Table of contents

Radial integrals for octupole transitions are calculated by various methods in the framework of the Coulomb approximation. A comparison of numerical results shows that in many cases accurate values may be obtained from analytical semiclassical expressions in terms of Anger functions, or by using two functions tabulated in the dipole case by Edmonds et al. (1978).

Calculations have been made of the hydrodynamics and kinetics of an expanding cylindrical C VI plasma layer. It was shown that for the evaporation of a approximately 0.1 mu m thick layer from the surface of a carbon fibre with a heating laser pulse energy of approximately 2.6+or-0.6 J cm^-1, a peak gain coefficient approximately 3 cm^-1 (18.2 nm) is attained for approximately=0.2 ns. These results are in disagreement with experimental data.

K-shell electron correlation effects within Li⁺(¹S), Li(²S), Li(²P), Li^-(¹S), Be²⁺(¹S) Be⁺(²S), Be⁺(²P) and Be(¹S) are examined in position and momentum space. The assessment is made in terms of Coulomb 'holes' Delta f(r₁₂) and 'shifts' Delta f(p₁₂), partial Coulomb 'holes' Delta g(r₁₂;r₁) and 'shifts' Delta g(p₁₂;p₁), and the correlation-induced changes in various statistical correlation coefficients, Delta tau . For each space, tau are chosen which reflect the angular and radial components of correlation in different regions of the doubly occupied K shell. From such an overview, the changes in the K alpha K beta correlation effects arising from the interaction of an outer electron could be ascertained and rationalised. In position space, the K-shell results for Delta f(r₁₂) and Delta g(r₁₂;r₁) were only marginally affected. Nevertheless, the Delta tau values did suggest changes of emphasis between the angular and radial correlation components. For the complementary momentum space, however, Delta f(p₁₂), Delta g(p₁₂;p₁) and the Delta tau values were found to be quite sensitive to the occupation and nature of the outer orbital, whether 2s or 2p. The momentum results were, therefore, the most informative. Besides highlighting the influence of occupied outer orbitals on the correlation effects within a doubly occupied inner shell, the present study demonstrates, very forcefully, the necessity for such examinations to be carried out in momentum space.

The authors present theoretical results on the stability of Be₂²⁺, BeH²⁺, BH²⁺, BeHe²⁺ and BeLi⁺. The wavefunctions and potential energy curves have been obtained with large basis sets to account properly for the ionic structures and the correct shape of the curves. The ground-state potential curve of the first three molecules is dissociative with a local minimum. However, the previously predicted BH²⁺ does not seem to exist since its potential is found not to support any vibrational levels. BeHe²⁺ and BeLi⁺ are thermodynamically stable. The tunnelling widths of the BeH²⁺ and Be₂²⁺ lowest rovibrational resonances are negligible at room temperatures. Using the stable light dications as reactants, highly exothermic physicochemical reactions are possible.

Line broadening of calcium Rydberg states in the presence of Xe, Ar and He perturbers is recorded using a two-photon absorption and ionisation detection technique. The dependence of the broadening parameter gamma on pressure for the lower Rydberg states ns ¹S₀ (n=6-12) and nd ¹D₂ (n=4-10) is calculated. The abnormal behaviour of some of these states with respect to the broadening parameter is discussed.

The semiclassical approach in the framework of the nonlocal theory of resonant electron-molecule collisions is discussed. It is shown that in the simplest case of electron collision with a two atom system, which is characterised by repulsive energy curves, the integral equation of the nonlocal theory may be rearranged in the form of an inhomogeneous second-order differential equation. This equation, which determines the inelastic scattering amplitude as a function of the energy of the atoms in the final state, is in close agreement with the key equation of local theory (the 'boomerang' model of Herzenberg (1968)). The WKBJ solution of the equation obtained is discussed and the results of calculations for a model system and given. Some interference patterns in the dependences of the cross sections on the final energy are obtained. The mechanism causing these patterns is described.

On the basis of Monte Carlo simulation of Auger cascades in many vacancy krypton atoms after a transfer ionising collision with 1.4 MeV/nucleon multiply charged ion (Fe¹⁵⁺) the charge spectra of recoil ions corresponding to 1-5 captured electrons have been calculated and compared with the experimental data, testifying to the important role of Auger cascades in charge spectrum formation.

Low-energy electron scattering off an HF molecular target is investigated in the static exchange approximation. The scattering by the static potential is treated numerically exactly whereas a separable approximation is adopted for the exchange interaction. A perturbative correction of the asymptotic part of the wavefunction is introduced in order to accelerate the convergence with respect to the cut-off radius of the numerical integration of the Schrodinger equation. Finally the dependence of the separable approximation for the exchange potential on the size of the basis set is investigated.

Using the unitarised eikonal Born series (UEBS) method, the authors have calculated the values of the state multipoles describing coherence effects between amplitudes for the excitation of the n=2 states of atomic hydrogen by electron and positron impact, in the energy range between 100 and 400 eV. They also present UEBS results for various alignment and orientation parameters and for the position of the centre of charge of the excited atom, for both electron and positron impact.

Wannier's classical theory has been successfully applied to studies of fragmentation functions when the interaction potential was homogeneous. Here, the methods of classical mechanics are applied to cases with a nonhomogeneous interaction potential. In this case an analytic solution is not available. However, the numerical solution can be fitted to an analytic expression. A system is discussed that in its final state consists of a highly excited atom, an electron and a positron (A*+e^-+e⁺). The interaction potential consists of the Coulomb term plus monopole to quadrupole and/or polarisation terms. Near threshold, the cross section is fitted to the form sigma =aE^kappa . Results are given for H and He.

The author reports quite accurate calculations on the rotationally elastic, inelastic and summed differential, integral, momentum transfer and energy-loss cross sections for the positron-CO scattering below the positronium formation threshold (7.2 eV). In the body-fixed adiabatic nuclei approximation, a single-centre expansion scheme is used to solve the scattering problem in the close-coupling theory. Fully converged cross sections are obtained via the multipole-extracted adiabatic nuclei (MEAN) method. The polarisation effects are included approximately in a parameter-free way via the positron correlation polarisation (PCOP) potential proposed by the author. The PCOP model is based on the short-range correlation energy, in _corr, of one positron in a homogeneous electron gas of the target. In the outside region, the in _corr, is joined smoothly with the correct asymptotic form of the polarisation interaction, where they cross each other for the first time. The analytic form of in _corr is derived from variational calculations including full positron-electron interactions in the Hamiltonian. The new PCOP results are compared with the author's earlier calculations based on the electron correlation polarisation (ECOP) potential and the R-matrix results of Tennyson and Morgan (1987). The PCOP total cross sections are in very good agreement with the measurements in the whole energy region considered: in particular, at very low energies (below 2 eV), where the ECOP values are in disagreement with experiment. The author's rotationally inelastic cross sections are also in good agreement with the R-matrix calculations.

The structural and electronic properties of small sodium clusters (Na_N, with 2<or=N<or=14) have been investigated. A Hubbard Hamiltonian with s orbitals only is used to model the electronic interactions. The eigenstates of the system are determined by a perturbation method and in the Hartree-Fock approximation. The energy minimisation is accomplished by means of the simulated annealing method to avoid local minima. The two electronic interaction parameters are taken from experiment and a simple scaling relation with distance is assumed. The resulting cluster radii, symmetries, magic numbers and ionisation potentials compare very well with experiment and with other, more elaborate, theories.

The Raman photon echo (RPE) in atomic vapour, allowing for depolarising atomic collisions and hyperfine structure of the atomic levels, has been studied theoretically. A new method of determining the electronic and electronic nuclear relaxation rates of the multipole moments of gas atoms with nonzero nuclear spin using the RPE has been proposed. The RPE method also makes it possible to measure the derivatives of these relaxation rates with respect to the impurity gas pressure. A method of determining the energy separation between the hyperfine structure sublevels has been developed. This method is based on the quantum beats of the RPE intensity and may be used when the hyperfine splitting is less than the Doppler linewidth. A possibility of identifying the forbidden transitions in both the electronic and total angular momenta has been shown.

Optical second harmonic generation has been observed on or near the 4s ²S_1/2-6s ²S_1/2, 4s ²S_1/2-np ²P_{1/2, 3/2} (n=5, 6) and 4s ²S_1/2-nd ²D_{5/2, 3/2} (n=3,4) two-photon resonances in potassium vapour, as laser scanning in the 660-940 nm wavelength range. The second harmonic intensities and ionisation currents have been measured as a function of pumping laser detuning from the two-photon resonances. On or near the two-photon resonances, the intensity and vapour-density dependence of second harmonic generation have also been investigated. The saturation of second harmonic generation has been observed. Some calculations and analyses have been made with the chi ⁽³⁾ model. The results have been compared with the measurements. Based on these analyses, the mechanisms for second harmonic generation in isotropic vapours are discussed.