Table of contents

An efficient method of partitioning the Zeeman Hamiltonian is described, and applied in detail to the hydrogen atom. A speedy and accurate series method for handling the radial Schrodinger equation is used in the numerical calculations.

An approximation to the Zeeman Hamiltonian of the hydrogen atom leads to the definition of eigenstates useful at low-medium field strengths.

Various polarisation selection rules guiding phase conjugate emission via degenerate four-wave mixing are discussed. It is shown that, in resonant gas media, depolarising collisions may induce phase conjugation in cases when this non-linear process in otherwise forbidden. However, Doppler broadening often limits the efficiency of non-collinear collision-induced four-wave mixing and introduces a very rapid decrease of the reflectivity with increasing angular separations between pump and probe beams.

Measurements have been made of the radiative lifetimes of the 6²P₃2/⁰ and 6²P₁2/⁰ levels of neutral gold and of the branching ratios of the five transitions originating from these levels. The lifetimes were determined by measuring the fluorescence decay following pulsed laser excitation of gold vapour produced by cathodic sputtering in a rare-gas discharge. The lifetime and branching ratio results are combined to derive absolute f values for the 2428 and 2676 AA principal resonance lines and also for the 3123, 5065 and 6278 AA lines. The new f value for the 3123 AA line is used together with the solar data of Ross and Aller (1972) to obtain a revised value of the gold solar abundance.

The profile of the satellite on the blue wing of the 6S_1/2-5D_5/2 transition of caesium perturbed by argon has been described by the semiclassical model developed by Szudy and Baylis (1975). Both the potential difference between the upper and lower levels of the Cs-Ar quasimolecule and the induced oscillator strength have been adjusted to give the best fit to the experimental profile. The results appear to be a good extrapolation towards smaller interatomic distances of the data obtained in a recent analysis of the quasistatic part of the line by Sayer et al. (1980).

A simple and accurate method of calculating resonance line shapes in photoionisation is described. The procedure involves calculating directly the complex photoionisation amplitudes at only three energies and then using the information in these amplitudes to determine the entire line profile. The method is demonstrated in a calculation for neon.

A time-of-flight mass spectrometer has been crossed with atomic beams of Ca and Sr and a source of monochromated synchrotron radiation in order to obtain the ratio of double-to-single photoionisation cross sections in the region of the 3p (or 4p) ionisation thresholds, between 25 and 35 eV. Attempts are made to correlate the structure observed in both elements with recently published photoabsorption, photoelectron and ejected-electron spectra.

The three-photon ionisation of strontium atoms via resonance in the last photon absorption with some autoionising states was observed in the excitation range 5560-5640 AA using a tunable dye laser. The spectra as a function of the laser intensity are reported. Very rapid line broadening and ion enhancement together with very asymmetric line profiles are observed.

Cross sections for the ionisation of ground state hydrogen atoms by 38-1500 keV H⁺ and 125-2200 keV He²⁺ ions have been determined using a crossed-beam technique. Collision products produced in the intersection of the primary ion beam with a highly dissociated thermal energy beam of hydrogen are identified by time-of-flight spectroscopy and counted using a coincidence technique. Cross sections sigma (H⁺) for proton impact, which involve much smaller uncertainties and cover a wider energy range than previous measurements, now permit an accurate assessment of the validity of the various theoretical predictions. Cross sections sigma (He²⁺) for He²⁺ impact, the first such measurements, are also compared with theory and measured cross section ratios sigma (He²⁺)/ sigma (H⁺) are considered in terms of the Z² scaling predicted by the Born approximation.

Differential measurements were performed for the direct and the charge transfer processes in Li⁺-Na and Na⁺-Li collisions by using a time-of-flight method over the energy range 750-2000 eV and for scattering angles 0-8 mrad. A semiclassical analysis based on the potential curves of Habitz and Schwartz (1975) shows that a larger difference of the potential energies between the lowest sigma states is required to obtain good agreement between the theory and the experiment.

The cross sections have been measured for K-shell ionisation of ₉₀Th by 1.2-6.0 MeV protons and 2.5-16 MeV alpha particles. The results are derived from comparisons with well known Coulomb excitation cross sections for nuclear rotational transitions. The measured cross sections for protons are in good agreement with the predictions of both the SCA and PWBA models provided both models are supplemented by the same Coulomb deflection, binding and relativistic corrections. The cross sections measured for alpha particles are on the average 30% higher than predicted by the two models mentioned above.

The Thomas binary encounter approach (BEA) has been applied to obtain modified energy transfer limits in ion-atom collisions. These limits can be used in the Gryzinski (1965) model to calculate cross sections of single charge transfer and single impact ionisation. A suitable three-dimensional correction factor is found to follow directly from the Thomas BEA, providing a consistent means of calculating both cross sections from the electron removal cross sections.

Modified energy transfer limits and a suitable correction factor are obtainable from an electron capture condition based on the Thomas binary encounter approach (BEA). With the effective radial distance of the electron in the target atom taken as R_max, the radial distance of maximum electron density in the subshell concerned, these limits and the correction factor are applied in the Gryzinski (1965) model to the calculation of cross sections of single charge transfer for protons incident on H, N, O and the noble-gas atoms and He⁺ ions incident on N, O and the noble-gas atoms. The influence of the effective charge of the He⁺ ion is also investigated.

Double charge transfer from helium and argon to fast bare nuclei is investigated with the continuum distorted wave (CDW) approximation. The calculations can be done with the powerful code for one-electron capture of Belkic et al. (1980) after quite simple modifications. They imply that the 'passive' electrons form a frozen core during the collision. The authors' results are compared with the available experimental data and previous calculations. Above the energy limit where CDW is valid for one-electron transfer they conclude that the model is meaningful for large values of the projectile and target nuclear charges.

Analyses the elastic scattering of electrons and positrons by atomic hydrogen at intermediate energies (50-500 eV) by using the basic ideas of the eikonal-Born series method to construct from first principles a local third-order optical potential. In addition to the first-order (static) interaction, the authors' optical potential contains a complex second-order part, obtained from improved values of the second Born approximation to the scattering amplitude, and also the leading contribution of the third-order part. Exchange effects are taken into account by means of a local pseudopotential. A full wave treatment of the optical potential is then performed, and a detailed comparison is made with recent absolute measurements of e^--H differential cross sections. Total (integrated) elastic and total (complete) cross sections for both electron and positron impact are also discussed.

The problem of removal of divergences from the nonrelativistic diagrams describing the break-up scattering amplitude of three charged particles is treated. It is shown that the off-shell scattering amplitude of two and three charged particles can be represented as a product of two terms, one of which includes the full information about divergences. The integral equation for determination of this term is obtained.

The asymptotic (with respect to n) cross section for the excitation of hydrogen atoms by electron impact has been studied in the energy range 20 eV to 1 keV. The effect of electron exchange has also been incorporated using various first-order approximations. The cross section for excitation into different angular momentum states are found to obey a scaling law. Results are presented for 90 degrees polarisation Lyman alpha radiation together with the alignment parameter. The ratio of the angular distributions for alpha s and alpha p states are also discussed.

The electron impact excitation cross section for the resonance transition of Mg II is calculated in a four-state close-coupling approximation. The interaction Hamiltonian contains a model potential that accounts for core polarisation effects. A detailed analysis of the resonance series converging to the 4s ²S and 3d ²D thresholds is made using quantum defect theory techniques. The total cross section is compared with experiment and other calculations, and rate coefficients are presented for electron temperatures of 5000-20000K. Cross sections are also calculated for the transitions 3s-3d, 3p-3d, 3s-4s, 3p-4s, 4s-3d and 3p ²P₁2/⁰-3p ²P₃2/⁰.

The triple (and double) differential cross sections for electron impact ionisation of helium and molecular hydrogen are investigated to a much higher accuracy than obtained before. In a coplanar symmetric geometry ( theta =45 degrees ) both electrons emerging from the ionisation event-with equal velocities- are detected in coincidence. Over the impact energy range from 200 to 2800 eV the authors present absolute triple (double) differential cross sections containing an experimental error smaller than 20(10)%. A detailed comparison is made with results from various theoretical approaches.

The decay of electronically excited H₂ and D₂ singlet states between 12.2 and 16 eV has been investigated by electron-photon coincidence measurements. Asymmetric structures representing a detectable exponential decay are not due to the lifetime of the initially excited states, but result from a threefold cascade decay. The lifetime of the considered n=3 gerade singlet states delivering light in the visible region is about 33 ns. The influence of predissociation is clearly observed.

Values of the energy-dependent cross sections for momentum transfer, annihilation and rotational excitation and deexcitation for positrons in N₂, H₂ and D₂ are chosen so that, when used in a simple slowing-down calculation, the forms of recently measured lifetime spectra are best reproduced. The success of this procedure suggests that, for essentially the whole time taken for thermalisation in these gases, the positrons undergo collisions involving only rotational excitation or momentum transfer. A reliable estimation of the cross sections for these two processes for collisions of positrons with energies below 0.5 eV with homonuclear diatomic molecules should follow from the application of fuller theoretical analysis of a wider range of experimental data.

Derives a Fokker-Planck equation for the Wigner function describing the centre-of-mass motion of a trapped ion in a monochromatic travelling wave of light. The driving term and the coefficients of diffusion are found by solving certain sets of ordinary differential equations. In the low-intensity limit, the author investigates the rate of change of the mechanical energy of the ion and analyse a linearised Fokker-Planck equation valid in the vicinity of the steady state. The quantum mechanical properties of the harmonic oscillator implicit in the equations for the Wigner functions are discussed.