Table of contents

The exact solution of the Shrodinger equation for a non-relativistic charged particle simultaneously embedded in both a uniform magnetic field and in a laser field (the latter in the dipole approximation) is obtained in a purely algebraic manner. The relative directions of propagation of two fields as well as the polarisation state of the laser are arbitrary. The connection with previously known solutions for specific cases is established.

The three-dimensional anisotropic charged harmonic oscillator in a uniform magnetic field and in a laser field in the dipole approximation is shown to be an exactly solvable quantum-mechanical problem. It can serve as a model for atoms or molecules interacting with magnetic and laser fields of arbitrary strength. The solution is derived in an entirely algebraic manner.

The importance of basis functions with f symmetry in diagrammatic many-body perturbation theory calculations of molecular electron correlation energies is investigated. Calculations are reported for the carbon monoxide molecule using two basis sets, the first containing functions of s, p and d symmetry and the second containing functions of s, p, d and f symmetry. The dependence of the component of the correlation energy which is associated with basis functions of f symmetry on the internuclear distance is examined.

The energy spectrum of the simplest one-positron-atom system (H+e⁺) is investigated within Bohr's atomic model. It is found that there is no bound state below the threshold for positronium formation, but the model does indicate a series of autoionisation levels. The adequacy of classical models in describing simple atomic systems in the states where correlation effects are dominant is discussed as well, and possible improvements to the model are outlined.

High-resolution two-photon spectroscopy was applied to investigate the 5s19d Rydberg level of strontium. Transitions from the ¹S₀ ground state to the 5s19d ³D₃ state were observed for ⁸⁷Sr (I=9/2), caused by local mixing between the 5snd ¹D₂ and ³D₃ states induced by hyperfine interaction. A calculation shows excellent agreement with experiment.

He²⁺ ions at velocities less than 6.6*10⁷ cm s^-1 changing to He⁺ in an Ar gas target are found to produce more Ar²⁺ than Ar⁺ recoil ions. This phenomenon demonstrates the importance of transfer ionisation in such collisions.

The authors obtain a classification of the 'b' and 'c' resonances in electron scattering by neon, which supports previous conjectures made on the basis of the grandparent model.

The authors have determined in ab initio, approximate, non-adiabatic polarisation potential for the e-H₂ system. The scattering results using this polarisation potential together with an accurate static model exchange potential are found to be in very good agreement with a variety of recent experimental measurement. In addition, an analytic fit of the polarisation potential is presented.

Some useful ways of improving the speed and accuracy of finite-difference methods for eigenvalue calculations are proposed, and are tested successfully on several problems, including one for which the potential is highly singular at the origin.

The CPMET method in its iterative version is a prototype for unitarily invariant and size-consistent approximations. It is shown how size consistency can be verified for simpler and therefore less time-consuming iterative methods. The idea of loosely linked reference determinants allows one to use these simple methods as a basis for the definition of the energy and the CI coefficients of systems for which multi-configurational reference functions are required.

Electron correlation has been examined in detail for the 2¹S, 2³S, 2¹P and 2³P states of He and comparisons are made with the ground state. The correlated and Hartree-Fock descriptions of these (1s, nl) excited states are provided by the wavefunctions of Tweed (1973) and Davidson (1965), respectively. Coulomb holes, various partial Coulomb holes and several one- and two-particle expectation values are presented. The concept of partial Coulomb holes and their collective presentation has surfaces as been demonstrated to be particularly informative. These surfaces enabled us to interpret correlation effects in different regions of space and so provide a rationalisation of the correlation mechanisms in each state. For all four states, correlation caused a significant inward shift of density from the outer regions due to a reduction in nuclear shielding. However, in the inner regions around the nucleus, the influence of correlation was more complicated and depended not only on the multiplicity of the state but also on its symmetry.

Electron impact excitation rate coefficients for the ³P-¹D, ³P-¹S and ¹D-¹S forbidden transitions of S III have been calculated in a seven-state close-coupling approximation. Bound-state energies of S II have also been calculated. Effects such as configuration interaction in the target and the inclusion of g waves in the close-coupling formalism were considered, and the rate coefficients are estimated to be correct to within about 15%. The agreement with quantum defects obtained from measurements is better than 5% in most cases.

The 6sn s ¹S₀ sequence of Ba is strongly perturbed around n=18 by the 5d7d ³P₀ doubly excited state. The authors have probed the configuration mixing by observing the strong decrease in the measured ¹S₀ lifetimes around the short-lived valence perturber state. The measurements have enabled the authors to resolve the problem of the designation of the studied states. Wavefunctions obtained from multi-channel quantum-defect theory were used to calculate theoretical lifetimes in good agreement with the experimental values.

The model potential method of Valiron et al (1979) is used to compute the ionisation energies and dipole moments for the KHe and KNe molecular systems. Structures in the excited states could be predicted using the Fermi model of a free electron scattered by the rare gas, and the accuracy of the curves estimated using asymptotic methods is discussed. A calculation of the velocity dependence of the cross section for the fine-structure transition K 4p ²P_1/2 to K 4p ³P_3/2 is given and compared with experiment. The computed potentials are used to determine the profile of the K fluorescence line perturbed by He and Ne in the framework of the quasi-static theory.

A recent calculation of Komarov and Ostrovsky (1979) seemed to have settled a controversy regarding the different experimental values of the H(2s)/H(2P) sharing ratio in the predissociation of the D ¹ Pi _u state of H₂ near threshold. This calculation was based on a correct physical picture of the dissociation process, but the dynamical treatment rests on invalid assumptions. In the present work, a more rigorous quantum mechanical treatment is presented, and a branching ratio of 0.70 is obtained. The authors' results show how dangerous it is to infer the correctness of a calculation solely from the 'goodness' of a single physical quantity.

The arc spectrum of tellurium excited by an electrode-less discharge of a 2540 MHz microwave unit is described. The resonant cavity spectrum in the wavelength range of lambda =2000-6600 AA has been remeasured and analysed. This has resulted in classification of 66 spectrum lines many of these being observed for the first time.

The potassium 4p ²P_3/2-7s ²S_1/2 (5802 Å) transition when broadened by the noble gases exhibits satellites and line asymmetries which are clearly of a collisional origin. Measurements of complete line profiles are given for broadening by He, Ne, Ar, Kr and Xe within 16 cm^-1 of the line centre at densities up to 10¹⁹ atoms cm^-3 as observed for a microwave-excited electrodeless discharge. These are compared to computed unified theory line shapes and are found to agree when appropriate parameterised potentials are chosen for the potassium-noble-gas interactions. Such analyses yield experimental potentials for the 7s state that show extrema at interatomic separations from 14 to 18 Å that are slightly repulsive for He and Ne, and attractive for Ar, Kr, and Xe. These features are not correctly predicted by the pseudopotential calculations of Pascale and Vandeplanque (1974) and are only qualitatively accounted for with potentials of the Fermi type.

Relative intensities of KMM Auger transitions in samarium and relative energies and intensities of KMM Auger transitions in polonium were studied. A satisfactory agreement between the measured and the calculated relative intensities of the strongest KMM lines were established.

The structure of the angular distribution of photoelectrons from polarised targets exposed to polarised radiation has been analysed in the dipole approximation. It is shown in particular that a measurement of this angular distribution without a spin polarisation measurement of the photoelectron constitutes a complete experiment for polarisable one-electron atoms and many other atoms. The completeness of the type of experiment proposed permits one to extract both magnitude and phase of all participating dipole transition amplitudes from an experimentally observed angular distribution, and represents therefore the basis for a sensitive comparison between experimental and theoretical work.

Absolute total electron detachment cross sections for collisions of H^- and D^- with He, and F^- with He, Ne, Ar, CO, N₂ O₂ and CO₂ have been measured in the energy range from about 1 up to 300 eV. The results for H^-(D^-)+He are found to be in very good agreement with the recent theoretical calculations of Gauyacq (1980) and Taylor and Delos (1982). The thresholds for electron detachment in F^- collisions are found to be approximately 7 eV for all targets except O₂, which has a threshold of about 4 eV. For this latter case, evidence of some charge transfer producing O₂^- (or O^-+O) is also observed.

A brief survey of the present status of dispersion relations in atomic physics is given. The singularity structure of the forward scattering amplitude is explored on the basis of the Landau rules and is compared with results following from potential theory. The connection between the type of singularities and the adopted form of atomic wavefunctions is revealed. The question of existence of unphysical and anomalous cuts is raised. The electron-hydrogen dispersion relations are analysed numerically and the need for a new determination of the real part of the amplitude at intermediate energies (10 eV<or approximately=E<or approximately=100 eV) is stressed.

The free-free absorption coefficient of the negative ion of atomic helium is evaluated using a multichannel theory based upon the R-matrix method for electron-atom collisions. The results are expected to be accurate to at least 10%.