Table of contents

It is demonstrated explicitly that diagonalization of the 4d⁹ nf channel in neutral atomic Ba (Ba I) leads to a 4d⁹4f ¹P resonance below the 4d⁹-ionization threshold in complete agreement with a direct Hartree-Fock calculation for the 4d⁹4f ¹P level. The wavefunction for the 4d⁹4f ¹P level is energetically and spacially very close to the 4d⁹5f level from average-of-configuration Hartree-Fock.

A relativistic generalization of the random phase approximation is used to study the response of helium-like ions to electromagnetic perturbations. The spectrum of the response function in this relativistic theory contains allowed and forbidden transitions. The computed E1(2^1,3P₁-1¹S₀), M1(2³S₁-1¹S₀) and M2(2³P₂-1¹S₀) transition rates agree well with those obtained by refined calculations and with experiment. Accurate values are also obtained for the excitation energies of ions with large nuclear charges, where relativistic effects are of major importance.

The fine-structure splittings of the L₂-MM(³P_0,1,2) and L₃-MM(³P_0,1,2) Auger multiplets of argon excited by 2.5 keV electrons were measured. Relative intensities were determined in good agreement with existing calculations using the mixed coupling scheme, i.e. jj coupling for the initial L₂ or L₃ vacancy and LS coupling for the final vacancies. Moreover, the natural line width of the L₂-MM and L₃-MM normal lines was determined to be Gamma _2,3=(100+or-20) meV.

An accurate and efficient procedure in the calculation of multi-photon ionization processes is proposed. In addition to the calculational advantage, this procedure also enables the relative contributions of the resonant and non-resonant intermediate states to be studied.

Characteristic K X-ray emission has been observed from projectile and target atoms during 4 MeV bombardment of ion-implanted Si hosts for projectile atomic numbers less than but near that of the target atom (Z approximately 35). The results are interpreted as evidence for 2p vacancy sharing early in the heavy ion-atom collision event. The dependence of the sharing ratio on Delta Z/v is similar to that previously reported for 1s shells.

Fine structure is observed in the dissociative attachment cross section for O^-/SO₂, S^-/SO₂ and S^-/OCS. It is attributed to the vibrational excitation of the neutral fragments SO, O₂ and CO. Appearance of such structure on these sigma (E) curves is shown to be a consequence of the kinetic energy of the negative ion fragment.

Accurate potential energy curves have been calculated for both F₂ and F₂^- with a view towards examining the stability of the molecular negative ion near the equilibrium position of F₂. It is found that the F₂^- curve does cross the F₂ curve in the Franck-Condon region of the ground vibrational state, and the implications of this crossing on the low-energy electron impact of F₂ are discussed.

A 70 eV electron energy loss spectrum for the molecule HF is reported, in the range 11-16 eV. The v¹ Sigma ⁺ valence state and two Rydberg series, each containing extensive vibrational structure are analysed.

Momentum-transfer theory shows that an additional factor involving heat flux must appear in the generalized Einstein relation as first proposed by Wannier (1953) for gaseous ions in an electrostatic field. The correction is most significant in cases where the ion mass is comparable with the mass of gas atoms.

Ab initio results (obtained by means of a numerical, multi-configuration Hartree-Fock procedure) are reported for the highest spin states of 3dⁿ4s and (3d+4s)ⁿ4p in neutral V and Cr. The importance of allowing the 3d orbital in 3dⁿ4p to differ from that in 3d^n-14s4p is demonstrated, particularly in Cr where the two configurations are nearly degenerate. More extensive correlation studies are reported for Cr. Energy differences and oscillator strengths are in fair agreement but the dominant component of z⁷P, usually designated 3d⁵4p, is the 3d⁴4s4p configuration.

Amplitudes for radiative transitions in fast-rotating molecules and heavy-ion collisions are derived. The effect of the Coriolis force on the electron motion and on the radiative transition amplitude is formulated. The term in the transition amplitude dependent on the angular velocity reflects the influence of the rotation of the states.

The semiclassical theory of potential scattering leads naturally to an inclusion of complex-value classical paths which describe classically forbidden processes. However, only a few of the existing complex trajectories contribute. The authors discuss which of these forbidden paths should be retained and a detailed investigation of the complex contributions is given for the special class of repulsive inverse-power potentials. The oscillatory structure of the quantum-mechanical differential cross section is resolved as an interference pattern of classically allowed and forbidden trajectories. The region of validity of the complex-trajectory approximation is investigated. For potentials with a minimum the forbidden trajectories contribute to scattering angles larger than the rainbow angle. This is demonstrated for a superposition of inverse-power potentials.

An accurate quantum calculation of the Lyman- alpha profile is given using the unified one-perturbation theory of Tran Minh and Van Regemorter (1972) and Tran Minh et al. (1975). A real 'dissection' of the electronic collisional broadening is made and the effects of the different approximations usually made in semiclassical approaches are discussed qualitatively. It is found that simple dipolar semiclassical calculations are valid not too far from the line centre but that quantum theory must be applied in the far wings. The correct average over the perturber velocities must be made for the entire profile.

Double electron photoionization is calculated for an open-shell system for the first time. Many-body perturbation theory is used in obtaining results for the ³P ground state of neutral carbon. Contributions from the 2p² pair and 2s2p pairs are included, with the 2s2p cross section dominating.

A theory of electron-atom scattering in the presence of moderately intense laser fields is applied to resonant electron-atom scattering. The radiation field is treated in the lowest perturbative order and the electron-atom interaction is approximated in terms of the N-state close-coupling model. A resonance in the elastic electron-atom cross section in the absence of the laser field at the energy E_R leads to two resonances in the free-free absorption cross section when the laser field is present. These two resonances are due to resonant scattering of the electron by the target atom before or after a photon is absorbed. If the field-free electron-atom resonance occurs in the lth partial wave, and if the photon polarization vector is taken parallel to the initial electron momentum, the angular distributions corresponding to these two resonances are proportional to cos² theta P_l²(cos theta ) or to P_l²(cos theta ). Furthermore two versions of soft-photon approximations are tested numerically on a two-state close-coupling model. For a CO₂ laser these soft-photon expressions agree within an error of only 2% with the results of the exact calculations. Application of this soft-photon approach to the Ar+e^-+h(cross) omega _CO(2) experiment of Andrick and Langhans (1975) yields a free-free transition rate of 150 s^-1. These results agree to within 2% with the numerical calculation of Geltman (1972), who employed more realistic model potentials.

The polarization of the 2p to 2s resonance lines of ⁶Li and ⁷Li excited by electron impact, are calculated by the Glauber approximation using Franco's (1971) procedure of explicity including all the target electrons. They are found to agree to within 2% with the recent experimental data of Leep and Gallagher (1974) for energies greater than 60 eV. Even for lower energies the differences are not as pronounced as in the case of total cross sections.

The differential cross section and spin polarization of electrons scattered from mercury were measured as a function of scattering angle at collision energies of 1.4, 2.4 and 3.9 eV. In the general shape of the angular dependence, the results agree well with recent theoretical calculations. In addition, results are given for spin polarization by resonance scattering in the energy range 4.4-6.1 eV. Absolute values of resonance energies are determined by referring to the ³P₁ level of Hg at 4.89 eV.

The observations of resonances is reported for inelastic electron scattering by krypton in the autoionization region. The differential excitation functions of the states 5s, 5s', 5p and 6s+4d have been measured in the forward direction by means of a high-resolution electron spectrometer with a static gas target. Numerous features are detected and compared with the observations performed in the transmission and electroionization spectroscopies. Interpretations are proposed in terms of Feshbach resonances and Wigner cusps. The energy calibration is achieved by means of the lower-lying resonances 4s²4p⁵5s²(²P_1+or-12/) which appear, in this energy range, in the channel 6s+4d via multiple scattering effects. These secondary processes are also investigated in other rare gases, and the expected pressure dependence is shown.

A photoionization chamber has been used to determine the absolute intensity of 304 AA radiation emitted during electric-field quenching of fast He⁺(2s) metastable ions. The measurements have been used to redetermine with greatly improved accuracy cross sections previously measured in this laboratory for formation of He⁺(2s) metastable ions by (a) one-electron capture in the passage of 10-60 keV ³He²⁺ ions through H, H₂, He, Ar, Kr, K, N₂ and O₂ and (b) direct excitation of 6-30 keV ³He⁺ ions in passage through He, Ar, Kr, H₂, N₂ and O₂. The measurements show that, while relative cross sections for the different target gases remain unchanged, previously estimated absolute values should be increased by a factor of 2.2. Cross sections revised in this way are believed to be accurate to within 15% for H₂ and 20% for the other target gases.

Under conditions in which direct radial or rotational coupling mechanisms are not expected to contribute and for collision velocities for which the MO model is expected to apply, it is proposed that experimental data concerning K-vacancy production in heavy ion-atom collisions can be described in terms of direct Coulomb excitation. Agreement between experimental ionization cross sections and the predictions of the BEA universal curve is demonstrated, with transient molecular effects allowed for by a simple binding-energy modification. This binding-energy-modified BEA is also used as a basis of an explanation of the observed enhancement of K-vacancy production at or near level matching.

A new band system of SrH and its corresponding system in SrD has been observed in absorption at 3067 and 3070 AA, respectively. The system has been named as J² Pi -X² Sigma . It has also been interpreted that the J² Pi state is a member of an nd complex of which the G² Sigma state as observed by Khan (1966) is also a member.

Autoionizing states of N₂ have been studied using a new electron-electron coincidence technique, by simultaneous measurement of their excitation energies and ejection energies in decay to states of N₂⁺. This method gives a much clearer understanding of molecular autoionization than methods which measure either energy separately. Low energy electron impact is employed using hemispherical analysers to select the incident beam and to detect scattered electrons which have excited a selected autoionizing level. In coincidence with these scattered electrons, ejected electrons are analysed by measurement of their flight times along a field-free region. Transitions between the selected autoionizing level and levels of N₂⁺ appear as peaks in the time spectra. The design, construction and operation of the spectrometer are discussed and factors limiting the time and energy resolutions are considered. Results are presented showing time spectra for electrons ejected from five autoionizing states of N₂ and good agreement is observed between the predicted and measured resolutions of the spectrometer.

The composition dependence of the thermal diffusion factors alpha _T of the binary gas mixtures NO-CO, N₂-C₂H₂, NO-C₂H₂ and CO-C₂H₂ have been studied by the swing separator method. All these systems have the special feature that the gases involved are linear molecules so that interpretation of the data in terms of inelastic theories is feasible. Attempts have been made to interpret the data by using both elastic and inelastic theories of thermal diffusion. The results indicate the influence of inelastic collisions on thermal diffusion in polyatomic gas mixtures. However, the inelastic theories available cannot explain satisfactorily the experimental data. It has been suggested that a more sophisticated theory involving more rigorously defined inelastic cross sections will be necessary for a proper interpretation to be made of the effects of inelastic collisions on thermal diffusion.