Table of contents

An analytical expression for the quadrupole moment in the 2P_1/2 state of the hydrogen (muonium) atom has been obtained, the contribution of both the discrete and continuous spectra being taken into account. It is shown that the relativistic effects in the present case are of supreme importance.

Carbon and oxygen X-ray emission spectra of carbon monoxide excited by electron impact have been studied using grazing incidence technique. Core electron binding energies were determined from an energy calibration using XUV emission lines and core-state bond lengths were determined from a Franck-Condon analysis.

The decay channels of the nu =0 and nu =1 levels of the c⁴ Sigma _u^- state of O₂⁺ have been studied using an electron-ion coincidence technique and the Daresbury 2 GeV electron storage ring as a source of continuum. There is a significant difference between the two which cannot be explained solely in terms of the relative importance of tunnelling and fluorescence.

The authors reconsider the energetic ordering of the Lambda -doublet components of the OH radical. Recent experimental and theoretical work suggests that the assignments of the symmetries of the levels have been previously made incorrectly. The implications of the revised assignments are that collisions with H₂ molecules lead to anti-inversion in the ² Pi _3/2 ladder of the radical and inversion for J<or=⁷/₂ in the ² Pi _1/2 ladder.

The processes of near-threshold ionisation and excitation of helium by electron impact have been studied experimentally by two different techniques to establish how the available excess energy is partitioned between the resulting free or bound electrons. The partitioning is found to be non-uniform and to be in agreement with that obtained from classical trajectory calculations.

A method has been developed for the evaluation of Coulomb integrals containing the product of the Coulomb wavefunction (r/K) with either the hydrogenic or Slater-type orbital (r/nlm) in closed form in terms of the Gauss hypergeometric function for an arbitrary nlm state. The results are of direct use for charge transfer in the impulse, the continuum distorted-wave and the continuum intermediate state approximations.

An equation is constructed in two-dimensional complex space, in the set of solutions of which solutions of the Dirac equation for a particle in a Coulomb potential are present. These solutions are found by a purely algebraic method.

A recently derived approximation scheme for the polarisation propagator has been applied in a study of discrete K-shell excitations in N₂ and CO. The new scheme referred to as second-order algebraic diagrammatic construction (ADC(2)) provides a direct approach to excitation energies and transition moments and gives a consistent second-order and first-order treatment for transitions to singly and doubly excited states, respectively. The essential computational requisite is a Hermitean eigenvalue problem in the space of single and double excitations on the Hartree-Fock ground state. Spin-free decoupled ADC(2) working equations for the singlet-singlet and singlet-triplet transitions have been formulated and employed. As the only additional approximation, the mixing between configurations with a different number of excited core-level electrons has been neglected. The calculated excitation energies of both the core-valence and core-Rydberg transitions are in very good agreement with the experimental data and are distinctly improved with respect to previous theoretical work, including extended configuration interaction treatments. The authors emphasise the accuracy achieved for the oscillator strengths which yield a very satisfactory description for the intensity ratios of the dipole-allowed transitions. The absolute dipole oscillator strengths are in excellent accord with the experimental values of Kay et al. (1977).

A systematic study is made of the theory of inner-shell ionisation which occurs during a resonant nuclear scattering process. Some previous treatments had emphasised the effect of nuclear recoil as an electronic excitation mechanism while others focused on the effect of the direct projectile-electron Coulomb interaction. In the present approach both excitation mechanisms are accounted for. One contribution to the transition amplitude is closely related to the approximation introduced earlier by Briggs and Lane (1981). The other components provide a generalisation of the Blair formula to include recoil effects non-perturbatively. This formula had been suggested as the basis for a determination of nuclear resonance widths. The observation of Briggs and Lane that recoil excitation can lead to a smearing of the resonance profile is extended to apply to the Blair formula. These results may be useful in future resonance studies to determine whether one is observing an intrinsic nuclear width or some average effect of a sequence of closely spaced projectile-atom resonances.

A crossed-beam technique incorporating time-of-flight spectroscopy together with electron-ion and ion-ion coincidence counting of the collision products has been used to study the ionisation of He by H⁺, He²⁺ and Li³⁺ ions. Cross sections for single and double ionisation have been determined with high precision at energies within the range 50-2380 keV amu^-1. Cross sections for single ionisation have been compared with the predictions of the first Born approximation, while cross sections for double ionisation have been considered in terms of the 'two-step' and 'shake-off' mechanisms. The authors have also measured the corresponding cross sections for one- and two-electron capture and for the transfer-double-ionisation process; the latter is shown to be the main He²⁺ production mechanism at the lower impact energies.

For pt.V see ibid., vol.14, p.2995 (1981). The process of Auger neutralisation of slow positive ions on metal surfaces is considered within the dipole approximation of the inter-electron interaction. An asymptotically correct expression for the transition probability per unit time is derived, and calculations are performed for the He⁺-W(100) system.

The authors derive the classical equations of motion for the three-body problem in a constant uniform magnetic field. They solve these for an ion-atom collision with the field along the initial projectile velocity on the assumption that the field quantum (h(cross) omega _c) is small compared with the ionisation potential of the target, using the classical trajectory Monte Carlo method. The example chosen is He²⁺ on H at laboratory energies of 25, 50 and 75 keV amu^-1. Increases of as much as 17% are found in the field-free capture and ionisation cross sections as the field increases up to 4.7*10⁴ T.

Translational energy spectra for single-electron capture by O²⁺ colliding with He, H₂ and H₂ target gases have been measured at 6 keV impact energy. For O²⁺-He, the dominant collision reaction is due to capture into the first excited state (2p³²D₅2/) of O^+., while for O²⁺-N₂ and H₂ the dominant reactions are due respectively to capture into the higher excited states (2p³²P₃2/) and (2p⁴⁴P₃2/) of O^+.. Contributions from metastable states of O²⁺ are also detected. Measurements of collision energy dependence of the relative cross sections of state-selective electron capture in O²⁺-He collisions are also reported.

Capture cross sections into highly excited p states (n<or=10) of Ar¹⁷⁺ have been measured in collisions of 250 MeV Ar¹⁸⁺ ions on a gaseous N₂ target. A high-resolution high-transmission crystal spectrometer of a new type has been specifically designed for this experiment. The measured absolute cross sections are in good agreement with CDW calculations.

The problem of scattering of a charged particle by a neutral two-particle atom is studied on the basis of the rigorous three-body theory in the integral equations approach. The Coulomb rescattering of the incident particle by individual particles of the atom is shown to generate long-range terms in the effective (polarisation) potential for interaction between the particle and the atom. However, cancellations of the long-range effects of the Coulomb rescattering are found to occur. As a result, the polarisation potential does not contain the terms which decrease at large distances as rho ^-2 and rho ^-3.

The electron impact excitation of He-like ions is studied using an equivalent-electron frozen-core approximation that was developed in an earlier paper on He. The resulting collision equations are solved using a five-state Coulomb exchange distorted-wave approximation that includes the dominant monopole exchange distorting potential exactly. Results are presented for scaled total collision strengths for all inelastic transitions between the 1¹S, 2³S, 2¹S, 2³P and 2¹P states of Li⁺, Be²⁺, B³⁺, C⁴⁺, N⁵⁺, O⁶⁺, Ne⁸⁺, Mg¹⁰⁺, Si¹²⁺, Ca¹⁸⁺ and Fe²⁴⁺, as well as for the infinite Z limit, for incident electron energies from threshold up to 4.0Z² Ryd. The present results are in good agreement with those of close-coupling and distorted-wave calculations by other workers for transitions from the ground state in O⁶⁺.

Recent improvements to the authors' time-of-flight spectrometer are described in detail, in particular the redesign of the gas-target entrance and exit necessary for the essential elimination of a systematic error which affected the measurements near cross section minima. The modified spectrometer was used to measure the cross section of argon in the energy range of 0.08 to 20 eV. For the Ramsauer minimum of argon at 0.345 eV the authors obtain a cross section about twice as high as that of earlier experiments but in excellent agreement with another current experiment as well as with recent theoretical results and with cross sections derived from swarm experiments. At higher energies, the authors' results agree well with other experimental and theoretical data.

The cross section for excitation of the metastable states of Hg by electron impact has been studied as a function of incident energy over the range 4.6 to 17.2 eV. The use of a low work function detector and an incident electron beam of high resolution (20 meV FWHM) have allowed the observation of a wealth of resonance structures, many being observed for the first time. Tentative assignments of many of these resonances are given. The present measurements also indicate the existence of a new metastable state of Hg at an energy of 10.54 eV.

The authors have measured the angular distribution of electrons ejected by the He^**(2s²)¹S autoionising state after its electron impact excitation via the He^-(2s2p²)²D resonance. Taking into account interference with electrons from the direct ionisation of helium, analysis of this angular distribution provides evidence for angular momentum exchange between ejected and scattered electrons during the post-collision interaction.

The authors consider the effect of the quadratic Doppler term on the inhomogeneous complex optical susceptibility of a two-level system for a weak beam in the presence of a saturating, counter-propagating wave, with both waves at the same frequency. There is a red shift of the resonance frequency in the absorption as well as in the dispersion. That red shift is a function of the temperature of the medium and the degree of saturation. An asymmetry of the absorption and dispersion curves is also observed which becomes stronger when the authors approach relativistic velocities of the particles. They also study for comparison purposes the effect of the quadratic Doppler term in the particular case of only one strong beam, without the weak one. They show the different behaviour of the red shift between the two cases.

The authors discuss a numerical study of the formation of gains in the H_alpha transition of hydrogenic Ti XXII during the recombination of freely expanding, energetic, columns of titanium plasma. The simulations predict that for laser gains greater than or about 10 cm^-1 the expansions must commence with initial radii less than or about 1 mu m. They conclude that unless this constraint is significantly relaxed either by certain physical effects neglected in the numerical study or by some technique with this specific aim (e.g. the use of alloy plasmas instead of pure titanium ones) the scheme studied is not within the realms of current technology. It may be possible, in the future, to produce such plasmas through the implosions of hollow or structured shells.