Table of contents

A Sturmian basis is chosen to represent the off-shell Coulomb wavefunction in order to assert the usefulness of this type of representation for continuum problems. The authors present results on the convergence properties and near-shell behaviour of the off-shell function that confirm the effectiveness of the expansion.

The highly excited even levels of the lutetium atom are studied by two-step resonance ionisation spectroscopy. Four Rydberg series, 6s²ns ²S_1/2 (n=17-76), 6s²nd ²D_3/2 (n=14-105), 6s²nd ²D_5/2 (n=19-87) and 6s²ng ²G_9/2 (n=21-61), are measured and analysed. A very weak perturbation from an unassigned J=³/₂ even valence level is observed around 6s²52d ²D_3/2 as a dip of ion signals induced by a decrease of radiative lifetimes in the perturbed Rydberg states. Small dips observed in nearby 6s²ns ²S_1/2 and 6s²nd ²D_5/2 Rydberg states are interpreted as the influence of the quadrupole hyperfine interaction of ¹⁷⁵Lu (I=⁷/₂).

The angular distribution of fragment ions from O₂ following a selective K-shell excitation has been measured using a rotatable time-of-flight mass spectrometer. The asymmetry parameter characterising this distribution was found to be close to -1 at 531 eV ( pi * resonance). The parameter was about 1.2 at 541 eV (1s to 3p excitation), indicating that the transition to the sigma * resonance occurs at a probability significantly higher than that to the 3p orbital.

A consistent derivation of the impulse approximation for collisions of strongly interacting ions and atoms is provided in the classical trajectory approximation with off-energy-shell motion in intermediate states and distorted waves representing elastic scattering in the initial channel.

Previous measurements of atomic collision parameters for e^--Na collisions show unresolved discrepancies in the data obtained using coincidence and superelastic scattering techniques at 22.1 to or from 20.0 eV. Results from a new superelastic experiment on Na at 20.0 eV are presented together with reanalysed data from a previous superelastic experiment and good agreement is now found for measurements of all atomic collision parameters reported from this and previous superelastic experiments at 20.0 eV and a 10 degrees scattering angle. Only partial agreement is found with the corresponding coincidence data.

The variation of Al XII resonance line intensities with the electron density has been measured in a recombining, laser-produced plasma. A detailed, quasi-steady-state model of the population kinetics for these ions is used to successfully describe the measured X-ray spectra. The collisional transfer of population between excited states must be included in detail to account for the observed line intensity dependence for late-time, recombining coronal plasmas.

By representing the states of the atomic f shell as the result of coupling four similar but independent quasiparticles, the Coulomb energy matrix can be calculated by extending standard angular-momentum theory from SO(3) to SO(7), where SO(7) is the special orthogonal group of transformations among the seven orbital states of an f electron. Both S (the total spin) and N (the number of f electrons) are good quantum numbers, and the coupled quasiparticle states can be put into a one-to-one correspondence with the classic states of Racah (1942). Each quasiparticle spans the elementary spinor (¹/₂¹/₂¹/₂) of SO(7). Only one of the four quasiparticle operators changes sign under the spin-quasispin interchange, and this is reflected in the properties of Racah's operators e₂ and e₃, the second of which decomposes into symmetric and antisymmetric parts. The selection rule Delta S=0 often reappears as a property of 9-W symbols, the analogues in SO(7) of the familiar 9-j symbols of SO(3). Several examples are worked out to illustrate the method.

Partial widths for autoionisation to 2l and 3l sublevels have been calculated for O⁶⁺ (3,3) and (4,4) states by a configuration-interaction method. The Gamma (2s)/ Gamma (2p) branching ratios of states under the N=3 threshold are shown to exhibit a characteristic dependence upon the quantum numbers K and L used in the doubly excited-state basis classification. An interpretation is provided by an analysis of predominant configurations in these states.

Energy levels of doubly excited states of beryllium-like ions are calculated using the conventional configuration-interaction method. The interactions of the two outer electrons with the inert 1s² core electrons are represented by a model potential. The resulting energies of 1s²3l3l' and 1s²4l4l' doubly excited states, designated in terms of the K, T and A correlation quantum numbers, are used to identify the Auger electron spectra derived from the collision of multiply charged ions with atoms and molecules. The systematics of energy levels is analysed and compared with the supermultiplet structure of doubly excited states of helium-like ions.

The MQDT of hyperfine structure is greatly simplified in extremely high Rydberg states. A four (or three)-channel quantum defect model is derived for the hyperfine structure of extremely high Rydberg states of msnd configuration in odd alkaline-earth isotopes, in which the transformation matrix U_i(F)i' is given analytically except for one parameter which measures the mixing of singlet ¹D₂ and triple ³D₂ induced by spin-orbit interaction in even isotopes, and in which the four eigen quantum defects mu _i' (i'=1, 2, 3, 4) are related to the quantum defects of the fine structure in even isotopes by mu _i'=mod( delta _i'). This model has been used to interpret the observed sublevel structure of 5snd (n>100) configurations in ⁸⁷Sr. An excellent agreement between the theoretical and experimental results was achieved.

The effect of superequilibrium populations in the excited states of CO₂ degenerate vibrational modes under vibrational-translational non-equilibrium is investigated.

Differential cross sections for elastic electron scattering by SO₂ were measured by utilising a modulated, crossed-beam method and calibration against He. The energy and angular ranges were from 5 to 50 eV and from 12 to 156 degrees , respectively. The present results are compared with earlier data of Orient et al. and it is suggested that their cross section values should be increased by about a factor of two.

The author reformulates and extend the Percival-Seaton theory (1958) for inelastic electron scattering from quasi-one-electron target atoms. Special attention is given to the consequences of neglecting explicitly spin-dependent effects such as the spin-orbit interaction between the continuum electron and the target. The general theory is applied to analyse the so-called 'integrated Stokes parameters' for the light emitted after excitation by generally polarised electrons. Some tests for the validity of this simplified model are proposed and illustrated by numerical results from a semirelativistic calculation for electron-caesium scattering.

The results of an experiment on dissociative ionisation of the CO₂ molecule by electron impact are reported. The contributions of double collisions with electrons and of ion-molecule reactions are analysed, observing the behaviour of the relative abundances of the simultaneously collected O⁺, O₂⁺, CO⁺ and C⁺ ions as functions of electron energy. The orders of magnitude of the rates for ion-molecule reactions and for double collisions with incident electrons are given.

The authors study a quasicontinuum system for which there exists a sparse dressed state composed of the ground state and a single state of the true continuum. In general, the loss of population from the ground state to a true continuum via the quasicontinuum is total. In a special case, however, population is trapped in the ground state and the sparse dressed state is replaced by a trapping state containing no contribution from the true continuum.

Simple analytic expressions are derived for the dipole matrix elements for atomic quasiclassical (WKB) states.