Table of contents

Ab initio calculations are presented for the photon catalysed autoionisation of the metastable 1s2s2p⁴P_5/2⁰ state of lithium. It is shown that, when the photon frequency is resonant with the 1s2s2p⁴P_5/2⁰-1s2p²²P_3/2 transition frequency, the probability induced by a 1 ns laser pulse becomes larger than the 'natural' probability for laser powers larger than 1.5*10¹¹ W cm^-2.

New experimental photoabsorption cross section and angle-resolved photoelectron spin polarisation data for mercury are presented. For the first three autoionisation resonances above the Hg 6s threshold an analysis is performed in terms of transition amplitudes to the singlet and triplet continua. These quantities reveal that photoionisation dynamics is largely influenced by a rather complicated spectral behaviour of the triplet matrix element, including three changes of sign. Results are compared with quantum defect differences and ratios of oscillator strengths from the discrete spectrum.

The ratio R_z of double to single ionisation cross section by charged particle impact is equal at high velocity to an integral over the energy, E, of the photoionisation ratio R_gamma (E) weighted by the density of ejected electrons, rho _z(E).

Cross sections for the production of highly charged recoil ions for 15.5 MeV u^-1 U⁷⁵⁺ on argon have been measured. Since bare and hydrogen-like argon recoil ions are produced with sigma >10^-18 cm² the feasibility of developing a permanent recoil ion source on a high-energy heavy-ion accelerator or even in a heavy-ion storage ring is discussed.

Angular correlations between scattered electrons which have excited the 2¹P state of helium and the 58.4 nm photons emitted in its subsequent decay have been measured at an incident electron energy of 22.0 eV and for scattering angles up to 120 degrees . The data reported, at 800 meV above the 2¹P excitation threshold, are considerably closer to the threshold energy than all previous angular correlation studies for any electron induced excitation or ionisation process. The higher values of the parameter lambda derived from the angular correlations relative to those previously measured at 24.0 eV are consistent with simple threshold predictions.

An explanation is given of the unexpectedly high ozone yield of discharges in N₂/O₂ mixtures. In addition to the direct electron impact dissociation of O₂ a substantial number of additional oxygen atoms are provided by reactions involving excited nitrogen molecules and nitrogen atoms. The calculations based on recent electron impact cross sections are found to be in good agreement with measurements of ozone yield and the time constant of ozone formation.

An ansatz for the calculation of the structure of heavy atoms using a relativistic model is described and developed. After using a series of Dirac-Fock calculations to define a suitable set of single-particle states, the set of orbitals is Schmidt orthogonalised and the one- and two-body Hamiltonian matrix elements calculated. This list of matrix elements is then processed and redefined to eliminate any reference to the inner electrons which are assumed to constitute a fixed core. These matrix elements are then utilised for large-dimension configuration-interaction calculations using Slater determinants as the basis functions. As an example, the ansatz is applied to the calculation of the low-lying states of Pb I and demonstrate the importance of configuration mixings within the ground-state manifold.

The quantum defects are studied for the doubly excited states of two-electron atom with one electron excited much more than the other. The dipole approximation for the electron-electron interaction allows the Rydberg series of the doubly excited states to be constructed with account for the electronic correlations. The latter are particularly important due to the high degeneracy of the atomic states when the electron-electron interaction is switched off. Most attention is paid to the semiclassical region of high total orbital momentum of the atom where closed analytical expressions are obtained for the quantum defect in the dipole approximation as well as for the Coulomb and exchange correction accounting for the residual electron-electron interaction in the first order of perturbation theory. The quantum defects for the Rydberg series of the helium atom and helium-like ions are calculated in the framework of asymptotic theory and compared with the data from energy level numerical calculations for the D and F series of He, Li⁺, Be²⁺, B³⁺.

The spectrum of a cadmium plasma produced by a 1.5 J, 25 ns ruby laser is dominated in the 80-110 AA region by 4p-5s transitions in ion stages XII to XIV. A number of lines from the transitions 4p⁶4d-4p⁵4d5s, 4p⁶-4p⁵5s and 4p⁵-4p⁴5s have been identified with the aid of Hartree-Fock and Dirac-Fock calculations. In each case isoelectronic extrapolations were also used to assist the analysis.

The author obtains expressions for Rydberg level broadening and shift cross sections and rates suitable for elastic collisions of ground-state rare-gas atoms with Rydberg atoms. The semiclassical impact broadening theory is used. The interaction of a Rydberg atom with a rare-gas atom is approximated by the superposition of polarisation potentials and the Fermi pseudopotential. It is shown that similar expressions for the broadening and shift cross sections due to the perturber-Rydberg-electron interaction can be obtained in the JWKB approximation for the Rydberg-electron wavefunctions and for the square-well model potential. Consequently the shift and width of the Rydberg level are not very sensitive to the shape of the potential but only to its normalisation. Expressions for the shift and width in the square-well potential approximation suitable in the high-density limit of the perturbing atoms are obtained. Limits of the applicability of the expressions for the broadening and shift cross sections are discussed. Calculated broadening and shift rates agree well with experimental results for the broadening and shift of Rydberg levels with small, medium and large principal quantum numbers by all rare gases excepting neon.

The total K-excitation cross section has been measured over a wide range of projectile charge states q for the very heavy collision systems ₅₄Xe^q+ to Xe(Z₁+Z₂=108) at 2.2 and 3.6 MeV u^-1 and for ₆₂Sm^q+ to Xe(Z₁+Z₂=116) at 3.6 and 4.7 MeV u^-1. High projectile charge states corresponding to up to N-like ions were used and single collision conditions in the gas target were applied. Coupling cross sections for the L to K and M to K vacancy transfer within the very heavy quasi-molecule-derived from the variation of the K X-ray emission-are reported. The role of rotational vacancy transfer in these relativistic quasi-molecules is discussed.

Total cross sections are calculated using a two-centred atomic orbital expansion for electron capture by He⁺ ions of energies between 1 and 400 keV from neutral lithium atoms. Two models are compared: a two-electron model in which the electrons originally in the 1s level of the He⁺ and the 2s level of Li⁰ are treated explicitly, and a one-electron model in which the captured electron moves in effective potentials due to the He⁺(1s) and Li⁺(1s²) cores. It is found that the two-electron model agrees well with the available experimental data up to 100 keV, while the one-electron model provides only fair agreement. Above 100 keV, capture from the K shell of Li⁰ is the dominant process and is estimated by a two-state approximation.

The electron capture process X^q+-H to X^(q-1)+-H⁺ is considered at intermediate-low velocities in a simple atomic collision model with quantum-defect wavefunctions for the final states of the captured electron. Electron translation factors are included in a consistent first-order approximation which allows a factorisation of the trajectory parameters in the coupling matrix. Close-coupling results are presented for a sequence of incident ions in charge state q=6. The results depend strongly on the configuration of core electrons. This effect is explained and contrasted with the fact that the capture cross section is so much larger than the geometrical cross section of the core that core penetration in practice may be ignored. The calculated results are in good agreement with available experimental data.

Polarisation potentials, explicitly calculated for real channels outside the coupled space, have previously been included only for diagonal matrix elements in the momentum-space coupled-channels calculation. The effect of the inclusion of the polarisation terms in off-diagonal matrix elements is investigated for electron-hydrogen scattering in the 1s, 2s, 2p channel space at 54.42 eV.

Free-free radiative transitions are influenced by scattering resonances which occur by the temporary formation of a negative ion in an electron-atom collision. The authors have measured this effect in the case of the helium resonance at 19.3 eV using a CO₂ laser as the radiation source. Resonance structures have been obtained at scattering angles of 20 degrees , 70 degrees , and 160 degrees and were compared with a calculation based on the soft photon approximation for free-free transitions and the Fano formalism for the description of the resonance. The agreement is very good.

Various models based upon the binary encounter first Born approximation have been developed to test absolute (e, 2e) experimental cross sections of the neon atom. All models are shown to possess the same qualitative behaviour but vary widely in their order of magnitude. Except for the description of the recoil peak, the Coulomb-wave formalism with an adjustable screening parameter is found to give a satisfactory approach for the quantitative description of such experiments.

A source of electrons has been constructed suitable for low-energy and high-resolution studies of electron-molecule collisions. The source is based on the principle of threshold photoionisation of Ar using synchrotron radiation. The experimental system is described in some detail. The characteristics of the apparatus have been tested in a series of electron transmission experiments involving He, Ar, N₂, O₂ and CF₄, down to energies of 50 meV. Scattering by CF₄ shows marked excitation phenomena in vibrational channels. The photoionisation technique shows considerable promise for beam experiments planned to start shortly.

In the process of cooling and trapping neutral atoms, the authors investigate a new laser configuration, which consists of two counterpropagating laser beams with orthogonal sigma ₊ and sigma _- polarisations. They show that such a configuration looks more promising than an ordinary standing wave (where the two counterpropagating waves have the same polarisation), and they explain this result as being due to angular momentum conservation which prevents any coherent redistribution of photons between the two waves. The authors' conclusions are based on a quantitative calculation of the various parameters (potential depth, friction coefficient, diffusion coefficient) describing the mean value and the fluctuations of the radiative forces experienced, in such a laser configuration, by an atom with a J=0 ground state and a J=1 excited state.

Degenerate four-wave mixing (DFWM) in excited atoms is investigated as a probe of the excited-state density produced by collision-assisted transitions. Sodium atoms are excited to the 3P state by absorption of light in the extreme blue wing of the D lines in the presence of He, Ne and Ar perturbers. The rare-gas pressure dependence of the DFWM signals, resonantly enhanced by the 3P-4D transition, is studied to determine the regime where the signals give an unambiguous measure of the excited-state (3P) density. The observed behaviour is compared with a simple model of DFWM in absorbing media. Good qualitative agreement is obtained and the results illustrate the role of absorption, saturation and finite laser bandwidth. The technique allows small excited-state densities ( approximately 10⁹ cm^-3) to be detected with good temporal and spatial resolution.

The population of the 3P state in atomic sodium produced by absorption in the extreme blue wing of the D lines is measured by excited-state degenerate four-wave mixing (ESDFWM). The strength of this ESDFWM signal is shown to give a measure of the pressure-broadened extreme-wing absorption coefficient alpha ( omega ). Relative values of alpha ( omega ) for the rare gases He, Ne and Ar are measured and compared with values calculated from theoretical interatomic potentials and from extreme-wing emission data. Very good agreement is found demonstrating the ability of this nonlinear technique to make accurate measurements of small excited-state densities with excellent spatial and temporal resolution. Advantages of the method for other applications are briefly discussed.