Table of contents

The relativistic corrections to the Lande factors for the ³P₁ and ¹P₁ levels of the configurations 5s5p of Cd and 6s6p of Hg are calculated from the relativistic Zeeman operator in the central-field approximation. The relativistic corrections are insufficiently accounted for by the Breit-Margenau operator when the coupling deviates from the pure jj scheme.

The technique of detecting ionization signals following two-photon excitation has been used to study even-parity Rydberg and autoionization levels in Sr I. New levels, including terms of the 5sns ¹S₀ and 5snd ¹D₂ series recorded to high n values, have been measured to +or-0.3 cm^-1. Configuration mixing effects have been observed in the ¹D₂ terms and the 4d²¹D₂ autoionizing term identified.

X-ray energies of multiplet transitions in helium-like sulphur projectiles passing through thin foils have been measured with high resolution. Small lineshifts have been found which can be attributed to the effect of dynamical screening of the projectiles by the target electron gas.

The absolute yield of Ar L Auger electrons emitted by 48-722 keV argon beams emerging from carbon foils has been measured. The Auger process is not the main effect which determines the difference between the mean charge of Ar projectiles in solid and gaseous targets in this energy range.

Simultaneous impact of electrons and photons on neon is shown to lead to resonant transitions between pairs of short-lived negative-ion states. One particular negative-ion state at 18/95 eV has been populated by transitions from two different lower lying states ( lambda =5897 and 4511 AA). The decay of the upper state was observed through the Ne I emission at 5882 and 6030 AA.

Transitions between continuum states of the e^--Ar system have been observed in the field of a focused CW CO₂ laser beam. The order of magnitude of the cross section agrees with theoretical predictions.

The electron translational effect is treated within a finite basis of wavefunctions of the transient quasimolecule. The resulting coupled equations do not show any spurious long-range interactions and are easy to apply.

An atom or molecule illuminated by a laser has its states modified by the laser. A set of laser-distorted states is defined and the equation for the amplitude of the wavefunction for change of population of these states obtained. The state equation is solved approximately in a novel gauge in the limit where the laser frequency is high, and the utility of this gauge is discussed. The equation is also investigated in the limit of intense fields where the effect of the laser is to make the internal interactions of the atom non-local. The effect of this non-locality is investigated at large distances.

Canonical transformation techniques are used together with variational and perturbative methods to describe the effects of the coupling between electrons and the radiation field in the Coulomb gauge. A simple transformation, equivalent to the use of Glauber's coherent states (1963) is treated exactly, and the 'direct' part of the currently used Breit correction (1932) to the electron-electron interaction in relativistic Hartree-Fock theory is derived variationally. More general transformations leading to electron self-energy terms as well as electron-electron interaction corrections are discussed and compared to the procedure used by Mittleman (1972). The case where the unperturbed Hamiltonian is of Hartree-Fock type gives the generalized Breit interaction.

Fourier integral techniques are used to generate non-diagonal first-order density matrices (1-matrices) from two-electron atomic wavefunctions containing exponential correlation terms. Although the techniques presented here can be applied to larger systems, the lengthy amounts of algebra limit their usefulness. The 1-matrices generated from two-electron correlated wavefunctions have been transformed to momentum space. The Compton profile obtained from a He wavefunction yielding over 99% of the correlation energy is presented.

The profiles of the boron resonance lines emitted from hollow-cathode and planar-cathode glow-discharge lamps have been studied for a variety of discharge conditions. For discharges operated in argon, krypton or xenon, the lines are extremely broad (halfwidths 0.13-0.27 AA) and are emitted very close to the cathode surface with unusually high intensity. The abnormal linewidths are due to Doppler broadening, which is unusually large because of a preferred excitation for those sputtered boron atoms possessing high kinetic energies of ejection. Investigations of the excitation process suggest that the energetic boron atoms are excited by inelastic collisions with rare-gas atoms. The high excitation efficiency at relatively low collision energies is interpreted in terms of curve crossings, which appear to be particularly favourable for some boron-rare-gas systems.

Using (100) reflections in a 40 cm curved mica crystal spectrograph of the transmission (Cauchois) type, the line corresponding to the electric quadrupole transition L₂M₃( beta ₁₇) in the L X-ray emission spectrum of lutetium 71 has been detected at lambda =1486.54 XU. This has been made possible by completely eliminating the unwanted tungsten, nickel and copper spectra.

Cross sections for the ionization of metastable helium atoms, predominantly in the 2³S state, at electron energies from the threshold to 1000 eV have been measured using the crossed electron and fast atom beam technique. Corrections are made for the presence of ground-state atoms and a very small concentration of highly excited atoms. The semiquantal binary encounter data agree satisfactorily up to 20 eV, but less well at higher energies. From 200 to 1000 eV, the measured data agree well with Bethe-Born cross sections of Briggs and Kim (1971) for He(2³S) provided a correction is made to take account of a spurious contribution to the ionization signal which is only significant at the higher energies. All the various Born cross sections, at high energies, are substantially lower than the present measurements and also the Bethe-Born data, but they are either comparable with or smaller than the measured cross section at its peak.

An integral form of the polarized-orbital method has been employed to investigate the positron-helium collision problem. This form does not satisfy the minimum principle. A change of sign of the s-wave phaseshift occurs in the low-energy region. The zero position is obtained at an energy slightly higher than that given by variational calculations. The total elastic cross-section values below the Ps formation threshold are in qualitative agreement with experimental results as well as with the more refined theoretical findings. The Ps formation cross section for the partial wave l=1 shows a resonance-like behaviour near 20 eV. A similar feature has also been noticed by Kraidy and Fraser (1967) around 23 eV.

A general discussion is given of the problem of experimental investigation of autoionisation by electron impact of Ar, Kr and Xe, and theoretical parametric forms derived for the cross sections. Experimental results are presented and analysed in terms of these parametrizations.

Absolute differential elastic cross sections were measured for electrons scattered between 2 degrees and 150 degrees by static gas targets of argon, neon, nitrogen and carbon monoxide. The energy ranges were 20-800 eV for argon and nitrogen, 50-800 eV for neon and 200-800 eV for carbon monoxide. The present nitrogen cross sections agree with those of Bromberg (1974) and Jansen et al. (1976) at the higher energies and with Srivastava et al. (1975) at the lower energies. Within a relative uncertainty of 5% the isoelectronic nitrogen and carbon monoxide cross sections were equal at all angles for 500 and 800 eV. At 200 eV the nitrogen cross sections were generally 5-20% larger at angles up to 120 degrees .

The single- and double-electron-loss cross section for H^- and He^- ions passing through H₂, He, N₂, Ne and Ar gas targets have been studied in the energy range 50-500 keV for H^- impact and 100 keV-3 MeV for He^- impact. H^- and He^- ions were formed from H⁺ and He⁺ by charge transfer in a gas cell and the cross sections were determined by the initial-growth method. The experimental cross sections for single-electron loss are compared with theoretical estimates, based upon the semi-empirical impulse approximation of Bates and Walker (1967). A comparison of the experimental results for H^- and those for He^- impact at equal impact velocities indicates an error of up to 50% in the basic, non-empirical part of the Bates-Walker theory.

Cross sections sigma _1*0 for one-electron capture by ³He⁺(2s) ions in H₂, He, N₂, O₂, Ar and Kr have been measured in the energy range 15-55 keV, and compared with the corresponding cross sections sigma ₁₀ for one-electron capture by ground state He⁺ ions. Previously measured cross sections sigma _1*2 for electron loss by He⁺(2s) ions have been renormalized and both electron loss and electron capture are shown to be of comparable importance in the energy range considered. Destruction of He⁺(2s) ions is also shown to take place to a significant extent by collisional de-excitation processes.

Differential cross sections for He(2³S)+Ar have been measured in the relative kinetic energy range from 20 to 480 meV. At low kinetic energies the small-angle oscillations have been resolved, so that it was possible to derive an accurate complex or optical interaction potential. The velocity dependence of the ionization and total cross sections calculated from this optical potential is in good agreement with experimental results. Orbiting resonances are predicted at low velocities. The derived potential is unusually soft for large internuclear distances, where the argon atom penetrates only the diffuse He(2s) orbital. The repulsion becomes much harder when the argon atom starts to overlap the He(1s) orbital.

The rotational excitation of CN by proton impact is studied using the decoupled l-dominant (DLD) method. When compared with the results of exact quantum close-coupling calculations, the DLD method is seen to offer a considerable reduction in computer time as well as a reasonable degree of accuracy, especially at high collision energies. Additionally, the method is considerably more accurate than the Born approximation and does not require the arbitrary Born cut-off factors.

The contributions of different non-spherical terms in the potential to the interaction second virial coefficient for the systems He-H₂, He-HD and He-HT have been calculated over a range of temperature. The quantum effects have also been considered. For the latter two systems, an additional non-spherical interaction term due to mass asymmetry resulting from the heteronuclearity of the molecules has to be considered. The effect of this term is quite significant and an estimation of it should be possible by having an accurate measurement of the interaction second virial coefficient over a range of temperature.

The time dependences of the mass-identified H₃⁺ ion densities have been measured during the decay period of a plasma produced in helium containing 0.01-0.1% hydrogen. Measurements were carried out at a temperature of 300K and over a pressure range 1.5-100 Torr. The time dependence was found to be exponential during the latter part of the afterglow period. Measurement of the decay time constants yielded the zero-field reduced mobility of mu ₀=31.6+or-1.0 cm² V^-1 s^-1.

For pt.I see ibid., vol.8, no.18, p.3044 (1975). The total intensity of an optically thick line can be greater from a differentially expanding plasma than from a non-expanding plasma of similar geometry, and can approach the optically thin value at sufficiently high streaming velocities. A population inversion, localized to one region of a differentially expanding plasma, can give rise to nett amplified spontaneous emission even though it would not do so in the absence of expansion. This enhances the prospects of a laser-produced plasma as a source of amplified spontaneous emission. Within the one line profile from a differentially expanding plasma, maximum amplified spontaneous emission and peak gain to an external signal need not occur at the line centre and need not occur at the same wavelength as one another.