Table of contents

The character formulae for isoscalar factors given by Fieck (1977) are corrected. The validity of the corrected formulae is shown by considering the point groups O and D₃.

The transitions 1s2s³S-1s2p³P of Si¹²⁺ have been excited using the beam-foil method, and the wavelengths measured for the first time, for beam energies in the range 21-57 MeV. The measurements may be used to test calculations of the ³P fine-structure splitting to +or-1%, and when better calculations become available, the Lamb shift contributions to the transitions energy may be evaluated to +or-11%.

The oscillator strengths of the principal series doublets of Rb 5²S_1/2-n²P_3/2 and 5²S_1/2-n²P_1/2(6<or=n<or=20) have been measured in absorption using a high-resolution spectrometer. The absolute f values and the ratios of the doublet components rho =f_3/2/f_1/2 are compared with available theoretical and other experimental results. At n=15 a pronounced perturbation in the monotonic increase of the oscillator strength ratio rho as a function of the principal quantum number n is found.

The impact of recent experimental and theoretical studies of 5p excitation in Ba I on the problem of the double ionisation anomaly is considered. Energy coincidences between the ejected electron spectrum and photoabsorption thresholds allow real intermediate states (other than the one originally proposed) for two-step autoionisation to be located.

In a crossed-beam experiment the vibrational excitation of H₂ by proton impact has been measured at a scattering angle of 0 degrees (angular resolution +or-0.5 degrees ) and in the energy range 15 to 120 eV (CM system). The spectra show resolved vibrational transitions nu =0 to nu ' with nu '=1, 2, 3 and 4. The measured energy dependences of the differential cross sections for all four vibrational transitions are characterised by an oscillatory behaviour superimposed on a linearly rising function. The results of first calculations for the nu =0 to 1 transition are in very satisfactory agreement with the experimental data.

The effect of taking proper account of zero-point vibration in adiabatic-nuclei calculations of cross sections for low-energy (1-10 eV) e-H₂ scattering is investigated. Explicit integration over internuclear separation leads to significant changes in integral cross sections over fixed-R results, where the molecule is frozen at its equilibrium separation. Increases ranging up to 5% for elastic scattering and 40% for rotational excitation are observed, with the largest changes occurring at the lower end of the energy range. Angular distributions are also affected. Adiabatic-nuclei cross sections are in good agreement with experiment over the whole energy range.

The first electron-photon coincidence data for molecules are presented. A considerable degree of coherence in the emitted radiation is observed even though the rotational structure is not resolved. The Stokes parameters obtained are related to the state multipoles defining the excited state.

A derivation of the relativistic, quantum electrodynamical expression for the alpha ²(Z alpha )²-vacuum polarisation correction to muonic energy levels is presented. Using this derivation based on the bound-state interaction picture, the expression for the other radiative corrections such as the Lamb shift alpha (Z alpha )³-vacuum polarisation, etc are obtained. This shows that the resulting expression for the alpha ²(Z alpha )²-vacuum polarisation contribution is consistent with those used in other calculations which are rigorously based on quantum electrodynamics. Furthermore, this approach avoids the problem of double counting.

An integral transform technique is suggested to evaluate the multicentre molecular integrals for Slater-type orbitals. For example, the two-electron-four-centre integral can be cast into a simple numerical integration over a three-dimensional space. The integrand contains a finite sum of spherical harmonics and spherical Bessel functions of order L whose arguments contain the complicated angular dependence of the geometry of the centres: L<or= Sigma _il_i,l_i being the angular quantum number of each individual orbital. A few examples are given.

It is pointed out that there is a need for values of the oscillator strengths of many transitions in Fe II and Fe III in order to interpret the sun's Fraunhofer spectrum in the vacuum ultraviolet. A description is given of a beam-foil experiment to measure the atomic lifetimes for some of these transitions. Results are presented for three multiplets in Fe II and one in Fe III with estimated accuracies in the range +or-5% to +or-22%. Measurements for a wider range of transitions were not attempted because of difficulties arising from the crowded nature of the spectrum, the poor spectral resolution and general weakness of the signals.

The photoabsorption of atomic Fe, Co, Ni and Cu has been determined in the energy range from 45 to 90 eV. For Co, Fe and Ni there is a strong asymmetric resonance at the 3p threshold which is attributed to 3p⁶3dⁿ to 3p⁵3dⁿ⁺¹ transitions. These transitions are split by the interaction of the 3p hole with the partly filled 3d shell and the 3p spin-orbit interaction and considerably modified by the interaction with the 3p⁶3dⁿ to 3p⁶3d^n-1 epsilon f transitions. The two channels are strongly coupled bia the 'super Coster-Kronig' decay 3p⁵3dⁿ⁺¹ to 3p⁶3d^n-1 epsilon f. The dominant absorption band is conserved in the spectra of the metals whereas the rich atomic fine structure at the 3p threshold and approximately 10 eV above is washed out. The 3d shell is filled in Cu and therefore the strong 3p-3d resonance is missing in the spectra of atomic and metallic Cu.

Anomalously small values of the intensity ratio alpha of resonance and intercombination lines of He-like ions have been observed in Nd- and CO₂-laser-produced plasmas. The values of alpha obtained are explained via calculations using a non-stationary ionisation model for the plasma (overheated for CO₂-laser plasma, and supercooled for expanding Nd-laser plasma). The measurements of the intensity ratio may be used to obtain information on the relative concentrations of H-, He-, and Li-like ions in the plasma. The results obtained allow one to answer the questions: when must the non-stationary character of the plasma ionisation state be taken into account for the observed spectra to be interpreted correctly; and when can one use more simple stationary models for the plasma concerned?.

The positions of the two satellites exhibited by the Lyman alpha line of hydrogen in the presence of an argon plasma have been determined in the frame of the quasi-static theory by means of an ab initio potential. One of the satellites appears to be due to a minimum in the potential of the levels arising in the (Ar⁺(3p⁵)+H(n=2)) configuration and the other satellite to an avoided crossing between these levels and the levels of the (Ar(3p⁵4s)+H⁺) configuration.

The bands associated with the Cs(6S to 7S) forbidden transition induced by collisions with various rare-gas atoms are studied (Xe to He). The ground-state potential curves being available, the quasi-static interpretation of the temperature dependence of the band profile gives the upper-state potential curves. Additional information about these curves is drawn from the analysis of the vibrational structure of the band. Finally, the values of the induced oscillator strength of the transition are obtained as a function of interatomic distance. A comparison with available theoretical results is presented.

Presents a method of determining the regions of atoms (using classical mechanics) in which, for a given type of collision by a charged particle, excitation or ionisation of the atomic electrons can occur. Calculations have been performed for ionisation and excitation processes using the classical binary encounter approximation. Two classical models of the atomic orbit are taken into account and three different approaches to the treatment of the collision between the atomic electron and a projectile are analysed. Numerical results are presented as the plots of the regions (called 'active'), where the ionisation and excitation processes may occur. Applications for the theoretical analysis are also shown.

Cross sections for the collisional ionisation of atomic hydrogen by energetic C⁶⁺ and He²⁺ ions have been calculated in the sudden approximation (simplified form of the first Magnus (1954) approximation). An impact parameter approach was used in which the projectile was assumed to follow a classical rectilinear trajectory. The problem associated with the anomalous large impact parameter behaviour of the ionisation probabilities in this approximation was dealt with by merging the sudden approximation calculations at small impact parameters with the Bang and Hansteen (1959) semiclassical approximation results at larger impact parameters where perturbation theory becomes applicable. The cross sections obtained in this manner are in reasonable agreement with classical trajectory calculations and available Glauber theory estimates for collision velocities above about 3 au.

Absolute differential elastic cross sections have been measured for electron scattering by helium, neon and argon atoms over the incident electron energy range of 0.5 to 20 eV, and over the scattered electron angular range of 15 to 150 degrees . The experimental method uses crossed electron and modulated atom beams, 127 degrees electrostatic energy analysis of incident and scattered electrons, quadrupole mass analysis of the target gas and single-particle counting techniques. It is shown how the use of a chi-squared optimisation analysis leads to better knowledge of apparatus behaviour and of cross section determinations. Absolute cross section values have been established by the methods of (i) the parametrisation of the relative elastic angular distributions in terms of the first three phases at 22 energies and 27 angles within the above ranges, (ii) a phaseshift analysis around the 19.3 eV (²S) resonance in helium and (iii) the measurement of elastic scattering signal ratios in Ne and Ar at any energy and scattering angle (here 19.2 eV and 60 degrees ) for known relative source pressures.

A semi-empirical molecular-orbital method for modelling the effective potential for electron-molecule scattering is applied to elastic scattering and rotational excitation of CO₂ at 20 eV impact energy. Agreement with experiment is reasonably good. The calculated rotationally summed integral cross section is 67.8 a₀².

Differential cross sections have been measured using coplanar kinematics for the electron impact ionisation of atomic hydrogen at incident energies of 100, 113.6, 250 and 413.6 eV for a variety of emitted electron energies and angles. The data are compared with differential cross sections calculated using: (i) the factorised distorted-wave half-off-shell impulse approximation (DWIA), (ii) the plane-wave impulse approximation (PWIA), and (iii) the plane-wave Born approximation with exchange (PWBE).

The ejected-electron spectrum of strontium vapour between 10.1 and 21.3 eV has been observed at 75 degrees to an incident energy beam of 23.5, 38, 50, 75, 150 and 500 eV energy. Comparison is made with the earlier ejected-electron measurements of Schmitz et al. (1976) the results of which have been considerably extended.

An ab initio L²-basis-set theory for low-energy electron-molecule scattering, recently proposed and tested for e-H₂ elastic scattering and rotational excitation, is applied to pure vibrational and vibrational-rotational excitation of H₂ by slow electrons (0.6-10 eV). An optical potential including direct, exchange and polarisation terms is expanded in a diagrammatic perturbation series and used in the framework of the T-matrix expansion method of Rescigno et al. (1974). Polarisation is shown to have a substantial effect at the lower end of the energy range, decreasing at higher energy and practically vanishing at 10 eV. Calculated integral and differential cross sections for both vibrational and vibrational-rotational excitation are in good agreement with experiment.

The usual expressions for the complex refractive index of a plasma are derived from mean-free-path-type formulae. They are not able to distinguish precisely between the mutual influence of electron-ion and electron-atom collisions and they ignore totally electron-electron collisions and electrostatic correlation effects, which are important in dense plasmas. In the frame of the kinetic transport theory more accurate expressions are obtained. It is shown that the difference can amount to 30% for the imaginary part of the refractive index of a dense argon plasma.