Table of contents

Polarization gradient cooling of ⁸⁷Rb in lin perpendicular to lin optical molasses is studied. The author's semiclassical force-and-diffusion simulations yield laser cooling temperatures in agreement with recent experiments by Gerz et al. (1993). Anomalies in the spatial distribution of the cooled atoms and in the heterodyne spectra are discussed.

It is demonstrated that ionization and excitation probabilities of a hydrogen atom by C⁶⁺ impact are influenced considerably by deep bound states of C⁵⁺, which have been disregarded in previous close-coupling calculations because of extremely small transition probabilities. The ionization cross section is overestimated by a few orders of magnitude at 1 keV amu^-1 if n=1 and 2 states of C⁵⁺ are not included in the expansion. The same problem is widely inherent in general diabatic-base-expansion treatments of slow collisions if low-lying states are not included fully.

Positron scattering from atomic hydrogen is analysed above the ionization threshold and resonances for coupled-channels calculations comprising bases of up to three physical states of hydrogen H(1s,2s,2p) and three physical states of positronium Ps(1s,2s,2p) are determined. The calculations have been performed over the entire energy range from the ionization threshold up to a maximum incident positron energy of 4 Ryd. Additional resonances appear as the models increase in complexity. For the six-state model the authors report the identification of one new resonance for the J=0 partial wave, four for J=1, one for J=2 and one for J=3. No resonances are found in the higher partial waves. The positions and widths of the resonances are given.

With the consideration of all the Feynman diagrams of the lowest order and electron spin correlation in the initial state, the one-photon decay rate of the Ps^- ion in its ground state is found to be 3.9*10^-2 s^-1. Comparison is made with the previous calculations of other authors.

Through the derivation of a bound to the gradient correction for the exchange energy, a new local density (LD) approximation to the exchange energy density functional and an analogous LD form for the kinetic energy density functional are proposed. Both these gradient-free functionals valid for spherically symmetric electron densities, yield excellent results for exchange and kinetic energies, using Hartree-Fock atomic densities.

New high resolution observations of the 2p inner-shell and of the doubly excited states of sodium covering the spectral region 320-400 AA are reported. The spectra were recorded in the first order of a three-metre spectrograph equipped with a 5000 line/mm holographic grating, using synchrotron radiation emitted by the 500 MeV electron accelerator as the background source of radiation. More than 200 new levels of Na I are reported, which are interpreted as inner-shell transitions, 2p⁵3s³P_2,1,0,¹P₁)ns, nd and the doubly excited 2p⁵3pnl, 2p⁵3dnl and 2p⁵4snl transitions. All the observed levels lie well above the first ionization threshold and are arranged into Rydberg series converging onto fifteen conspicuous limits. The inter-channel interactions between the overlapping series have been parameterized using multichannel quantum defect theory.

The spectrum of caesium was photographed in the 300-1240 AA wavelength region on a 3 m normal incidence spectrograph. The earlier analysis of the 5s²5p⁴-5s5p⁵ transition array of Cs IV was confirmed. The 5s²5p⁴-(5s²5p³5d+5s²5p³6s) transition array has been investigated. 41 of 48 levels belonging to the 5s²5p³5d and 5s²5p³6s configurations have been established. One hundred and two additional lines have been classified in this spectrum. Configuration interaction calculations involving all three odd parity configurations have been carried out to satisfactorily interpret the observed spectrum.

A systematic study of experimental Stark widths of 3s-3p and 3p-3d transitions in N III-Ne VI measured in a plasma produced by the gas-liner pinch device is reported. The scaling of measured Stark widths with the spectroscopic charge number Z (Z=3 for N III, etc.) shows appreciable deviations from that predicted by some theoretical calculations in the electron-impact approximation. A strong indication is found that proton collisions influence the linewidth of the 2s2p3s⁴P⁰-2s2p3p⁴D transition in Ne VI in a significant way.

Calculations of the Hartree-Fock and local-density approximation exchange energies for all atoms with Z<or=18 using the 6-311G** Gaussian-type basis set have been carried out. This particular basis set reproduces remarkably well both the Hartree-Fock and the local-density approximation exchange energies obtained from higher quality Slater-type orbitals. In addition, the Becke correction to the local-density approximation exchange energy has been studied. The best value of its sole parameter beta , for the 6-311G** basis set, is found to be 0.0051, which differs from its corresponding value of 0.0042 obtained with a Slater-type basis set. Results for other standard Gaussian basis are also reported.

Large scale ab initio calculations for the radiative data of Fe II have been carried out in the close coupling (cc) approximation employing the R-matrix method and a target state expansion consisting of 83 LS terms of Fe III. All bound states of Fe II with n<or=10 and l<or=7 are considered. The results include 1301 bound states in LS coupling, oscillator strengths for 35941 transitions among the bound LS states, and detailed photoionization cross sections for all bound states. Autoionizing resonances, as well as the coupling to excited core states, enhance the photoionization cross sections substantially. The calculations of oscillator strengths have been extended beyond the requirement of the Opacity Project to include a large number of fine structure transitions in Fe II, using an algebraic transformation of the LS coupled line strength and the observed energies. The present f-values compare favourably with available experimental values and the calculations by Kurucz. However, the present results differ considerably from earlier 16-state R-matrix calculations and the new radiative data yield Rosseland mean opacities that are 50% higher. Some special features in the monochromatic opacity spectra of Fe II are also noted.

In order to account properly for the dynamics of the electron angular correlations near the double ionization threshold the extended Wannier model is formulated in which the electrons recede from the reaction zone being at equal distances from the nucleus but in arbitrary directions. ¹S^e and ¹P^o states of the two-electron continuum are considered. The boundary conditions on the border of the reaction zone are imposed corresponding to the double photoionization process. The wavepacket propagation to the free electron regime is calculated numerically. The asymmetry parameter beta is in good agreement with recent experiments.

The semiclassical formula for the radial integrals of bound-bound dipole transitions derived in a previous paper is modified for obtaining more accurate estimates and extending to bound-free transitions. The possibility of these is due to the special definition of the mean value of energy generalizing that of the previous paper. It is shown that the definition is optimal in the sense of the best agreement between semiclassical and exact quantum mechanical results in a limiting case. Matching with the corresponding expression for free-free transitions is also obtained. Good agreement with experiment and more accurate calculations is found.

Photoelectron spectra have been recorded for the Cl 1s shells of a range of chlorinated hydrocarbons using synchrotron radiation with energy close to the ionization threshold. In addition to the single intense Cl 1s ionization, weak features are observed which are assigned to shake up satellites. The spectrometer used for fixed-frequency photoelectron measurements has also been used to record the threshold electron spectra of the molecules in the region of the Cl 1s edge. These are observed not to resemble fixed-frequency photoelectron spectra but are found to be closely similar to the Cl 1s X-ray absorption spectra of these molecules. A feature readily identifiable with direct ionization is also observed in the spectra.

Total and partial photoabsorption cross sections are computed for N₂ at the fixed equilibrium internuclear separation. By the present method the basic step is to compute the (real) polarizability alpha (i eta ) on the imaginary frequency axis, and the cross sections are then in turn obtained from the alpha (i eta ) values by inverting a dispersion relation. Many-body perturbation theory to second order in the Coulomb interaction is used to compute alpha (i eta ), and the continuum is represented by a finite set of discrete virtual states. In this way the difficult problem of obtaining continuum orbitals for a two-centre system is avoided, and a high degree of electron correlation will be included if there is reasonable convergence of the perturbation expansion. The present results are compared with experiment and with other calculations. For the total cross section there is good agreement with experiment, and the same conclusion applies to the 1 pi _u^-1 partial cross section. Problems related to the 1 pi _u to k pi _g partial channel cross section that are typical for other investigations, do not arise with the present method. However, for the 3 sigma _g^-1 and 2 sigma _u^-1 partial cross sections problems are encountered due to slow convergence of the perturbation expansion. The poor convergence seems to be related to strong interactions between the valence shells.

The authors propose a theoretical description of the l-mixing process of alkali-metal atoms in thermal collision with neutral perturber atoms within the framework of an eikonal approximation. The present work overcomes to some degree the weakness of those current theoretical methods neglecting the interaction between the perturber atom and the alkali-metal atomic core and treating the collision system as a two-body one. The present theoretical cross sections for the l-mixing processes of Na and Rb atoms exhibit a similar dependence on the principal quantum number n to that of the experimental results of Gallagher, Edelstein and Hill and Hugon et al.

The authors have investigated the cusp resulting from electron capture to the continuum of 1.25-5 MeV amu^-1 fully stripped hydrogen and oxygen as a function of the collision energy and the detector angular resolution theta ₀. It is revealed that the characteristic cusp shape parameters depend strongly on the experimental resolution. The experimental data are compared with the second-order Born theory and the impulse approximation. Both theories confirm the theta ₀ dependence of the shape parameters and give a reasonable description of the cusp asymmetry. However, theory tends to overestimate the absolute cross sections, in particular in the case of oxygen.

The authors have measured the alignment of Mg⁺(L₃) ions after electron capture by H⁺ in the energy range E_H+=40-240 keV (range of relative velocity nu _H+/ nu _L3=0.6-1.5) via the coincidences between the non-isotropically emitted L₃-M₁M₁ Auger electrons and the neutral H atoms. The experimental results show a strong resonance-like behaviour of the alignment values A₂₀^c as function of relative velocity with a maximum at about nu _H+/ nu _L3=0.8. This is in qualitative agreement with the theoretical values obtained with the Oppenheimer-Brinkmann-Kramers (OBK) approximation. The tendency of the experimental alignment results towards large negative values at high collision velocities is in accordance with the high-velocity behaviour of the OBK approximation and the semiclassical impulse approximation.

Doubly differential cross sections of single-electron capture were measured for He²⁺ impinging on a molecular deuterium target. The investigated collision energies are 4, 6 and 8 keV and the scattering angles range from 10' to 2 degrees 30' (laboratory frame). The exothermic capture leading to He⁺(1s)+D₂⁺* was found to be the most important process at low energies and angles, whereas the endothermic channels leading to dissociative capture become the main processes at high scattering angles, i.e. at small impact parameters.

The Sherman function has been calculated for the elastic scattering of electrons from zinc, cadmium and mercury atoms at incident energies up to 14 eV. The calculations were carried out within a relativistic polarized orbital approximation. Comparison with existing experimental data shows rather moderate agreement.

Inelastic cross sections for fast electrons scattered from Na atom targets prepared in the 30s Rydberg level are calculated in the Born plane wave approximation. The resulting cross sections for the dominant dipole transitions are in good agreement with a recent experiment and with an earlier semiclassical version of the Born approximation. Cross sections for dipole-forbidden transitions are also computed and are found to be a few orders of magnitude smaller than the corresponding dipole-allowed cross sections for incident electron velocities from 80 to 200 times the mean orbital speed of the target electron. Variations of the Born cross sections with target quantum defect are also investigated systematically by comparison with the corresponding transitions using hydrogen atom wavefunctions.

The total cross section for excitation of the nu '=0, 1, 2 vibrational levels and the total electronic cross section of the C³ Pi _u state of nitrogen have been determined for incident electron energies from threshold at 11.03 eV to 17.5 eV. The emission cross section of the (0,0) line (337 nm) of the second positive band of nitrogen has been also obtained in this energy region. Strong non-Franck-Condon excitation of vibrational levels has been observed. The present data are compared with previous experimental and theoretical results.

Results are presented which comprise the first detailed study of the post-collision interaction (PCI) in electron impact ionization of a molecular target. PCI shifts of three states were examined with broad agreement being found between our measurements and theoretical predictions based on studies involving atomic targets. Two of these states were the lowest members of two Rydberg series. Higher-lying members of these series did not show a PCI shift.

Within the Debye-Huckel model, and within the first Born approximation, the authors consider non-spherical screening of the electron-ion interaction and screened bound-electron wavefunctions to obtain collision strengths for the inelastic scattering of electrons by one-electron ions in dense plasmas. It is found that whether or not one uses unscreened bound-electron wavefunctions is less influential than the fact of having spherical screening or not, with the non-sphericity leading to an important increase of the collision strengths with respect to earlier calculations which considered spherical screening.

The author has analysed Rydberg energies within a perturbative approach. Taking a suitable combination of states allows one to analyse cores with incomplete shells at the same level of accuracy as cores with complete shells. Using experimental values for the Rydberg energies of states with l>or=3, the author has deduced accurate values of dipolar and quadrupolar polarizabilities, and in some cases octupolar polarizabilities as well.

The authors study the excitation of N₂⁺(B, nu ) in a RF pulsed discharge in low-N₂-content He-N₂ mixtures. The decay of the ion B state density and its vibrational distribution are measured by means of time resolved emission spectroscopy of the first negative system. The Langmuir probe technique is also applied to the measurement of the electron energy distribution function. It is found that, in the conditions of the experiment, the Penning reaction with He(2³S) metastable is the most important excitation source of N₂⁺(B, nu ). A non-Boltzmann N₂(X, nu ) vibrational distribution is then deduced from the ion B state one.