Table of contents

Attention is drawn to the connection between the scaling (or (2/1) Pade) approximation for improving the convergence of perturbation series, recently discussed by Wilson (1979) and Aitken's (1939) delta ² transformation. The invariance with respect to scaling of the (n+1/n) Pade approximant is shown to follow from its equivalence to a generalised Aitken (1939) or Shanks (1955) transformation. An algorithm for generating this series of Pade approximants is given.

Global identities for delta functions, given by Hiller et al. (1978) (HSF) are applied to the calculation of the hyperfine structure (HFS) of the ground state of Li. It is shown that use of the HSF identity together with configuration interaction type wavefunctions can yield values of the HFS constant f which are comparable in accuracy to that obtained by Larsson (1968) with a 100-term Hylleraas-type wavefunction. The implications of this result for HFS calculations for atoms with many electrons are discussed.

Circularly polarised synchrotron radiation has been used to produce spin-polarised photoelectrons from krypton and argon atoms (Fano effect (1969)). The results obtained experimentally in the autoionisation and the continuum regions are presented and discussed. In the case of argon they are compared with Lee's (1974) theoretical prediction.

(n+p)-photon ionisation of atoms is studied when n-photon ionisation is energetically possible. A non-perturbative treatment is described. The corresponding results agree with the predictions of perturbative theory in the weak-field limit. The effect of the light pulse shape is discussed.

The elastic scattering of positrons by mercury atoms in the energy range 0 to 25 eV has been investigated using the relativistic Hartree-Fock wavefunction to describe the target. Polarisation is taken into account using an empirical formula. Phaseshifts for the s, p and d partial-wave scattering are presented. The total scattering and momentum transfer cross sections have been calculated. Relativistic effects are found to be negligible for positron scattering in contrast to electron scattering by the same target.

The simplest four-body system with Coulomb interaction is investigated using the adiabatic basis. The wavefunctions of this basis are the solutions of the quantum mechanical two-centre problem (one electron in the field of two fixed nuclei). An infinite set of ordinary differential equations, describing the hydrogen molecule in the adiabatic representation, has been constructed. An approximate finite system of equations has been solved and the first three vibrational states of the hydrogen molecule were obtained. Some advantages and the convergence of the proposed method are discussed.

It is shown that the evaluation of hook spectra is limited towards small values of Nfl for all three presently known methods: the hook technique itself, the hook vernier and the phase method. Criteria are presented for calculating these limits. In an experimental test it is demonstrated that systematic errors do occur for small values of Nfl in the hook technique, as often supposed, but can be avoided by taking notice of the estimated limits or by using one of the alternative methods.

Lifetime measurements of single fine-structure levels of the multiplets e⁸S, z⁶F, z⁴F and z⁴D of neutral manganese are described. The levels were excited stepwise by a pulsed dye laser from metastable levels populated by a discharge. The lifetimes were determined directly from the exponential decay of the fluorescence using the delayed coincidence technique. The results are compared with beam-foil data and used to derive absolute gf values by means of the branching ratios of Greenlee and Whaling (1979). Comparison of these gf values with the data compilation of the National Bureau of Standards shows large scatter. Using the results of the sun model of Holweger and Muller (1974) the mean photospheric solar Mn abundance has been determined from twelve different lines to A_Mn=5.47.

The zirconium spectrum produced in a condensed vacuum spark source was recorded on the 3 m normal-incidence spectrograph of the physics department, Lund University (Sweden) from about 250-2550 AA, using aluminium and gold coated gratings. Thirty lines are classified as 4s²4p³-4s4p⁴ transitions, establishing all eight 4s4p⁴ levels of Zr VIII. New identification of 18 4s²4p³-4s²4p²4d transitions led the authors to determine seven more 4p²4d levels, thus completing the knowledge of this configuration's structure except for (³P)⁴F_9/2 and (¹D)²G_9/2 that do not combine with the ground levels.

Twenty five 4s²4p²-4s4p³ transitions are identified in Zr IX establishing all the ten 4s4p³ levels of this spectrum, recorded on the 3m normal incidence vacuum spectrograph of Lund University.

Calculations of discrete oscillator strengths, of AC Stark shifts and of single and multiphoton ionisation cross sections are presented for helium atoms. The wavefunctions of excited (bound and continuum) states are determined in the framework of the central field approximation; two optimal parametric potentials are determined for singlet and triplet systems. The reliability of the wavefunctions is shown by comparing oscillator strengths for various transitions and photoionisation cross sections for the 2¹S, 2³S, 2¹P and 2³P states with previous accurate theoretical results. Cross sections for two- and three-photon ionisation of 2¹S and 2³S metastable states are calculated for linearly polarised light for a large range of photon frequencies. The AC Stark shifts of resonant transitions are evaluated. Cross sections and shifts are calculated by means of perturbation theory; the infinite summations over intermediate states are evaluated by means of perturbed-function solutions of single-particle equations. The results are compared with experiment as well as with previous calculations.

A simple retarding ionisation chamber has been attached to a standard 127 degrees photoelectron spectrometer to produce a high-resolution UPS instrument. This instrument has been used to record the Ne I and He I spectra of the title compounds. Spin-orbit splitting has been measured for all ² Pi states and Renner-Teller effects in the vibrational structure of ² Pi bands for CO₂ and COS. Vibrational enhancement in Ne I spectra is attributed to autoionisation. The spectra obtained are compared with recently reported threshold studies of these molecules.

It is shown that fluctuations of the predissociation linewidth with vibrational quantum number, normally expected on semiclassical grounds, will be suppressed over any energy region where the relevant repulsive curve is roughly parallel to the bound-state potential curve. This conclusion simplifies recovery of the repulsive curve by an RKR based method. Direct inversion of data for the heterogeneous predissociation of the B state of Br₂ yields a curve in good numerical agreement with that obtained by the numerical Franck-Condon technique.

Ratios of deuteron- and proton-induced ionisation cross sections for Co, Ni, Cu, Zn, Ge and Se (Z=27 to Z=34) in the incident energy range 0.2 to 0.9 MeV amu^-1 obtained from thin target measurements are presented. These experimental ratios are compared with those obtained from theoretical calculations of the Coulomb deflection effect and the importance of other effects that may affect this comparison is discussed. It is concluded that the recent SCA calculations of Kocbach (1976) are a significant improvement on previous approximations and give a fairly good fit to the present results.

The study of the temperature dependence of the Cs-He absorption spectrum between 5350 and 6850 AA gives information about both the Cs(7S and 5D_5/2)-He interaction potentials and the oscillator strength of the 6S-5D_5/2 collision-induced transition. These experimental data are found to differ widely from the available theoretical results.

The reactions of Cl⁺ with NH₃, H₂S, O₂, CH₃Cl, NO, COS, C₂H₄, H₂, CH₄, and N₂O have been studied at room temperature using a selected ion flow tube (SIFT). No evidence was found for the presence of long-lived excited states of Cl⁺. Ground-state rate constants are given for the above reactions together with product ion branching ratios where applicable.

It is shown that the standard version of Thomson's (1924) well known theory of electron-ion recombination in an ambient gas is based on an untenable assertion. Retaining his concept of a trapping radius a corrected classical theory is developed. In view of its simplicity it is unexpectedly good at reproducing results obtained by the diffusion method. The electron energy exchange frequency per unit ambient gas number density is judged to be inappropriate to the phenomenon of electron-ion recombination in an ambient gas. During the course of the investigation it is demonstrated that of the several different classical diffusion models of recombination which have been put forward the correct model is the earliest, that due to Pitaevskii (1962).

A theoretical model based on the CDW approximation of Cheshire (1964) for K-shell electron capture by stripped nuclei from multi-electronic atoms at high energies has been derived. The influence of the projectile-passive-electron interaction has been properly taken into account in addition to the well known nucleus-nucleus interaction. As an application of this model, differential cross sections for H⁺+He(1s²) to H(1s)+He⁺(1s) are calculated and compared with other recent theoretical and experimental data.

The dissociative attachment is treated as a three-body rearrangement process. The interaction of the electron with the molecule is replaced approximately by the interaction with each of the atoms as if they are simple field centres. Faddeev (1960) equations are used to calculate the cross section. The interaction of the electron with the atom is represented by a separable potential the parameters of which are determined from the electron affinity and the scattering length of the electron. The results are compared with available experimental data.

The spheroidal s- and p-wave phaseshifts for the scattering of electrons from H₂⁺ are calculated by the Kohn variational method in the prolate spheroidal coordinate system. Two-centre Coulomb wavefunctions are employed as the long-range basis functions. Adiabatic quantum defects are obtained by extrapolation of the phaseshifts to the low-energy limit. Two-electron excited resonances found in the p-wave scattering are analysed.

Utilising a crossed-electron-beam-metal-atom-beam scattering technique differential excitation cross sections at 10, 20, 40 and 54.4 eV incident energies and at scattering angles ranging from 10 to 120 degrees have been measured for the following transitions: (i) elastic scattering, and (ii) excitation from ground state (3²S) to (a) 3²P, (b) 4²S, (c) 3²D+4²P and (d) 4²D+4²F+5²P+5²S states in Na. Comparisons with recent theoretical and experimental results have been made. Using previous theoretical and experimental results as guides the present differential cross sections have been extrapolated between angular regions of 0-10 degrees and 120-180 degrees . Integral and momentum cross sections have been computed from these cross sections.

Coulomb-Born-exchange calculations of the scaled or reduced cross section for ionisation from the 4s, 4p, 4d and 4f sublevels of hydrogenic ions with Z= infinity are made for several impact electron energies. The results are fitted to a simple form which is readily integrated over a Maxwellian distribution function. By following the prescriptions given in the authors earlier work (1978) the results can be easily used to obtain approximate cross sections for direct ionisation from the n=4 sublevels of any ion with finite Z for which Z/N>or approximately=2, where N is the number of bound electrons per ion.

The effect of interatomic interaction on cooperative resonance fluorescence in the case of well separated components is discussed. It is shown that for two atoms the new components appear both in the emission spectrum and in the intensity correlation function. An increase of the number of interacting atoms leads to an increase in the number of additional spectral components.

The interaction of a two-level atom with two monochromatic electromagnetic fields having arbitrary amplitudes and resonance frequencies is considered. It is shown that the time averaged upper-level population can exceed 0.5 and even reach 0.7. Its dependence on the frequency difference reveals resonance satellite peaks, which are concerned with three- and five-photon combination scattering. The widths and the resonance spectrum shifts of these peaks are found. The frequency spectrum of the spontaneous emission of an atom contains fine structure under some conditions. This structure does not exist in the case of one monochromatic field. An asymmetry in the intensity distribution for the Rabi satellites due to a field non-monochromaticity is predicted.