Table of contents

Current calculations of the Lamb shifts and relativistic corrections for two-electron ions are critically analysed and the need for reliable values of average excitation energies k₀ for two-electron ions over a wide range of states and Z is re-emphasised.

Radiative lifetimes have been determined for thirteen levels in Fe II using the technique of laser-excited fluorescence from sputtered metal vapour. Results for the quartet terms, z⁴P⁰, z⁴D⁰ and z⁴F⁰, are new, while those for the z⁶D⁰ term are in agreement with the results of previous electron excitation experiments.

It is shown that a first-order charge transfer theory recently introduced in the literature can be derived from the Schwinger variational principle. The formal properties of the Schwinger principle are also used to establish a hierarchy of higher-order approximations.

Effective collision strengths are presented for excitation of the ground state of O VII to the 1s2s³S^e and 1s2p ³P⁰ states for electron temperatures from 10⁴ to 10⁷K. The calculations were carried out using the R-matrix code. Results are given for both five-state and eleven-state calculations. For the 1s²¹S^e-1s2s ³S^e transition resonances converging to both the n=2 and n=3 levels are important.

New collision strengths for the electron impact excitation of the fine-structure transitions within the 1s²2s²2p² configuration of Si IX have been calculated. These have been obtained from a suitable transformation of LS coupling reactance matrix elements computed with the R-matrix program. The calculations have been done at a large number of energy points in an energy region below 90.0 Ryd. The collision strengths exhibit a complex pattern of resonances in almost the entire threshold energy region. The effective collision strengths have been obtained after averaging the entire collision strength spectrum over a Maxwellian distribution of electron velocities. The results are tabulated in an electron temperature range below 2.5*10⁶K.

Full distorted-wave calculations have been carried out on np⁶ to np⁵nd² transitions to test the formalism for complex atomic configurations, and to provide a more meaningful comparison with experiment than was possible from earlier calculations. Agreement with experiment is certainly improved, but some discrepancies remain.

Computation of the ground-state hyperfine splitting of neutral muonic helium ( alpha - mu ^- e^-)⁰ has been carried out. Account of two terms in the expansion of this quantity in power series of a small parameter beta approximately m_e/m_mu approximately 1/200 results in the energy splitting value Delta v=4462.9 MHz. It is in good agreement with the experimental datum, Delta v=4464.95 (6) MHz, of Orth et al. (1980). The authors' value is also to be compared with previous theoretical results: Delta v=4462.6 MHz (Lakdawala and Mohr, 1980); Delta v=4465+or-1.0 MHz, Delta v=4478.7 MHz (Huang and Hughes, 1980); Delta v=4493.3 MHz (Borie, 1979).

The investigation of expanding laser-produced plasmas has been carried out. X-ray spectra of multicharged F VIII and F IX ions were observed in the wavelength range lambda approximately 11-17 AA for the regions of expanding plasma, remote from the target surface by up to 10 mm. The calculations of resonance and intercombination line relative intensities were made, taking into account recombination of the population of ion levels in the case of sharply non-equilibrium plasma ionisation states. The results of the calculation are utilised for an investigation of expanding laser-produced fluorine plasmas.

The density dependence of the dielectronic satellite lines in the emission spectrum of Si XII in laser-imploded microballoon targets is observed. With increasing electron density, in the range 1.2*10²² to 8*10²² cm^-3, the increase in the intensities of the (q,r) and (a-d) satellite lines relative to the (k,1,j) satellite lines is found to be in agreement with theory, when the effects of inner-shell electron collisional excitation are included.

Electron momentum density contour maps of the first-row homonuclear diatomic molecules, A₂, are obtained from near-Hartree-Fock wavefunctions. Both the total momentum density and momentum density difference (molecule-isolated atoms) maps present trends that may be related to the binding in the molecules. These results are compared with the corresponding charge density maps in position space (Bader et al., 1967).

A correction for the self-interaction contribution, which depends only on the total number of electrons, is introduced in the local density approximation. Energies and scattering form factors for rare-gas atoms, positive alkali ions and negative halogen ions are calculated both for the Kohn-Sham form of the exchange potential and for the Slater one, and compared with those calculated from the Hartree-Fock equation. Both the corrected KS and S equations give results very close to the HF ones. For negative ions bound states are obtained without artificial modifications of the potential.

The passage of an electronic resonance through the continuum threshold in the presence of a Coulomb potential is analysed. The approach is based on Feshbach's projection operator formalism combined with analytic continuation and a threshold expansion of the complex level shift function. Resonances and bound states are obtained as the poles of the analytically continued fixed-nuclei S matrix, S(k), for electron scattering. Close to threshold the behaviour of resonances and bound states is completely determined by the long-range Coulomb potential which causes an essential singularity of S(k) at k=0. Both the attractive and the repulsive Coulomb potentials are considered. The relationship between the Feshbach formalism and the quantum defect theory is established. The relevance of these results for the understanding of nuclear dynamics in electron-ion scattering is illustrated by a model calculation.

The authors have used saturated absorption spectroscopy and a sigmameter to obtain the absolute wavelengths of the 2p ³P_0,1,2-3d ³D_1,2,3 transitions near 587.6 nm in ⁴He I. The precision is about eight parts in 10⁹. By comparing with recent two-photon measurements of direct excitation from the 2s ³S₁ level, they obtain values for the wavelengths of the 2s ³S₁-2p ³P_0,1,2 transitions. Precision calculations for these transitions are compared with experiment. In particular, the authors discuss the QED and relativistic corrections. The relative energies of many triplet levels in He I are analysed.

By combination of plane-polarised synchrotron light and a close-ruled holographic 3 m concave grating, of high resolution, the authors have observed-in addition to the first resolved Zeeman structures to be reported below 2000 AA-magnetic rotation patterns of lines of Rydberg series. They have used these patterns as a basis of a flexible and accurate method for measurement of relative f values, usable in the Schumann region. Some results, at somewhat longer wavelengths, for Sr I are compared with earlier results obtained by the 'Hook method'.

An analysis of recently observed magneto-optical spectra is presented. A vernier method is described in which, by considering the full profile and including both the contribution due to birefringence and that due to dichroism, an enhanced accuracy can be achieved in the determination of relative oscillator strengths as compared with that achievable in methods based on Faraday rotation alone.

The quantum defect method, generalised to the spin-orbit coupling scheme, has been used to calculate the oscillator strengths of the principal series for atomic caesium, taking into account the core polarisation produced by the valence electron. The good agreement obtained with respect to experimentally measured values has led the authors to carry out new calculations of generalised two-photon ionisation cross sections for the atomic caesium 6S ground state with linearly and circularly polarised incident radiation in the deep minimum region, related to destructive interferences, near the intermediate 7P resonance. A theoretical method previously developed was used; excellent agreement was obtained with respect to recent experimental data. The degree of spin polarisation of the ejected electrons has been calculated in the same region of incident photon energy.

The experimental study of a laser-induced energy transfer for a dipole-dipole collisional interaction in a Na-Ca mixture is reported. A specific excitation scheme is used in this experiment for which the virtual level is optically excited. A comparison with the case of a dipole-dipole interaction in the Eu-Sr mixture where the virtual level is excited by collisional energy transfer is presented.

Ab initio methods are used to compute photodissociation and radiative processes occurring in interstellar C₂ from the X ¹ Sigma _g⁺ ground state. To this end, self-consistent-field+configuration interaction wavefunctions and potential energy curves of the X ¹ Sigma _g⁺ and the first four ¹ Pi _u states and transition moments between the ground and excited electronic states are computed. Photodissociation cross sections for absorption into the second and third ¹ Pi _u states are calculated and the corresponding interstellar photodissociation probability is found to be 0.73*10^-10 s^-1. Ground-state transition probabilities and absorption oscillator strengths f₀₀ of the Phillips (A ¹ Pi _u-X ¹ Sigma _g⁺) and 2 ¹ Pi _u-X ¹ Sigma _g⁺ systems are determined.

At collision energies below 1 keV electron transfer processes were studied in the collision system Ne²⁺-Xe. By applying high-resolution translational spectroscopy 12 different reaction channels are separated corresponding to pure electron capture reactions as well as transfer ionisation processes. In the energy range studied, the mechanism of the transfer ionisation is described as a two-step process: after one electron is captured by the Ne²⁺ ion, the Xe⁺ target ion is ionised by a Penning-type process at nuclear distances near 5 a₀. The influence of metastable projectiles on the measured kinetic energy spectra is discussed.

Adiabatic switching factors are introduced into the molecular model of atomic collisions. These factors are so designed as to modify the perturbed stationary-state equations only in the asymptotic region, where an analytic WKB-type solution can be found. This solution can be used to impose the correct boundary conditions at any arbitrarily large (but finite) internuclear distance. The scattering amplitudes are then uniquely defined and are readily obtained by a simple matrix transformation, once the solutions of the standard perturbed stationary state equations are known. The problem of spurious long-range couplings and of possible divergences in the cross-section calculations are thereby avoided. An application to a typical ion-atom collision such as H⁺/Li, where no meaningful result can be obtained without the introduction of translation factors, illustrates the feasibility of the approach.

Electron capture in proton-lithium collisions at centre of mass energies below 10 keV is examined in the PSS approximation using a model potential for the e+Li⁺ interaction. It is assumed that the level H(n=2) is preferentially excited. Eigenvalues of the molecular Hamiltonian are found variationally, and radial and rotational coupling elements are calculated directly from these in prolate spheroidal coordinates. The coupling elements show strong dependence on the choice of coordinate system and have non-zero asymptotic limits. The authors use an adiabatic switching factor in the asymptotic region to ensure convergence of the cross sections. These are evaluated in the impact parameter approximation. Calculations involving both six and seven molecular states are performed. Radial coupling dominates the process at higher energies while rotational coupling is more important below 500 eV.

The alignment and orientation of the 2¹P state of helium by electrons of incident energy 24.0 eV has been studied both experimentally and theoretically. Experimentally, the angular correlation between the 2.8 eV scattered electron which has excited the state and the 58.4 nm photon resulting from its decay has been measured for electron scattering angles up to 120 degrees . Theoretically, the alignment and orientation parameters have been calculated using a five-state R-matrix method. There is excellent agreement between theory and experiment at all scattering angles.

Energies and widths of the series 2s2p⁶nl for neon and magnesium and 3s3p⁶nl for argon are calculated by analysing the resonant behaviour of the scattering matrix of the ion+electron collision theory. The results obtained agree well with the experimental results and confirm the conclusions of previous calculations for sodium.

The anomalous effects frequently observed when hydrogen fluoride gas is studied in scattering experiments have been further investigated using time-of-flight mass spectrometry. The molecular identity of the spurious gases as a function of time has been determined. Appreciable amounts of H₂, H₂O, N₂ and O₂ were observed together with other species dependent on the past history of the inlet system. Careful sample handling and conditions are needed to ensure that pure HF enters the spectrometer.