Table of contents

For a hydrogen atom in a uniform magnetic field the authors calculate wavefunctions and oscillator strengths for bound states in a region reaching far into the quasi-Landau regime, At the zero-field threshold quasi-Landau photoabsorption peaks are due to groups of individual states which have large amplitudes at small electron-atom separations, but which are not localised around the plane perpendicular to the field and cannot be related diabatically to low-field states. However, in going from the low-field region to the quasi-Landau region the oscillator strengths closely follow trajectories obtained by naively extrapolating the behaviour of the low-field K=0 states.

The author presents the first calculations on the total cross sections of e⁺-CO vibrationally elastic scattering in the energy range from 0.25 eV up to the positronium threshold. In the adiabatic-nuclei approximation, a single-centre expansion approach is employed to determine the fully converged scattering parameters. The e⁺-CO interaction energy is composed of an accurate repulsive static term and a parameter-free attractive correlation-polarisation potential. The results for the total cross sections are compared with recent measurements; the agreement is fair except below 2 eV, where theory underestimates the experimental data by about 50%. Results on the differential, momentum transfer and rotational excitation are also reported in the same energy region.

The authors present a discussion of the results for the positronium fraction, F, in gaseous methane against density and electric field, recently investigated by Sharma et al. (1985). It is shown that these data are consistent with a significant contribution to F by both spur and Ore/spur-like processes in molecular gases at densities above a few amagats.

Red shifted conical emission induced by a laser blue shifted with respect to the D₂ line of sodium is observed and interpreted in terms of Cherenkov emission. Measured cone angles and wavelength shifts agree well with model calculations taking into account saturation and dispersion effects.

The dipole polarisability alpha and the higher polarisabilities C, B and gamma have been calculated for Li⁺, Li and Li^- at close to the Hartree-Fock limit. All the electric polarisability tensors up to the fourth rank have been included in this study. The contributions of these tensors to the simple potentials Li-e^-, Li-H⁺, Li-He²⁺ lead to deductions about the chemical behaviour of the lithium atom.

A modified 'shake-down' model describing both Auger and photoelectron lineshapes is derived from the two-potential formulation of post-collision interaction following photon impact. The model gives the correct expression for the lineshapes at high photoelectron energies and finite Auger energies. The distortion of the lineshapes is shown to be given by the overlap between the photoelectron wavefunctions in the final and smeared intermediate states, for which a construction is explicitly given.

The process of post-collisional Auger decay induced by electron impact is discussed. Close to the excitation threshold the situation is framed within the atomic and molecular resonance theory, and discussed from the point of view of electron Compton scattering. The connection between post-collisional effects in Auger decay and the Compton defect is indicated. For restricted classes of processes, the scattering amplitudes is derived. The derivation is based on the Gell-Mann-Goldberger two-potential formulation combined with the complex coordinate method, and provides an extension of the treatment recently proposed by us for photon-induced Auger decay.

Differential spectra for electron production in the forward direction have been obtained by collision of H₂⁺ and He⁺ ions with H₂, He, Ne and Ar gases. The measurements where made with ions of energy 0.8-2.8 MeV. The target consisted of a jet of gas from a multicapillary array, thereby providing a well localised and directed gas beam. A 30 degrees parallel-plate electrostatic analyser having a FWHM resolution (dE/E)=10^-3 was used to study the energy of the ejected electrons. A cusp-shaped peak centred on v_e=v_i was observed for all the electron spectra, suggesting that these electrons were ejected with only a small energy relative to the parent ions. This process has been called electron loss to the continuum (ELC) since in some ways it mimics the now well established phenomenon of charge exchange to the continuum (CEC). In contrast to data for ELC from heavy ions the cusps observed here were generally asymmetric. They did, however, show a width approximately independent of projectile velocity, a feature which seems to distinguish ELC from CEC. Both the cusp shapes and widths are compared with theoretical predictions and other experimental measurements.

The width Gamma of the electron loss of the continuum (ELC) cusp for He⁺ colliding with He is calculated using the Born approximation. Good agreement is found with the recent data of Man et al. (1986).

Closed-form analytic expressions for the dynamic autoionisation widths of capture-ionisation processes are obtained at large internuclear distances. The relation of these results to collisional dynamics of capture-ionisation processes is discussed.

In the collision system Neⁿ⁺+Ne⁰⁺ to Ne^n'++ne^q++(n'-n+q)e^-, absolute cross sections sigma (n,n',q) for the production of recoil ions were determined by a coincidence technique, in which the final charge states of the projectile (n') and the recoil ion (q) were measured simultaneously. This method permitted the investigation of the velocity dependence (at energies of 75-360 keV u^-1) of different reaction channels, such as pure multiple ionisation of the target, and multiple ionisation accompanied by both capture of one or more target electrons into the projectile system and loss of projectile electrons. In the velocity range investigated here direct multiple ionisation at large impact parameters is the dominant process in creating recoil ions with low charge states (q=1,2,3), whereas highly charged ions are mainly produced in close collisions. In these symmetric collisions multiple ionisation of the projectile and the target behave quite similarly.

A drift tube of the four-gauze Tyndall-Powell type has been used to obtain accurate mobility data for ground-state Xe⁺(²P_3/2) ions, metastable Xe⁺(²P_1/2) ions and molecular Xe₂⁺/Xe₃⁺ ions in xenon at room temperature (294.1+or-0.2K) over a wide E/N range (E is the electric field strength and N is the gas number density). The measurements were made at E/N values from 50 to 10000 Td in the case of the Xe⁺(²P_3/2) ions, and from 50 to 250 Td in the case of the Xe₂⁺/Xe₃⁺ ions. The mobility data for the Xe⁺(²P_1/2) ions were derived by a manual deconvolution of the Xe⁺(²P_3/2)+Xe⁺(²P_1/2) peak into two Gaussian curves. Owing to low resolution, the data for the Xe⁺(²P_1/2) ions could only be derived for E/N values less than 900 Td. The mobility of the Xe⁺(²P_3/2) ions was found, over the common E/N range, to be systematically 6% lower than that of the Xe⁺(²P_1/2) ions. The charge transfer cross section sigma _c, calculated for Xe⁺(²P₃2/) ions in xenon over the energy range 0.38 to 4.46 eV (lab) using the theory of Skullerud (1969), is estimated to be in error by less than +or-5%.

A semiclassical quantal theory of threshold electron impact ionisation is developed. Singular wavefunctions, classical differential cross sections and matching procedures are avoided. The Kohn variational principle is applied perturbatively to evaluate the scattering amplitude. The absolute electron-helium singlet ionisation total Wannier cross section is estimated to be 2.37 (Ea₀/e²)^1.127a₀², in good agreement with the experiment of Pichou et al. (1978). Both helium electrons and the first four partial waves are included. Doubly differential cross sections p_0.8(E_r,¹/₂ pi ) at E=0.8 eV are found at 2.58*10^-19 cm² sr^-1 eV^-1, in good agreement with the experiment of Pichou et al. (1978). Triple differential cross sections are compared favourably with the recent experiment of Fournier-Lagarde et al. (1985).