Table of contents

The experimental ion yield curve for photoionisation of atomic fluorine from the ionisation threshold ( approximately 711.6 AA) to 635 AA is reported. Early members of three autoionising series converging to the ¹D₂ ionic state are observed. Their peak widths are all less than the instrumental resolution (0.28 AA). The largest peak, at 680.7 AA, has been observed previously in emission, but surprisingly not in absorption. It is assigned to (¹S₀)3s, which is 'forbidden' to autoionise. An interpretation is offered, which implies superradiance or laser action from this state.

Auger profiles including PCI effects are calculated using the MBPT formulation of Amusia et al. (1979). Results are compared with (semi)classical results of Niehaus (1977).

This letter reports a two-state model calculation of the cross section of the electron detachment reaction H_A⁺+H_B^- to H_A⁺+H_B+e. Centre B is chosen as electron coordinate reference and the asymptotic parts of both the interaction and the energy difference are considered. Agreement with experiment entails the interpretation of the detachment process, as a dipole transition to the continuum under the Stark effect induced by the proton.

Total cross section measurements and translational energy spectroscopy have been performed at collision energies of 1q keV for one-electron capture processes by S¹¹⁺, S¹³⁺ and Kr^q+ (7<or=q<or=25) ions from He atoms. When the measured cross sections are plotted as a function of the crossing radius R_c deduced from the observed energy gain, it is found that the cross sections in general increase with R_c and almost all of them lie between the ¹/₂ pi R_c² and pi R_c² curves. This dependence on R_c is very different from previous results for ions with relatively low q(q<10).

A novel approach to dielectronic recombination is presented. The process is analysed from the viewpoint of electron scattering, using a suitably chosen, complex potential. An exactly solvable model illustrates the new approach and highlights the essential physics of the process: a core-excited resonant state produced by electron impact can decay through the competing channels of autoionisation and radiation. The analysis also clarifies electric-field effects, which are of considerable current interest.

A systematic study of finite dimensional matrix approximations to products of quantum mechanical operators is made. It is shown that the matrix representation of the product of two operators is equal to the product of the matrix representation of each of the operators if the domains of the matrices are suitably chosen. The importance of these results to relativistic molecular structure calculations and to perturbation theory studies is emphasised.

The non-degenerate finite-order many-body perturbation theory is developed with respect to a bare-nucleus reference function. The state consistency and size consistency of this approach to the calculation of molecular electronic structure is emphasised. The bare-nucleus many-body perturbation theory is applied to three simple model systems involving four hydrogen atoms in various spatial arrangements. The results are compared with those obtained by many-body perturbation theory developed with respect to a self-consistent-field wavefunction and full configuration interaction. The effects of quasidegeneracy in the three model systems on the convergence of the perturbation expansion are investigated.

A semi-relativistic variant of the SW-X alpha method is derived. Its foundation from the fully relativistic treatment is shown. All the relativistic shift effects, direct (Darwin term, mass-velocity term) as well as indirect effects (via the potential) are considered. Spin-orbit coupling is omitted. The method is equivalent to the semi-relativistic method of Wood and Boring (1978) but simpler in numerical realisation. The computational effort is only slightly increased compared with the non-relativistic SW-X alpha scheme. Results for the uranium atom and I₂ and C₂ molecules are reported and compared with results from fully relativistic calculations.

The relativistic time-dependent local-density approximation (RTDLDA), a time-dependent generalisation of the relativistic local-density approximation (RLDA), is presented in a form suitable for the description of electric multipole excitation of closed-shell atomic systems. The development of the theory from the relativistic density functional formalism is outlined. A discussion is given of the angular decomposition of the RLDA and RTDLDA equations to those for the radial wavefunctions. Results of applications of this theory to calculations of atomic photoionisation parameters for xenon and electric dipole susceptibilities for various heavy atoms are compared with predictions of other theories and with experimental measurements.

The spin-orbit term of the Pauli approximation has been included directly in a SCF method of the numerical multiconfigurational Hartree-Fock type. The details of the procedure are presented, and the structure of the modified Hartree-Fock equations is given and discussed. The application to the study of some resonance lines of the Kr-like ions up to Mo VII is used as a test of the method which is compared with more sophisticated calculations and with known experimental results.

The hydrogen Rydberg states in the presence of weak magnetic fields are analytically investigated by using quantum mechanical first-order perturbation theory. The unperturbed hydrogenic wavefunctions, which diagonalise the quadratic Zeeman interaction within the subspace of states with fixed principal quantum number n, are obtained by separation of variables on the Fock hypersphere in momentum space. By considering n as a large parameter, the comparison equation method is employed to find the uniform asymptotics of eigenfunctions and asymptotic expansions of quadratic Zeeman energies corresponding to the outermost components of the Zeeman n manifold. The results obtained are compared with other theoretical predictions.

The non-exponential decay of an initially prepared state is a well known theoretical result. Experimental confirmation is difficult (and absent) because of the smallness of the effect. The authors treat the commonly occurring case in which the initial state is prepared by an adiabatically switched external field. They find that there can still be non-exponential decay but that it is now characteristic of the way the field is switched and not of the structure of the system as in the initially prepared state case.

The relative intensities of L-shell X-rays, I(L_l)/I(L_alpha ),I(L_beta )/I(L_alpha ),I(L_gamma )/I(L_alpha ) for Ta, Au, Pb and Bi, have been measured at incident photon energies of 17.8, 25.8, 46.9 and 59.5 keV. The emitted X-rays were measured with a Si(Li) detector system. The results for Pb at 17.8, 25.8 and 46.9 keV and those for Ta and Bi at all the four energies are being reported for the first time. The experimental results are in reasonable agreement with theory.

Einstein A coefficients for the electric dipole transitions between the rotational levels up to 700 cm^-1 of the ground vibrational state of H₂S are calculated. The Einstein A values are used to compute the mean radiative lifetimes of the levels.

The author gives a theoretical description of the relation between scattering amplitudes and angular correlations in projectile-photon and photon-photon coincidence measurements of excitation of two atoms in a single collision. The information content of both types of experiments is analysed. The author discusses the additional effects of fine structure and of the identity of the atoms.

Techniques of atomic fluorescence were used to determine cross sections for relaxation and decay of atomic multipole moments associated with the 4²P_1/2 and 4²P_3/2 states in potassium. 4²P_1/2-1/2or ²P_3/2-3/2 Zeeman substates were selectively populated by irradiating potassium vapour, mixed with N₂ or H₂ and located in a kilogauss magnetic field, with monochromatic and circularly polarised radiation. Collisions with the buffer gas molecules caused the population of the other Zeeman substates and the emission of a Zeeman fluorescent spectrum, the sigma components of which were resolved with a piezoelectrically scanned Fabry-Perot interferometer. Measurements of the intensities of the Zeeman fluorescent components yielded cross sections for relaxation and decay of the dipole, quadrupole and octupole moments.

The authors present calculations of electron excitation rates in heavy-ion collisions, in which the electron-electron interaction is treated within a relativistic self-consistent field approach. In comparison with earlier calculations neglecting electron shielding, the authors obtain increased probabilities for K-hole and delta -electron production, whereas the total positron yield decreases. Computed probabilities are compared with available experimental data.

Explicit expressions for capture of an electron to and from arbitrary hydrogenic states are presented for various multiple-scattering approaches, namely approximate forms of the strong-potential Born (SPB) and impulse approximations (IA) as well as the continuum distorted-wave (CDW) approximation. In the latter case, however, the authors limit the calculation to an initial 1s state. The transition amplitudes are reduced in all cases to closed-form analytical formulae leaving a single numerical integration to obtain the capture cross sections. The author discusses some symmetry and scaling properties of the post (+) and prior (-) forms of the different methods and, as a first application of the newly gained analytical amplitudes, the author extracts some new asymptotic results. The necessary mathematical tools for the evaluation of a common matrix element appearing in the SPB, IA and CDW are provided in an appendix, where it is shown how the relevant transition amplitudes can all be obtained from one generating integral.

One-electron capture from the K shell of a target ion (atom) by impact of a bare ion at intermediate and high energies has been studied in the continuum distorted wave approximation. Double-scattering structures on differential cross sections d sigma /d Omega , predicted by Rivarola and Miraglia (1982) for large velocities, are analysed for non-relativistic high energies in near-symmetric and very asymmetric monoelectronic systems. Multielectron systems are also considered in order to determine the influence of passive electrons on these two-step processes. Theoretical results are in good agreement with the first recent experimental data on the Thomas's differential cross section peak from Horsdal-Pedersen et al. (1983) for the H⁺-He system. The authors compare the continuum distorted wave approximation for asymmetric systems at intermediate velocities with experimental differential cross sections and impact parameter probabilities from Horsdal-Pederson et al. (1983). The systems considered are: p+Ne at 1.5 MeV laboratory energy and p+C at 0.4 and 0.6 MeV laboratory energies. The agreement between theory and experiment is remarkable.

The single- and double-electron capture cross sections for Ne²⁺ in He, Ne and Ar have been measured in the energy range 0.5-3.0 MeV. For the single-electron capture cross section, comparison with the Rapp and Francis (1962) theory has been made.