Table of contents

A calculation of the 1s2s¹S-1s epsilon p¹P transition oscillator strength in helium is reported. Agreement to well within 1% between the dipole-length and dipole-velocity values is obtained. The results are believed to represent a considerable improvement in accuracy on the close-coupling calculations of Jacobs (1974).

The degree of polarisation of electrons ejected from unpolarised atoms by unpolarised and linearly polarised light can reach 70% and 100%, respectively, and its measurement gives detailed information on atomic structure. Results of calculations for Ar and Xe atoms are presented.

The variations of the interaction time can modify the amplitude of the resonance peaks in three- and four-photon ionisation of caesium. The predictions of the theory and the experimental data are compared and the agreement is excellent.

Diode laser measurements on the nu ₄+ nu ₅ band of dideuteroacetylene are reported. Various lines of interest in this band have been measured to an accuracy of 0.001 cm^-1 and the self pressure broadening and helium pressure broadening coefficients determined. These measurements are of relevance to the recent observation of optically pumped laser action in this molecule.

High-resolution studies of sulphur-induced titanium KLⁿ-satellite X-ray lines are presented and the origin of the KL⁰ peak is investigated. In collision systems where the projectile is lighter than the target, photoionisation of the target K shell by target K beta X-rays can give rise to significant K alpha _1,2 contributions in the observed KL⁰ intensity.

The second Born contribution to electron capture has not yet been unambiguously detected even though it almost surely dominates over the first Born term at sufficiently high energies. Three methods are suggested whereby it might be possible to detect the second Born contribution to the cross section for forward electron capture from a hydrogen atom or molecule or from a helium atom by a lightly charged (bare) ion incident with an energy in the range 10-80 MeV per nucleon. The first method involves measuring the cross section for 1s to 3d capture to check that this cross section exhibits a 1/v¹¹ velocity dependence. The second method involves measuring the angular distribution for scattering angles ranging from zero to a few minutes; at a small but non-zero angle there should be a sharp peak in the angular distribution. For the target a He atom or H₂ molecule, a third possibility is to detect a peak in the energy spectrum of electrons ejected from the target in a direction perpendicular to the beam direction, in coincidence with capture.

The authors studied the threshold ionisation of H atoms, He⁺ and Li²⁺ ions. In particular, they calculated the orbital angular momenta attained by the two escaping electrons. This was done by (i) using the classical three-body collision theory and (ii) by expanding the two-electron wavefunction given by Rau (1971), in spherical harmonics. The anomalous behaviour of the angular distribution function of the two electrons for residual ion charge Z>or=3 is not reflected in the calculated angular momenta.

S-wave phaseshifts for e⁺H are calculated for incident energies of the positron less than the threshold for positronium formation using an adiabatic approximation introduced by Spruch (1975). In this approximation the separation R (but not R) of the proton and the positron is fixed. The results obtained are close to those obtained by Drachman (1965), using the H₂⁺ potential of Dalgarno and Lynn (1957), and agree qualitatively with the highly accurate results of Bhatia et al. (1971).

General formulae have been obtained for one- and two-centre spin-spin nuclear attraction integrals over Slater-type orbitals with arbitrary quantum numbers occurring in the evaluation of the spin-spin interactions in molecules.

Statistical correlation energies due to the Gell-Mann-Brueckner high-density functional were calculated for the atoms Mg to Ar and the molecules N₂ and F₂, and then corrected for statistical errors by means of a global correction based on the idea of the finite Kohn-Tong electron gas. 92% of the corresponding atomic and 80% of the corresponding molecular correlation energies were obtained. For the same systems correlation energies were calculated on the basis of the phenomenological Ma-Brueckner functional (1968): for molecules about 90% of the total correlation energy is recovered, again the results for atoms being even better.

A contribution is made to the development of fully relativistic configuration interaction using Brueckner-Goldstone many-body techniques. The theory is applied to the magnetic hyperfine interaction and the quadrupole hyperfine interaction in the Gd³⁺ ion. For the general half-filled-shell case several formulae are given. In particular, the quadrupole analysis shows how this method provides a unique insight into the contributions to this complicated interaction. For the magnetic hyperfine interaction the authors made a detailed comparison with results obtained with direct-integration techniques, in particular the UHF method. Two-body magnetic interactions represented by the Breit interaction proved to be negligible in effect.

The excitation energies and the probabilities for the radiative and radiationless decay of the autoionising states belonging to the 2s²2p, 2s2p² and 2p³ configurations have been calculated for three-electron atomic systems on the basis of perturbation theory with respect to inter-electron interaction in a broad range of values of atomic number Z. Using the results of the diagonalisation calculations of the autoionising states of two-electron systems, the authors have obtained estimates of corrections to the autoionisation widths calculated by perturbation theory. The calculations of the radiative transition probabilities for Fe and Ni ions are in good agreement with the 1976 results of Nussbaumer.

A previously developed exact semiclassical method is used to evaluate multi-photon spectra and Floquet characteristic exponents (dressed-atom energies) for several multi-level systems interacting with both strong oscillating and static fields under varying degrees of saturation. The usefulness of characteristic exponent plots in extracting the physically significant features of the spectrum, without the necessity of evaluating portions of the complete spectrum, is demonstrated for a number of model three-level systems which are not directly amenable to study by the often used rotating-wave approximation. The effect of neighbouring non-resonant states on two-photon resonance profiles is also discussed using three to six level systems (H and Na atoms) as models.

The pulsed output obtainable from a mode-locked laser has usually been described using classical radiation theory. A quantised-field representation is developed and utilised in a discussion of multiphoton ionisation. The assumptions made in deriving the results are critically examined, and a modification of the theory to deal with other kinds of pulsed radiation is indicated.

The relative differential cross sections for elastic scattering of H⁺ by a caesium vapour has been measured. The reduced cross sections exhibit a characteristic rainbow oscillatory structure. The rainbow reduced-scattering angle is 63.5+or-1.4 eV deg. A best Morse potential has been determined by fitting the experimental data with semiclassical differential cross sections. The well depth of the Cs(6s)H⁺ Morse potential is 0.77+or-0.05 eV at 10+or-1 a₀, the exponential parameter being 2.5+or-0.1. For comparison the Cs(6s)H⁺ molecular potential has been calculated using a pseudopotential approach with a large LCAO-MO basis. The interaction of the valence electron with the Cs⁺ ion is represented by a Hellmann pseudopotential. When the non-linear coefficients of the trial wavefunction are optimised, satisfactory agreement is obtained with the experimentally determined potential.

A projected valence bond method is used to obtain adiabatic and diabatic states for the study of the H⁺+Cs collisional system. By freezing an adjusted Hartree Fock type Cs⁺ core the electronic problem is reduced to that of treating the outer electron of the (H-Cs)⁺ quasimolecule. The present treatment is designed to correct some serious defects of previous theoretical approaches. The calculated well in the adiabatic potential energy curve correlated with H⁺+Cs satisfactorily accounts for the rainbow structures observed recently by Scheidt and colleagues (1977, 1978) in the measured elastic differential cross sections at low energy.

The continuous X-ray spectrum generated by the bombardment of a thick target of aluminium by 450 keV protons has been determined in the photon energy range 2.5-8.5 keV. Comparison with theory shows that the contribution from stopping electrons is considerable below 5 keV, and that electrons additional to the 1s electrons make a significant contribution.

It is shown that the shapes of the peaks in the forward electron velocity distributions, observed when protons and H₂⁺ ions of 0.5-2.5 MeV are transmitted through thin carbon foils, are consistent with 'charge exchange to continuum theories'. Two possible reasons, concerning instrumental resolution and the treatment of the background, are advanced for the inability of others to confirm these theories.

For pt.I see ibid., vol.9, p.1401 (1976). Experimental angular distributions of electrons formed by transfer ionisation in slow (E_coll=5/10/20 eV) He²⁺-Hg collisions are reported. Also angular distributions of super-elastically scattered He⁺ ions were measured at collision energies of 10 and 20 eV. The ion angular distributions confirm the reaction mechanisms proposed earlier. The electron angular distributions are anisotropic with asymmetries strongly depending on collision energy. An analysis of angular dependent energy shifts found in the electron spectra yields finer details of the ionisation mechanism which had previously been proposed to be of Penning type.

Wallace's (WSOEA) second-order eikonal approximation is used to calculate elastic differential cross sections for e^-or+-H and e^-or+-Ar scattering at 100 and 500 eV in the angular range 0-20 degrees . The predictions of the WSOEA are compared with those of the partial-wave calculation (PWC), in addition to the results obtained in the first Born approximation (FBA) and Glauber's first-order eikonal approximation (FOEA). The WOSEA shows a definite improvement upon the FBA and the FOEA and yields cross sections in good agreement with the PWC results.

The wavefunction of a low-energy unbound electron in the field of a closed-shell diatomic molecular ion is calculated using a single-centre expansion in the static-exchange approximation. The evaluation is reported of the electronic capture width necessary for the calculation of the dissociative-recombination rate of the CH⁺ ion in its ground state, with a two-centre molecular integral program. The agreement between calculated values for a very low energy (0.01 eV) and extrapolated values from Rydberg states, found both for the quantum defect and the electronic-capture width, establishes the reliability of the direct calculation.

It is shown that the measurement of wing intensities of those Stark-broadened Lyman lines, for which the upper level is thermally populated with respect to the free electrons and fully stripped ions, enables accurate deduction of electron density in a high temperature plasma. The parameters which characterise the plasma microfield distributions are defined in such a way as to allow the use of existing tabulated values of the electric microfield to construct accurate theoretical profiles. The method is illustrated using emission lines from laser-irradiated solid polymer targets.

A laser-based method of preparing a plasma with only a single atomic-state population significantly out of equilibrium is described. This technique allows determination of collision rates for first excited states largely free from the problems of multiple-level excitation and cascade inherent in most fluorescent scattering techniques in plasmas, or from the absolute intensity measurements in the VUV inherent in emission spectroscopic techniques. The results obtained for the n=2 state of hydrogen, however, pose severe problems in interpretation in terms of presently accepted electron collision rates.