Table of contents

A propagator for the log-derivative is formulated as a continued fraction and it is shown, with a numerical example, how to find eigenvalues from its asymptotic value.

The authors show that the compact representation of the complete Dirac spectrum provided by finite analytic basis set methods allows the study of relativistic electron correlation effects. The instantaneous Coulomb and low-frequency Breit interactions are treated using a unified formalism. The principles of this computational method are illustrated by calculations of the electronic structure of the argon ground state. The calculations reveal that the terms in the perturbation expansion corresponding to the self-consistent modification of the one-electron orbitals due to the Breit interaction result in the most significant relativistic many-body effect.

An important effect to be considered in forward-scattering experiments is demonstrated. Ring-shaped structures are observed in the transverse intensity distribution of the forward-scattered light induced by an intense CW single-mode dye laser in an excited neon system. These structures depend upon the laser detuning from the absorber line centre, the magnetic field strength and the neon gas pressure. A preliminary interpretation of the phenomenon is given and its consequences for the interpretation of forward-scattering spectra are pointed out.

Cross sections for two-, three-, four- and five photon detachment from H^- have been calculated in the framework of perturbation theory applied at the minimum nonvanishing order. The role of correlations for continuum states has been investigated.

A laser of wavelength 600 nm, pulse length 0.6 ps and focused intensity approximately=3*10¹⁵ W cm² is used to multiply ionise CO molecules. The fragmentation channels, for example (CO³⁺) implies C²⁺+O⁺, are isolated using a form of triple coincidence called covariance mapping. The angular distribution of the various atomic ion pairs is found to be strongly peaked along the E vector of the highly linearly polarised laser and to a degree that increases slowly with ionisation stage.

The first classical trajectory Monte Carlo calculation of the electronic spectra arising from both target and projectile ionisation is presented and compared with experimental measurements of the differential cross section for electrons emitted in C⁺+He collisions. The theoretical treatment is based on an independent-electron model in which the interactions between the electrons are approximated by quantum model potentials. Good agreement is obtained between theory and experiment and structures appearing in the measurements can be explained in terms of the electron emission from either, or both, target and projectile. Further, the relative importance of the electron capture to the continuum and the electron loss to the continuum peaks is analysed for different impact energies and the conclusions obtained from this analysis are found to be in agreement with experimental works.

The authors have measured the azimuthal ( phi _p) and polar angle ( theta _p) scattering distribution of projectiles in coincidence with recoil ions in charge states from 3+ to 8+ for the collision 0.53 MeV u^-1 F⁸⁺+Ne to F⁶⁺+NeqR⁺+(q_R-2) e^-. The coincident scattered particle distributions as a function of the azimuthal and polar angles phi _p, theta _p are obtained for different degrees of ionisation, ranging from one to six electrons ionised. Transverse projectile and recoil momenta have been experimentally determined. For a high degree of ionisation the authors observe strong deviations from two-body nuclear kinematics, i.e. for a given collision the resultant transverse momentum of all electrons emitted into the continuum increases with the number of ejected electrons and has a direction which is mostly not coplanar with the scattering plane.

State-selective single-electron stripping processes of Ar²⁺ ions in collisions with He and Ar atoms have been studied experimentally at laboratory impact energies of 6 and 8 keV by means of translational energy spectroscopy. These measurements show that the dominant reaction channels are due to ionisation of long-lived highly excited states with excitation energies very close to the ionisation potential of Ar²⁺. Weaker processes are also observed due to ionisation of metastable and ground state Ar²⁺ ions.

'Convoy' electrons have been measured in coincidence with emergent H^o and H⁺ for incident beams of 0.5-2.5 MeV/H^- and 0.15 MeV/H⁺ traversing very thin ( approximately 1 mu g cm^-2) carbon foils. It is found that the cusp-shaped peak is strongly suppressed in the 0^o spectrum of electrons measured in coincidence with emergent H^o. The observation supports the picture that the sudden appearance of the projectile Coulomb field upon exit plays an essential role in forming the cusp-shaped peak.

Using beams of polarised electrons and polarised hydrogen atoms the authors measured the spin antiparallel-parallel ionisation asymmetry from threshold to 500 eV. Their measurements are generally more precise than those of earlier studies and, more significantly, are characterised by an energy spread that is at least an order of magnitude smaller than that of the earlier work. They compare their results with several theoretical models as well as with the results from similar experiments using polarised alkali atoms.

A simple interpretation of experimental results on simultaneous electron-multiphoton excitation of helium based on the use of the instantaneous collision approximation (ICA) is given.

The stability and structure of 10 states of B₂^- have been investigated with highly correlated MRD-CI wavefunctions. This radical possesses the largest number of stable states (positive electron affinity) so far reported for a diatomic anion: no less than eight states lie below B₂.

The authors have measured Mg I transitions at 740-1126 cm^-1 with a precision of 0.0002 cm^-1. Results include several new lines as well as resolving discrepancies between previous measurements. Their superior resolution allows a complete determination of the fine structure in the 6F and 7F states.

The evaluation of the relativistic continuum-continuum radial matrix elements which occur in the theory of bound Compton scattering and electron-atom bremsstrahlung is discussed. A method is developed for their calculation which involves deformation of the integration contour into the complex plane. The method is fast, accurate and applicable to a wide range of electron angular momenta, photon energies and photon angular momenta. As an adjunct to the method, second-order WKB forms for relativistic point-Coulomb continuum radial eigenfunctions are developed.

The Hanle resonance investigated as an optical field induced circular birefringence has been applied to studying the collisional depolarisation in the 4f ⁶6s²⁷F_J (J not=0) levels of Sm by rare gas perturbers. The measured cross sections for the destruction of orientation in 4f ⁶6s²⁷F_J (J=1-6) by He and Ar perturbers are reported. The data for the ⁷F₁ level are compared with previous works.

Strong-field photoionisation from high discrete levels of the hydrogen atom is considered. The secondary coherent population of Rydberg levels via Raman-type transitions is described. The space and time structure of the wavepackets arising is investigated. Sharp jumps of the semiclassical phase of the corresponding wave function are predicted. These features of the wavefunction are used to explain the reasons for its stability with respect to photoionisation in a strong ionising field. An experimental scheme with almost sudden laser excitation of atoms to Rydberg levels is proposed. This scheme is shown to be applicable for observation of the strong-field interference stabilisation of atoms.

The R-matrix method is used to obtain cross sections for the photoionisation of the 2p³3s ^3.5S⁰, 2p³3p ^3.5P excited states of atomic oxygen. the results for the 3p ³P state are in disagreement with previous theoretical work. Cooper minima are found in the O⁺(⁴S⁰) in d ^3,5D⁰ partial cross sections for the 2p³3p ^3,5P excited states. The cross sections for 2p³3s^3,5S⁰ states are dominated at high energies by the contribution arising from ejection of a 2p electron (the 3s behaving like a 'spectator' electron).

An angle-, energy- and spin-resolved photoionisation experiment was performed in the region of the 6s6p² autoionisation resonances of thallium. Using monochromatic circularly polarised synchrotron radiation the energy dependence of the spin-polarisation parameters A, xi and alpha and the angular asymmetry parameter beta of the differential cross section were determined. In the wavelength region investigated these dynamical parameters show a pronounced variation which agrees well with the results of the random phase approximation with exchange' calculation by Cherepkov (1980). A detailed discussion of the resonance behaviour for the autoionising states is given in terms of dipole-matrix elements and phaseshift differences which are extracted from the experimental data.

The dependence of the v=0 to 1 vibrational excitation of diatomic molecules in their ground state by ion projectiles as a function of scattering angle is described using a potential model represented by multipolar and exponential interactions and assuming pure electronically elastic scattering. The linearly forced harmonic oscillator model is assumed in the calculation of the corresponding vibrational energy transfer. This treatment lends itself to solutions in closed analytical form and enables one to investigate the dependence of the vibrational energy transfer upon the molecule orientation. It is applied to He⁺ + N₂ and CO collisions; overall agreement with experiment is found whenever electronic transition processes may be neglected.

A relativistic one-centre coupled channel formalism for excitation and ionisation in atomic collisions of high-Z ions is presented which uses Coulomb-Dirac functions of the target ion as a basis. For the continuum wavefunctions time-dependent relativistic wavepackets are employed. As applications the authors calculate probabilities and cross sections for excitation and ionisation in the collision systems Z_p+U⁹⁰⁺ at E_lab=430 MeV/nucleon and Z_p+Xe⁵²⁺ at E_lab=82 MeV/nucleon and compare the results with experimental data.

The electron transfer reaction is considered as a three-body problem. The proposed approach is based on the Alt-Grassberger-Sandhas (AGS) three-body equations for Coulomb interactions. The AGS equations are written in the first-order K-matrix approximation, i.e. the free three-particle propagator is replaced by the corresponding delta function. Instead of the partial-wave expansion, the impact parameter representation is used. As a result, a system of algebraic equations for transition amplitudes is obtained which satisfies the two-particle unitarity. A few terms of the quasi-Born expansion, contributing to the effective potential, are considered.

Semiclassical calculations were performed for 10 keV amu^-1-1 MeV amu^-1 collisions of protons with H, He⁺ and Li²⁺ as well as for He²⁺, Li³⁺, C⁶⁺ and Ne¹⁰⁺ colliding with hydrogen. The calculated integrated cross sections for global ionisation and capture agree well with classical trajectory Monte Carlo (CTMC) calculations, other theories for capture and with experiment where available. The semiclassical method for the description of the electron processes encounters difficulties in the description of low-energy capture from highly charged hydrogen-like ions.

Two asymmetries in the differential cross section for elastic scattering of electrons from one-electron atoms are discussed. Numerical results for the standard left-right asymmetry (electrons polarised, atoms unpolarised) and the interference asymmetry (electrons unpolarised, atoms polarised) are presented for caesium and thallium targets. For electron-thallium scattering, it is found that the former left-right asymmetry is predominantly due to spin-nonconserving processes in the electron-core interaction, even for very low scattering energies. This result explains the relatively small importance of the fine-structure effect (based on exchange between the continuum and the target electrons) for this particular collision system.

The authors present experimental data on the autoionisation of neon and argon excited by electron impact. The energy of the incident electrons is chosen such that the two outgoing electrons in the final state have comparable energies. In this energy region state-state coherences and post-collision interactions (PCI) between the two electrons may be observed. Examples of observations of state-state interferences are presented and the effect of PCI in these experiments is discussed. A detailed analysis of the measurements is complicated due to the presence of overlapping neighbouring states.

The K-shell differential ionisation cross section, differential in energy and angle of one of the final-state electrons has been calculated using Dirac plane waves for both incident and scattered electrons, and the Darwin wavefunction and relativistic Sommerfeld-Maue wavefunction to represent the bound and continuum state, respectively. The authors present the results of their calculations incorporating spin-flip and without spin-flip of the atomic electron for Z=29, 47 and 79 with incident electron energies of 140 keV and 300 keV for different scattering angles. The results are compared with the available experimental and theoretical data.

The mass spectrometric apparatus and the method of studying the processes of dissociative ionisation of atmospheric molecules under collisions with electrons in crossed electron and gas dynamical molecular beams are described. The energy spectra and the angular distributions of O⁺(O₂, CO₂) and CO⁺(CO₂) ionised fragments were measured over the incident electron energy range 45-150 eV. On the basis of the known potential curves and the appearance potentials using the measured energy and angular distributions the possible mechanisms of ionised fragment production are discussed. It is shown that, besides the main process of CO₂⁺ predissociation, the fraction of ionised fragments is produced in the process of direct dissociation.

The article outlines a theory for the direct excitation of dipole-allowed vibrations, v=0 to v=1, in polyatomic molecules. The molecule is modelled by the Hartree-Fock function in contrast to earlier treatments using a point dipole model. The discussion retains the practice of these earlier treatments in using the Born approximation for the scattering amplitude. The theory is applied to the acetylene molecule. The results show an improvement over the point dipole model between values of the vibrational transition moments inferred from infrared intensity measurements and values of differential cross sections for low-energy electron impact excitation of these vibrations.

Representing the molecules with their Hartree-Fock functions and the scattering amplitude with the Born approximation, the results from calculations on CO and CO₂ show good agreement between the transition moments inferred from electron scattering experiments and infrared intensity measurements.

Results are reported for the rotationally elastic, inelastic and summed cross sections (differential, integral, momentum transfer and energy loss)from electron-CH₄ scattering in the 0.1-20 eV energy region. Electron exchange is treated exactly and distortion effects are included using the parameter-free model polarisation potential. Agreement with available experimental measurements is good The results are also compared with other theoretical treatments. The authors also report the value of the scattering length from the model.

The kaonic hydrogen molecular ion has been studied by the variational method. Binding energies and strong interaction effects are reported. It is found that the ion has only one bound state.

Laser oscillation in a sealed-off room-temperature CO-He-Xe laser was observed to perturb CO and Xe sidelight intensities in the laser discharge. Bands of the CO fourth-positive group (A ¹ Pi to X ¹ Sigma ⁺) with v'=1-5 and 7 were enhanced during laser oscillation, while those with v'=6 and 8 exhibited no change in intensity. Bandheads of the CO third-positive group (b ³ Sigma ⁺, v'=0, 1 to a ³ Pi _r) were either enhanced or inhibited during laser oscillation depending on the discharge conditions; tail intensities of the third-positive bands, however, were always inhibited. Near-infrared Xe lines were enhanced (by up to 190%) during lasing. The excitation routes for the CO bands and the Xe lines have been identified, and the effects of laser oscillation on those routes are explained in terms of altered rates during lasing of electron-impact processes and collision-induced energy transfers.

The work is aimed at drawing attention to peculiar properties and the considerable magnitude of the vortical part of the light force which make it important for light-induced effects of atomic drift, cooling and trapping in gas. The author introduces basic notions that specify the vortical force, determine its properties and the functional structure and give estimates in the weak-field limit. The vortical force like the light gradient force and the light frictional force sharply increases in the light field resonantly exciting the atom, but depends quite differently on the resonance detunings, wavevectors, polarisation and light wave phases. The results give insight into how to control the vortex effect of light under what conditions it dominates critically the effect of light friction and gradient forces.

A theoretical approach is described to represent a N-electron ion embedded in dense plasma, from which a computer code was constructed. The surrounding charge particles, ions and electrons are represented by a central potential. Including this potential in the N-electron Hamiltonian, the ionic wavefunctions are represented as a multiconfigurational expansion. In a single configuration model, the energy shifts are investigated as a function of the free electron density N_e. It is shown that, as long as energy band broadening does not intervene, the continuum lowering is proportional to N_e^1/3 and the line shift to N_e. Blue and red shifts are demonstrated. Aluminium ions are considered as an illustrative case.