Table of contents

Scattering of a non-spin-charged particle by a Coulomb centre located near a hard wall is considered in spheroidal coordinates. The problem is equivalent to the scattering of two plane waves symmetric with respect to the boundary by a potential which is a Coulomb one in the half-space plus its geometric image. The scattering amplitude is expanded in angular Coulomb spheroidal functions.

Probabilities for multielectron radiative transitions can be interpreted and computed efficiently by using state-specific Hartree-Fock and/or correlated wavefunctions. Results are presented from application to prototypical one-photon, two-, three- and four-electron transitions connecting ground and low-lying excited states to triply and quadruply excited states in Li, Be, Ne⁷⁺ and Ne⁶⁺.

The initial value problem and the field dressing associated with the time-dependent Stark effect are treated, which are of importance in applications. An exact solution of time-dependent field mixing of degenerate states as an initial value problem is obtained in the restricted subspace of a given principal quantum number of hydrogenic ions, with the field-dependent phases explicitly included. The field mixing in terms of the transformation from spherical to parabolic representation is clarified. The distinction between these two problems is stressed and previous work on the field-perturbed dielectronic recombination is re-examined.

Using a coincident energy gain spectroscopy, experimental evidence is given of the direct stabilization of two electrons following a double-electron capture process at keV energies. At variance with indirect stabilization, this mechanism does not require a transfer to highly asymmetric Rydberg series. Three examples are given in which both direct and indirect stabilization mechanisms are found to coexist.

The observation of a multiple laser induced continuum structure in the vicinity of a manifold of autoionizing states of calcium is reported and discussed. The lineshape of the induced structures is shown to be strongly dependent on the intensity of the lasers.

Full details are presented of the new method of Halley et al. (1992) for the calculation of near-threshold resonance-dominated spectra. This method combines the R-matrix and complex coordinate techniques and has necessitated an extension of quantum defect theory to calculate an appropriate irregular Coulomb function for complex energy and complex radial distance. The method is discussed in particular in its application to the near-threshold photoabsorption spectra of a non-hydrogenic atom in a laboratory strength magnetic field and applied here to the examples of strontium and barium. Excellent agreement is obtained with experimental measurements.

The results of detailed simulation of gain on the 3¹D to 2¹P transitions of He-like carbon and aluminium at 267 AA and 46 AA respectively are presented. The system investigated is that of a recombining laser-produced plasma, cooled by expansion into vacuum. Gain coefficients of approximately 4 cm^-1 are predicted. The He-like scheme is found to be comparable to the H-like, although there is evidence that the former may become the more favourable of the two as the systems are scaled to shorter wavelengths.

Electron collisional excitation strengths are calculated, using an eleven-state ab initio R-matrix method in LS coupling, for singly ionized atomic oxygen. All intermultiplet forbidden n=2 transitions from the 1s²2s²2p³ (⁴S⁰, ²D⁰, ²P⁰) and the 1s²2s2p⁴ (⁴P^e, ²D^e, ²P^e, ²S^e) levels are investigated. Effective collision strengths and rate coefficients are also presented for all possible forbidden n=2 transitions between these multiplets over the temperature range 5000-1,000,000 K. These results have wide ranging applications in many laboratory and astrophysical plasmas and, being more accurate, should be used in preference to previous values available in the literature.

Using the configuration interaction (RCI) method and including relativistic effects and relaxation, the K shell excitation and ionization energies of Na I are calculated. The authors calculation confirms the designations of Yavna et al. (1986) for 1s3s(¹S₀)3p ²P and 1s3s(³S₁)3p ²P levels, and suggests the exchange of identifications for levels 1s3s(³S₁)np ²P with 1s3s(¹S₀)np ²P (n=4,5). Theoretical excitation energies for doublets and the configuration average ionizing energy are in good agreement with experiments, differing by 0.6 eV only. Excitation energies for quartet levels 1s3s(³S₁)np ⁴P (n=3, 4, 5) and ionizing limits for 1s3s ¹S₀ and ³S₁ are predicted.

Multi-reference configuration interaction (MRD-CI) computations are reported on nineteen potential curves of Al₂³⁺, a trication having three valence electrons. The ground state X ² Sigma _u⁺ (4 sigma _g²4 sigma _u), dissociating into ground Al⁺+Al²⁺, is repulsive. In spite of its triple positive charge, three excited states arising from configurations with maximal occupation of bonding orbitals are metastable: 1² Pi _u (4 sigma _g²2 pi _u), 1⁴ Sigma _g^- and 1² Delta _g (both 4 sigma _g2 pi _u²). In the same order, the potential barriers D_eff, equilibrium geometries R_e and vibrational frequencies omega _e are: 0.28, 0.57, and 0.04 eV; 4.88, 4.79 and 5.20 a₀; and 298, 330 and 180 cm^-1. Experimental data for X ³ Pi _u (4 sigma _g²4 sigma _u²5 sigma _g2 pi _u) of Al₂ correspond to 1.55 eV (D_e), 5.10 a₀ and 286 cm^-1, respectively, showing that the local bonding in Al₂³⁺ is comparable to that in Al₂. For low upsilon ', the main depopulation mechanism of 1² Pi _u and 1⁴ Sigma _g^- involves radiative emission of type bound-continuum. For upsilon '=0, each maximum of the bands 1² Pi _u to 1² Sigma _g⁺ and 1⁴ Sigma _g^- to 1⁴ Pi _u appears in the same energy region ( approximately= 5800 cm^-1).

Various quantum decoupling and semiclassical methods are used to calculate cross sections for rotational and rovibrational transitions in O₂⁺ colliding with Kr. A general analysis of time scales for rotational and vibrational excitations and of coupling strengths for the corresponding potentials is used to review the standard decoupling methods that treat separately vibrations and rotations in atom-molecule collisions. When such decoupling is applied to strongly interacting ionic systems and to heavy particles, it is found to fail rather dramatically and to be brought back to reliable values only when fully coupled rovibrational equations are used to treat the dynamics.

The production of electron-positron pairs with subsequent inner-shell capture of the produced electron in relativistic collisions of heavy ions has recently been investigated within a coupled-channel formalism based on an expansion in atomic orbitals of the target. At close collisions a drastic failure of perturbation theory in first order has been found. For the case of the collision of U⁹²⁺+U⁹¹⁺ at an incident energy of 2 GeV/nucleon the authors demonstrate, that this non-perturbative behaviour is essentially due to the strong interaction of the K-shell bound state with the field of the projectile charge.

A purely classical method is presented for the calculation of the cross sections for transitions between the Rydberg atomic states n₁, L₁ to n₂, L₂ by electron impact. The state of the electron in the atom is described by the microcanonical distribution function. The cross sections are obtained in a quite appealing form after a very simple calculation. The accuracy of the approximation appears to be reasonable.