Table of contents

The effect of an electric field on some resonant states in the H^- ion below the n=2 threshold of H is studied using the stabilisation method and compared with recent experimental observations.

Parity favouritism is found to play a vital role in distinguishing three types of transitions in the photoionisation of diatomic molecules. It explains why the asymmetry parameter beta is larger for Sigma ^+or- to Sigma ^+or- than for Sigma to Pi transitions. For Sigma ^+or- to Sigma ^-or+ transitions, a value of -1 for beta is predicted.

A coincidence technique has been used in a fast intersecting beam study of H⁺-He⁺ collisions to determine cross sections for charge transfer in the centre-of-mass energy range 60-180 keV. Cross sections for ionisation have also been determined by reference to previously measured total cross sections for He²⁺ production from the combined processes of charge transfer and ionisation.

Three different types of entrance-channel data are described by the optical model; differential cross sections, total non-elastic reaction cross sections and spin polarisations. The model of McCarthy et al. (1977) fits its one free parameter, the absorption strength, to the reaction cross section. Differential cross sections are well described, but spin polarisations calculated for xenon compare badly with early experiments. They give good agreement with the recent experiment of Kessler et al. (1977).

The crossed-beam technique has been used to measure the absolute cross section for the process Ar⁺+e to Ar²⁺+2e at electron energies ranging from below threshold to 1000 eV. The cross section differs in shape from the trapped-ion data of Hasted and Awad (1972) but is in good absolute agreement with the semi-empirical calculation of Lotz (1968).

The photon-photon coincidence technique is used for the identification of events. The dissociation of a doubly excited state of D₂ into two D(2p) fragments has been measured by coincident detection of two Ly- alpha photons. The cross section for this process is estimated to be 6*10^-20 cm².

An extremely simplified version of the atomic shell model is used to obtain an explicit parameter-free approximation to the ground-state energy E_tot(Z) of any atom. The resultant values of E_tot(Z) agree to within a few per cent with those obtained from Hartree-Fock calculations for any Z in the interval Z=2 to Z=102, the agreement being of the order of 0.5% or better for Z>45. This is an order of magnitude better than Thomas-Fermi (TF) theory in this range. For Z to infinity , where TF theory becomes exact (Lieb-Simon theorem), we find (r_ij^-1)/(r_j^-1) to ²/₇, which is the exact TF value, and E_tot approximately -CZ^7/3 with C=0.74, which is within 4% or C_exact=0.77.

The spectra of molybdenum ions produced in Tokamaks in the wavelength range 10-200 AA have been reproduced in a plasma formed by laser beam irradiation of solid molybdenum targets. Lines from highly ionised stages of molybdenum (Mo XXX to Mo XXXII) have been distinguished by varying the laser beam intensity. Detailed analyses of the simpler ions, Mo XV (Ni-like), Mo XVI (Co-like), Mo XXXII (Na-like), and to a lesser extent Mo XXXI (Mg-like) and Mo XVII (Fe-like), have been achieved by comparison with ab initio calculations. A general interpretation of intermediate ion stages is also given but it is shown that most of these spectra are so complex, as a result of inner-subshell excitation, that detailed term-scheme analyses are nearly impossible.

The resonance fluorescence from a three-level atom irradiated by a monochromatic laser is studied. Detailed spectra are calculated by using the Heisenberg source-field approach, and introducing Liouvillian space techniques. It is predicted that in the steady-state limit the fluorescence from an atom having two closely spaced excited states will have an anisotropically polarised seven-line spectrum. Graphs are shown of the predicted spectra in the case of on-resonance excitation and of far off-resonance excitation.

The X-ray photoelectron spectrum of Kr in the neighbourhood of the main 3p lines has been calculated using diagrammatic many-body theory for the 3p self-energy and spectral function. Super Coster-Kronig 3p to or from 3d² epsilon f dipole fluctuations are found to shift the 3p lines by about 3 eV from their Delta SCF positions, the dipole shift being approximately 30% of the Delta SCF monopole relaxation shift. For determining the 3p linewidth the 3p to 3d4s epsilon p and 3p to 3d4p epsilon d Coster-Kronig transitions and the super Coster-Kronig transitions are approximately equally important. The calculated positions and widths of the 3p main lines are in good agreement with experiment.

The conventional theory of the Auger effect, based on Wentzel's ansatz (1927), is generalised to incorporate relaxation and final-state channel mixing. An application to neon KLL Auger rates in LS coupling is made. The bound-state orbitals are generated by alternative self-consistent-field methods and the continuum orbital is solved in the field of the doubly ionised residual ion including full exchange. In accordance with Kelly's many-body-perturbation-theory calculation (1975), it is found that the mixing between the final ²S channels affects the various partial rates by 10 to 40%. The effect of the non-orthogonal overlap elements is found to be of importance for the KL₁L₁¹S and KL₁L_2,3³P transitions. Since the Slater integrals involved are shown to be sensitive to the relevant orbitals, the importance of incorporating correlation effects in a systematic fashion is stressed.

Calculations of the K-LL, K-LM and K-MM Auger transition rates for the magnesium atom are presented. Configuration interaction between the residual ions and intercontinuum interactions between the final ²S states are included. It is shown that the former interaction modifies substantially the calculated K-LM and K-MM rates. While the effect of orbital re-organisation in initial and final states for K-LL rates is similar to the Ne K-LL case, the K-LM and K-MM rates are found to be more sensitive to this feature.

Accurate values of the relative transition probabilities A_{upsilon ' upsilon ''} for 41 bands in the N₂ second positive system have been measured using single-photon counting techniques on a repetitive pulsed source. A computer code simulating the band structure, including multiplet splitting, and intensity distribution enables any band intensity to be deduced from the fractional intensity seen at the peak in the P branch (band-head). The variation of the electronic transition moment with internuclear separation R_e(r) is obtained from the measured A_{upsilon ' upsilon ''} by means of a weighted least-squares curve-fitting procedure based on nth moment r centroids. It is best represented by a polynomial of degree 2, (1-1.336r+0.487r²) whose curvature is in the opposite sense to previously derived fits.

The continuum-distorted-wave approximation has been used to determine the electron-capture cross sections sigma (nl,n'l') when fast protons impinge on the H^- target system. The range of the projectile energy extends up to 2 MeV and the quantum labels (nl) and (n'l') denote the electronic states of the captured and passive electrons, respectively. The authors have considered the states n (and n')=1 and 2 with l=0, 1 and l'=0. From these results, values of the total capture cross section Sigma have been determined using the Oppenheimer n^-3 rule. The target system is described, in turn, by a simple variational wavefunction, a Hartree-Fock function, a split-shell description, and finally, a configuration-interaction wavefunction which allows for over 99% of the electron correlation energy.

An ab initio L² basis-set theory for electron-molecule scattering is proposed. All terms of the potential (static, exchange and polarisation) are included. The T-matrix expansion technique of Rescigno et al. (1974) is extended to include polarisation via a one-electron optical potential, which is expanded in a diagrammatic perturbation series. Integral and differential cross sections for e-H₂ elastic scattering and rotational excitation at electron energy 0.6-10 eV are calculated with the lowest-order polarisation diagrams. Very good agreement with experiment is obtained for elastic scattering over the whole range and for rotational excitation above 4 eV.

The close-coupling approach has been used to study e⁺-H₂ scattering in the low-energy region. The interaction of positrons with the H₂ molecule is represented as a superposition of electrostatic and polarisation potentials. The spherical and the nonspherical parts of both potentials are taken into consideration. Total and differential rotational-excitation cross sections are determined and compared with values found by earlier workers.

Utilising the crossed electron-beam-molecular-beam scattering technique, relative values of differential 'elastic' cross sections sigma ( theta ) have been measured at electron impact energies of 5.4 and 20 eV for the angular range 20 degrees to 130 degrees . The absolute values of these cross sections have been obtained by normalisation to the classical perturbation theory of Dickinson (1977) at 40 degrees scattering angle. These differential cross sections have then been used to calculate the integral and momentum-transfer cross sections. An energy-loss spectrum at 100 eV electron impact energy and 15 degrees scattering angle has also been obtained. Two weak features at the energy losses of 6.74 and 8.82 eV appear. Their energy positions are compared with the recent calculations of Kahn et al. (1974).

The electron-photon delayed coincidence technique has been extended to radiative lifetime measurements of excited simple free radicals. Dissociation processes of the parent molecules NH₃ and CH₄ leading to formation of the excited fragments NH and CH, respectively, have been studied. For the NH radical the following lifetimes are obtained: C¹ Pi (364+or-5) ns and A³ Pi (347+or-5) ns; and for CH: A² Delta (440+or-20) ns.

A uniformly convergent series expansion is derived for the intensity component which is polarised normal to the direction of electron incidence. Results are expressed in a form suitable for numerical calculation. Tables of total intensity and polarisation components are presented for electron energies U between 0.001Z² and 50Z² (keV), and photon energies up to 0.90U.

An improved theory of electron temperature relaxation in monatomic and molecular gas afterglow plasmas has been developed. The effects of the electron temperature dependence of the electron-neutral collision frequency and of the fractional energy transferred per inelastic collision are treated.