

Discrete diffraction management and control of light transfer via evanescent coupling in a circular array of twisted optical fibres is theoretically demonstrated. In particular, it is shown that fibre twist can be exploited to cancel discrete diffraction, to tune the coupling length in a simple two-fibre directional coupler and to control the optical tunnelling between two communicating defects in the array. Quantum-optical analogues of the proposed schemes for the control of discretized light in the array are also elucidated.

Solitons of dipolar spinor condensates confined in a one-dimensional optical lattice are studied in terms of the equation of motion of the spinor which is reduced to the sine-Gordon equation in the limit of strong dipole–dipole interaction and weak external magnetic field. It is shown that the solitons obtained analytically can be easily controlled by adjusting the light-induced dipolar interaction, which is realizable in the optical lattice created by red-detuned laser beams with modulating intensity. The possible creation and detection schemes are also proposed.

A scheme is proposed for the generation of a two-atom entangled state and an N-atom W state in two distant cavities via adiabatic passage. In the scheme, since only laser fields applied to an atom need be changed, the property can effectively reduce the number of operations for implementing the scheme in experiment. And there are no excited states involved during the operation process, so the atom's spontaneous emission can be omitted approximately. Furthermore, the scheme is robust against the amplitude fluctuations of some experimental parameters.

An algebraic model to describe inelastic collisions between an atom and a diatomic molecule within the semiclassical approximation framework is presented. For the interaction in the diatomic system a Morse potential is considered, while an exponential function is taken for the atom–diatom interaction. The original atom–diatom Hamiltonian is transformed into a time-dependent Hamiltonian for the diatomic system. In the interaction picture framework the interaction potential is approximated by a linear expansion in terms of the generators of the SU(2) group, the dynamical algebra for the Morse potential bound states. A minimization procedure to determine the time-dependent coefficients is proposed. The transition intensities are given in terms of matrix elements of the product of exponentials of the Morse potential dynamical group generators. A comparison of the algebraically obtained transition probabilities with the exact semiclassical results is presented.

Radiative lifetimes, accurate to ±5%, have been measured for 8 even-parity and 72 odd-parity levels of singly ionized erbium using time-resolved laser-induced fluorescence (LIF) on Er ions in a beam. This new set of measurements is more extensive than earlier LIF sets, and is in good agreement with those sets where they overlap. These lifetimes provide an absolute scale for a large, accurate set of Er atomic transition probabilities. Basic spectroscopic data on rare earth transition probabilities are needed for astrophysical research and for research on lighting products.

Electron-impact scattering data for argon and its ions continue to be of interest in studies of magnetically confined plasmas. In an earlier paper, Griffin et al (1997 J. Phys. B: At. Mol. Opt. Phys.30 3543) employed the results of 28-term and 40-term R-matrix calculations of electron-impact excitation in Ar⁺ to carry out a collisional-radiative modelling study of the impurity influx of argon in tokamaks. We have now completed a 452-term R-matrix with pseudo-states (RMPS) calculation of electron-impact excitation for Ar⁺ in order to provide more accurate excitation data; using these improved data, we have repeated the modelling studies presented in the earlier paper. We compare our excitation data, as well as the results of the collisional radiative calculations, with those arising from the 40-term R-matrix calculation and find significant differences.