Table of contents

Absorption measurements of the half-width of H_alpha in an atomic hydrogen atmosphere are reported. Hydrogen densities of about 4*10¹⁹ cm^-3 at 10⁴K were produced in a piston-driven shock tube.

The K_alpha ^h/K_{alpha alpha} branching ratios of S and Ar are determined in dependence on the projectile-charge state for 32 MeV S on Ar collisions. The branching ratios of the S projectile are about a factor of two larger than the branching ratios of Ar for all charge states (5⁺-13⁺), in contradiction to some existing experimental results. The values are within the error bars, at most weakly increasing with the incoming charge state. The results are explained by a dependence of the branching ratio on the 2s-shell population according to a theoretical treatment of Gavrila and Hansen.

A complete set of relations between geminals, which generate the N-particle antisymmetric function, is given.

Phaseshifts in the Coulomb two-centre problem of quantum mechanics are calculated and their asymptotic expansions are found in the limit of large and small distances between the charges. The asymptotic expansions for these phaseshifts are compared with the results of direct numerical calculations. The relationships between the one- and two-centre phaseshifts are considered.

The value of the Sommerfeld screening parameter sigma ₁ for the K level has been calculated for Z=1-103 assuming that the inner screening parameter sigma ₂=0 for this level. The method used for the calculations is believed to be superior to the original Sommerfeld method since the two-body relativistic energy expression has been used in closed form, thus avoiding series truncation errors which can be important for large Z. It is found that: (i) sigma ₁ increases with Z contrary to Blokhin's observation (1957) that it has a value of 0.3 independent of Z; (ii) Delta sigma ₁(Z)= sigma ₁(Z)- sigma ₁(Z-1) when plotted as function of Z rapidly diminishes in the region of low Z and then stabilises around 0.05 for medium Z; (iii) the curve of Delta sigma ₁ against Z shows clear evidence of filling and closure of the d and f shells; (iv) for the first transition series, Delta sigma ₁ shows an interesting variation consisting of a gradual decrease till the 3d shell is half filled, followed by a gradual increase till the shell is completely filled. Possible reasons for these observations have been discussed.

Electron-atom scattering in the field of a resonant laser is briefly reviewed and the theory of an experiment suggested by Compton, resonant laser induced negative ion production, is presented.

The kinetic energy spectra of the protons produced in the dissociative photoionisation of H₂ and directed predominantly along the electric vector of a photon beam have been obtained at certain wavelengths between 19.8 and 40.8 eV. The time-of-flight method described employed a specially constructed wide aperture analyser with multichannel analysis to extract the very weak signals available. The system has been used to measure the energy distributions of the protons formed through the ground state of H₂⁺ and at higher energies, where two-electron excited states of H₂ and other states of H₂⁺ also contribute to proton production. Protons identified with the lowest ¹ Sigma _u⁺ state (predominantly 2p sigma _u2s sigma _g) exhibit a peak in their kinetic energy distribution which appears to move to higher energies as the photon energy is increased. The contributions from direct and autoionising processes are assessed and related to other published results including electron impact data.

The Penning ionisation systems He(2³S)/H and He(2¹S)/H have been studied experimentally in the thermal range of relative collision energies. The authors report results for the energy distribution of the ejected electrons, total ionisation cross sections and associative to total ionisation cross section ratios. By reproducing these experimental data for He(2³S)/H in quantum-mechanical model calculations, the authors were able to derive an accurate complex potential for this system and to confirm the approximate validity of the theory used. The potential so determined is used to calculate measurable quantities, which are then compared with other available experimental data. The limitation of the theory, which becomes evident in the calculations, is discussed.

In the collision energy range 2-1000 eV, the ionisation processes occurring in collisions of He⁺ with Ca, Ba, and Ne⁺ with Ba, are studied by electron spectroscopy. From the observed electron spectra, and from the variation with collision energy, details on the respective mechanisms are derived, and numerical data on the relevant potential curves obtained. It is found that in the He⁺/Ba system ionisation occurs as atomic autoionisation of Ba⁺* after the collision is completed, while the He⁺/Ca system autoionises at small separations during the collision. In Ne⁺/Ba both these mechanisms are observed. For the analysis of molecular autoionisation spectra new semiclassical formulae are derived and their validity shown by comparison to the He⁺/Ca spectra.

The classical-trajectory Monte Carlo method has been used to calculate the multiple-ionisation cross sections for 1 MeV amu^-1 collisions of highly stripped ions in charge states q=2-20 with He, Ne and Ar targets. An independent-electron model was used to represent the rare-gas targets. For the highest charge state studied (q=20), the cross section for removing all the outer-shell electrons from Ne or Ar is approximately 2*10^-16 cm², whereas the cross section for removing both electrons from He is approximately 1*10^-15 cm². The calculated values are consistent with experimental observations of the cross section for the total number of electrons ejected from the target.

Thick targets of Ta, W, Pt, Au, Pb, Bi, Th and U have been bombarded with 3 to 6 MeV krypton ions. The generation of the characteristic projectile and target X-rays and of the non-characteristic radiation is examined as a function of the impact energy and of the target atomic number. The observed X-ray spectra show a significant change in the intensity of the various components of the target L radiation which is interpreted in terms of relativistic MO diagrams.

The R-matrix method calculations of Berrington et al. (1975) and Fon et al. (1978) are extended to the calculation of the integrated and differential cross sections for electron excitation of the ground-state helium atom to the 2³S and 2³P states in the energy ranges (21.4<or=E<or=30 eV) and (80<or=E<or=200 eV). For intermediate energies (30<or=E<or=80 eV) pseudo-resonances arise which make accurate cross sections difficult to calculate and this region is therefore not considered. The calculations are compared with recent theoretical calculations of the mutual angle with the nuclear charge has been fitted by.

Angular correlations between the escaping electrons after ionising collisions between an electron and single-, double- and triple-charged hydrogenic ions have been examined in the vicinity of the ionisation thresholds. Numerical investigations indicate, within a restricted classical model, an increasing spread of the mutual-angle distribution with an increase of the ion charge, but do not reveal the total disappearance of all angular correlations for an ion nuclear charge Z>or=3, as predicted by Rau (1971, 1976). The increase in the total spread of the mutual angle with the nuclear charge has been fitted by a Z^0.56 power law. Dependence of the angular spread on the total energy has been examined for e-Li²⁺ and excellent agreement with the analytical prediction of Vinkalns and Gailitis (1967) has been obtained. The final energy distribution generally appears to be uniform. It deviates from this simple behaviour when Z>or=3. The relation between the regular non-uniform distributions and those suggested by Temkin (1974) and Rau has been discussed.

Wannier's cold-gas ion-transport theory has been extended to finite gas temperature by the inclusion of thermal motions in a perturbation approach. The simplicity of the cold-gas collision integrals makes the use of very large basis sets feasible, and as a consequence very accurate calculations of the transport coefficients, over the range of electric fields where the perturbation theory gives convergent results, can be carried out. For light ions in a heavy gas, i.e. for non-neutral-atom mass ratios m/M<or approximately=0.25, convergent results are obtained for all values of the electric field, while for larger mass ratios the perturbation expansions diverge in the low-field range.

X-ray spectroscopic methods for the determination of electron densities N_e>10²⁰ cm^-3 in plasmas with high electron temperatures (T_e>or approximately=0.2 keV) are reviewed. The methods considered are based on the following measurements of: (i) relative intensities of resonance and intercombination lines in spectra of He-like ions; (ii) relative intensities of satellite lines in spectra of H-like ions; (iii) relative intensities of H-like resonance doublet components; (iv) Stark broadening of the lines of H-like ions. All these methods have been used in experiments on plasma heating by focusing a laser beam on plane massive targets containing elements with nuclear charge Z=12-19. The density distribution in the plasma was measured within the limits of N_e=10¹⁸-10²¹ cm^-3. The results obtained by all the methods are compared and the limits of application of the methods are determined.