Table of contents

The author reports a calculation for ¹S^e resonance parameters in electron-hydrogen scattering converging on the N=4 to N=9 hydrogen thresholds. Resonance energy positions and widths are calculated by using a method of complex coordinate rotations, with quantum numbers K, T, N, and n given to each resonant state. Results are compared with the recent R-matrix calculations for resonances below the N=4 and N=5 hydrogen thresholds, and with the hyperspherical coordinate calculations for resonances associated with the N=6-9 hydrogen thresholds. The present results are also used to interpret the grandparent model for high-lying doubly excited two-electron atomic systems.

The electron or ion impact excitation of the giant resonances is considered. The spectrum of ejected electrons is shown to be strongly distorted by post-collision interaction (PCI).

The parity of the number of zeros of one-photon multipole transition matrix elements for an electron in a point Coulomb field is examined, generalising on some multipole transitions in the long-wavelength limit. Results are obtained for the general multipole matrix element with fixed angular momentum, Delta l not=0, retardation being included. For low multipolarities, results are compared with those obtained in the long-wavelength approximation, and cases in which results differ are indicated. The method involves determining the sign of the matrix element both at very low photon energy (for unphysical situations below the photoelectric threshold not corresponding to real bound-bound transitions) and in the limit of high photon energy.

A double Rydberg formula has been developed for Wannier ridge intrashell states of atoms that have a non-nuclear core. A 'double-electron quantum defect' that decreases as the principal quantum number n increases is proposed.

The integral partial cross sections for the processes H+Ar, Xe to H(2s), H(2p), H(3s), H(3p), H(3d) have been measured for collision energies from threshold to 1.5 keV. They were obtained from an analysis of the intensity variation of the emitted light under the influence of a weak electric field, which was applied to the collision region. The method yields in addition a number of n=2 and n=3 off-diagonal density matrix elements (e.g. the sp coherence) and n=3 polarisation data.

The author discusses impact ionisation of polarised atoms (Li, Na, K and metastable excited He) by polarised electrons. The consequences of the 'maximum interference approximation' in theoretical 'distorted wave' calculations for the total cross sections and the spin asymmetry are investigated. Numerical results obtained with the DWBA method are compared with recent experimental data. The agreement between theory and experiment is only marginal. This indicates that more sophisticated methods are needed to enable correct theoretical predictions of the spin dependent ionisation asymmetry.

The R-matrix method is used to obtain electron impact collision strengths for various transitions from the ground state of neon-like S VII. The calculation includes the first fifteen target eigenstates in the R-matrix expansion of the total wavefunction. Effective collision strengths are given for electron temperature in the range (5*10⁴-10⁶ K) by assuming a Maxwellian electron velocity distribution for the incident electrons.

The R-matrix method is used to calculate electron impact excitation collision strengths in the F-like ion Ni XX from its ²P^o ground state. Configuration interaction wavefunctions are used to represent the sixteen LS-coupled target states which are retained in the R-matrix expansion. Effective collision strengths are then calculated for transitions from the fine-structure levels of the ground state to the fine-structure levels of a number of excited states for electron temperatures in the range (1-10)*10⁶ K.

Results are presented for positron-H₂ collision cross sections for energies below the positronium formation threshold. Calculations are performed with a variety of target representations, including full configuration interaction within a basis of Slater-type orbitals, and several models. The most sophisticated model includes the use of pseudo-excited electronic states of the target to account for intermediate-range polarisation effects. Short- and long-range polarisation effects are also considered. Calculations are performed for Sigma _g⁺, Sigma _u⁺, Pi _u, Pi _g, Delta _g and Delta _u symmetries which are combined to give differential cross sections as a function of energy. These calculations underestimate the Sigma _g⁺ eigenphase sum but perform better for the higher symmetries. The differential cross sections show a minimum for angles about 45' which should be amenable to experiment. A computationally more efficient reformulation of the positron-molecule scattering problem is presented within the R-matrix method. Corrections are given for some previously published results.

The mobility and the diffusion tensor have been calculated for He⁺ ions in He and Ne⁺ ions in Ne, at temperatures of 77-78 and 294 K, and at field-to-density values E/n₀ up to 2000 Td. For He⁺ ions in He, ab initio potentials were used, with a careful extrapolation to large distances. A slight adjustment of the mean potential resulted in agreement between calculated mobilities and the best experimental values to better than 0.5%. For Ne⁺ ions in Ne, a potential model with three adjustable parameters was constructed, and an overall agreement between measured and calculated mobilities to better than 1% was obtained. The model potentials probably give a good estimate of the gerade-ungerade splitting at internuclear distances from 7.5 to 10 au, but are not expected to be accurate at shorter distances.