Table of contents

A quantum mechanical study of a simple two-dimensional problem identifies lines in coordinate space about which the probability densities are localised. These lines in general do not correspond to extrema in the system's static potential energy surface. The subject of this study has been the hydrogen atom in externally applied parallel electric and magnetic and magnetic fields. Probability density plots of eigenstates, in which the electron experiences comparable influences from all three fields: Coulomb, electric and magnetic, are each found to display a localisation at a polar angle related to an invariant previously obtained in a classical study of the problem. The line about which localisation occurs is found to vary with energy and is in coincidence with a periodic trajectory found within a classical framework.

The authors have studied the two-photon ionisation of the E, v=0, J=0 metastable state of H₂ with resonance on the 4p pi ¹ Pi ⁺_u, v=1, J=1 level, in the 10⁴-10⁹ W cm^-2 laser intensity range. Anomalous dips are observed at resonance (4777.5 AA) on the ionisation signals. These dips are due to competition of photoionisation with very fast predissociation. The resonance profiles exhibit a strong power broadening (55 cm^-1 GW^-1 cm²) due to photoionisation of the resonant level whose cross section is measured to be (7+or-2)*10^-16 cm².

The total cross sections (TCS) for 0.7-400 eV positrons and 1.0-400 eV electrons scattered by benzene (C₆H₆) vapour molecules obtained by a relative method are presented. The values of TCS for positrons at low energies are very high, and the positron scattering at low energies is found to be sharply forward peaked. On the other hand, the TCS values for electrons decrease with decreasing energy at low energies. The positronium (Ps) formation cross sections are rather low in comparison with the TCS.

Excitation of the 3¹D state of helium by electron impact has been studied by measurement of the linear and circular polarisation of the decay radiation emitted perpendicular to the scattering plane in a polarisation correlation experiment. Results are presented over the angular range 40-120' at an incident electron energy of 40 eV. Recent data in the angular range 31.2-50' by Beijers et al. (1986-7) are confirmed but recent calculations do not agree well with experiment.

Two different calculational methods are compared for atomic data used to analyse transitions of the type 1s2s^k2pⁿ-1s²2s^k2p^n-1. Wavelengths, radiative transitions, autoionisation probabilities and intensity factors are compared. These parameters are needed for diagnostics of low-density plasmas as found in solar flares and tokamak devices.

Describes measurements of He II Balmer series line shapes and shifts, using both absorption and emission techniques, in a dense (N_e=2+or-0.5*10¹⁸ cm^-3), cool (T_e=3.4 eV) helium z-pinch plasma. Tunable coherent VUV radiation was generated around 1640 AA by Raman up-conversion in H₂ and around 1215 AA by frequency tripling in krypton. An absorption measurement was made on the near wing profile of the He II Balmer-alpha line (1640 AA) using the Raman up-converted light source. Emission measurements were made on the He II Balmer-beta (1215 AA) line profile; these were complemented by an accurate wavelength calibration using the tunable coherent VUV light from the frequency tripled light source. A shift in the wavelength of the Balmer-beta line centre of 0.7+or-0.2 AA to the red was observed. Discussion is given of this shift in relation to existing theories of the plasma polarisation shift.

In terms of illustrative physical representations of the collision between a fast multicharged ion and a hydrogen or helium atom, a simple method for estimating ionisation cross sections in the range of velocities 1<<Z¹¹²<or approximately= nu <or approximately=Z is proposed.

Presents the results of electron spectroscopy and total cross section measurements of one- and two-electron loss processes in collisions of halogen negative ions with various gases for collision energies below 5 keV. The electron spectroscopy study illustrates that the production of singly excited autoionising states of the halogen atoms can be important for energies as low as 100 eV and is responsible for positive ion production, as has been also recently shown for O^- collisions by Penent et al. These conclusions concerning positive ion production should also be applicable to collisions involving the parent atoms, being a characteristic of these systems rather than that of the negative ion.

By non-coincident electron spectroscopy, it is shown in N⁷⁺+Ar that the electron spectrum allows an identification of double, triple and quadruple electron capture into autoionising multiply excited states of nitrogen. The contribution of double capture processes is discussed in great detail and a comparison with double capture electron spectra previously obtained in N⁷⁺+He, H₂ is made. The triply and quadruply excited nitrogen ions decay through autoionising cascades into doubly excited states. The origin of the observed doubly excited lines as well as of some satellite 3ln'l' lines is discussed through possible decay modes of multiply excited states. All the experimental findings are compared with the predictions of Niehaus' (1986) classical overbarrier model as well as with a recent coincident energy gain spectrum of Roncin et al. (1987).

Charge exchange cross sections for collisions of fast K⁺, Rb⁺ and Sr⁺ ions with an Na vapour were measured for collision energies ranging from 5 to 30 keV. Sr⁺(5s) and Sr⁺(4d) ions providing the first state-dependent data in this mass region. The results are compared with theoretical near-resonance predictions. The application to collinear laser spectroscopy using state-selective neutralisation is discussed.

State-selective single- and multiple-electron capture processes for collisions of multiply charged Kr^q+ ions (q=3-5) and Xe^q+ ions (q=3 and 4) with rare-gas atoms have been studied by means of translational energy spectroscopy, at collision energies between q and 5q keV. The collisional energy gain spectra for single-electron capture by Kr⁵⁺ from He, Ne and Ar are interpreted qualitatively in terms of the 'reaction windows', which are calculated using a multichannel Landau-Zener model. The measured energy gain spectra are in good agreement with the calculations.

The formation of positronium atoms in the n=1 and n=2 levels in e⁺-H scattering has been studied. A second Born approximation has been employed to investigate the processes. The ground-state capture cross section has also been obtained by using a model in which the second-order effect is included and the first-order term is a solution of the coupled static equation. The convergence of the second-order term has been discussed. Differential and integrated cross sections are reported in the energy range 50-1360 eV. For ground-state capture, structure in the differential cross section near the scattering angle 45' has been noticed at all energies considered. For 2s- and 2p-state capture this structure was found at and over 200 and 500 eV, respectively. The present second-order integrated cross sections are always greater than those of the first Born approximation.