Table of contents

We have measured photoelectron angular distributions in the molecular frame (MF-PADs) for O 1s photoemission from CO₂, using photoelectron-O⁺–CO⁺ coincidence momentum imaging. Results for the molecular axis at 0, 45 and 90° to the electric vector of the light are reported. The major features of the MF-PADs are fairly well reproduced by calculations employing a relaxed-core Hartree–Fock approach. Weak asymmetric features are seen through a plane perpendicular to the molecular axis and attributed to symmetry lowering by anti-symmetric stretching motion.

The first application of the time-dependent close-coupling method to electron–molecule scattering is used to calculate electron-impact ionization cross sections for H⁺₂. The time-dependent Schrödinger equation for the six-dimensional wavefunction is reduced to a set of close-coupled equations on a four-dimensional numerical lattice in (r₁, θ₁, r₂, θ₂) centre-of-mass spherical polar coordinates. When the non-perturbative close-coupling results for low lM angular momenta are combined with perturbative distorted-wave results for high lM angular momenta, the resulting ab initio ionization cross sections are found to be in excellent agreement with experimental measurements in the intermediate energy range.

Application of a parallel-projection inversion technique to z-scan spectra of multiply charged xenon and krypton ions, obtained by non-resonant field ionization of neutral targets, has for the first time permitted the direct observation of intensity-dependent ionization probabilities. These ionization efficiency curves have highlighted the presence of structure in the tunnelling regime, previously unobserved under full-volume techniques.

Potential energy curves for interactions of antihydrogen atoms with singlet and triplet states of helium atom and helium cation have been calculated within the adiabatic approximation. The leptonic Schrödinger equation was solved variationally with the basis of explicitly correlated Gaussian functions. Adiabatic corrections have been calculated with the Born–Handy method. For the and systems, critical distances have been found to be below 1.7 bohr and 1.0 bohr and the rearrangement cross sections have been computed with the semiclassical method. The possible existence and structure of complex atomcules is discussed.

We present experimental and theoretical studies of the absorption spectrum of Na–K–He mixture. Semiclassical spectral simulations in the 400–850 nm wavelength range were performed, on the basis of available interaction potential curves for Na₂, K₂, NaK, NaHe and KHe molecules. Calculated absorption spectra were analysed and compared to the experimental absorption spectrum of Na–K–He mixture generated in a gravitational heat pipe oven.

We derive a quantum theory of matter-wave detection from microscopic considerations. We calculate both the short-time approximation and the long-time correction to the detection rate of matter waves. The detection rate can be related to the flux of the matter waves through a detector medium.

The interaction between resonances through autoionization continua and the interaction between autoionization continua were investigated theoretically and experimentally for the photoionization process of the 4p- and 4s-shells and for the population of 4p⁴ (³P)5s ⁴P_J, 4p⁴ (³P)5s ²P_J and 4p⁴(³P)4d ⁴D_J satellites of KrII. Cross sections for the satellite production and the angular distribution parameter of the fluorescence radiation were measured by photon-induced fluorescence spectroscopy after excitationx with linearly polarized monochromatized synchrotron radiation at exciting-photon energies between 28.45 eV and 29.95 eV with an exciting-photon energy resolution of 10 meV (FWHM). Measured cross sections are in good agreement with the computed ones. A refined assignment of resonances in the proximity of the 4s¹4p⁶5p resonance was performed. It was concluded that there is a strong influence of the core rearrangement, of the interaction between resonances through the autoionization continua and of the interaction between autoionization continua, on the investigated processes on the basis of the observed good overall agreement between the computed and measured quantities.

We consider double ionization by positron impacts near the break-up threshold. Threshold exponents are evaluated making use of the Newtonian classical method. Our results agree with those due to Kuchiev and Ostrovsky (1998 Phys. Rev. A 58 321–35), obtained by Hamiltonian formalism. We address the question of deducing threshold law in the presence of several guiding configurations, as the case with positron impact arises. General formulae for these degenerate cases are proposed and the issue of the comparison with experimental data near threshold is discussed. Finally, we argue that the disagreement between Wannier's threshold laws and the experimental evidence for the multiple fragmentation functions may be resolved on the basis of the shift from the proper threshold region.

Single-electron capture in O⁶⁺ + Na collisions at 1–9 keV/amu collision energy has been studied both experimentally and theoretically. Partial cross sections for electron capture into n = 5, 6, 7, 8 and n ⩾ 9 have been obtained from target recoil momenta measured by the technique of MOTRIMS and are compared with close-coupling results obtained from a two-centre extension of the basis generator method. An overall good agreement is found, concerning the relative magnitude as well as the energy dependence. Also the contribution of ionization could be extracted from the recoil spectra. The ionization cross section is very small but strongly increases with energy.

Single ionization of CO₂ by 6 MeV proton impact has been studied by measuring in coincidence the momentum vectors of the emitted electron and the charged nuclear fragment CO⁺₂, CO⁺ or O⁺, respectively. The experimental data have been compared with the predictions of a state of the art CDW–EIS (continuum distorted wave–eikonal initial state) calculation for molecular orbitals. A good qualitative agreement is observed even though the vibrational motion of the molecule is not taken into account in the model. The low-energy electron spectra show a rich structure which may be attributed to the presence of molecular excitation channels which undergo radiationless decay, via autoionization and also via predissociation. This interpretation is supported by photoionization studies of CO₂.

Large-scale configuration interaction (CI) calculations are performed to evaluate the energy levels, oscillator strengths and transition probabilities for all the transitions in Co XI from ground-state level to those of 3s3p⁶, 3s²3p⁴3d, 3s²3p⁴4s and 3s²3p⁴4d states in LSJ coupling scheme. Lifetimes of the 3s3p⁶ and 3s²3p⁴3d levels are also determined. The calculations include the major correlation effects. We attempt to correct the inaccuracies in the CI coefficients in the wavefunctions, which would lead to inaccuracy in transition probabilities by applying a 'fine-tuning' technique. The relativistic effects are incorporated by adding mass correction, Darwin term and spin–orbit interaction terms to the non-relativistic Hamiltonian in the Breit–Pauli approximations. The calculated energy levels are in close agreement with the NIST results. We have predicted many new spectral lines, which are to be experimentally observed in the future.

Inelastic collisions with hydrogen molecules are claimed to be an important channel of antihydrogen losses (Armour and Zeman 1999 Int. J. Quantum Chem.74 645). In the present work, interaction energies for the system in the ground state have been calculated within the Born–Oppenheimer approximation. The leptonic problem was solved variationally with the basis of explicitly correlated Gaussian functions. The geometry of H₂ was fixed at equilibrium geometry and the atom approached the molecule from two directions—along or perpendicularly to the bond axis. Purely attractive potential energy curve has been obtained for the first nuclear configuration, while a local maximum (lower than the energy at infinite separation) has been found for the second one.

A calculation scheme with significant modifications and improvements for determining the ionization balance and the ion temperature evolution in an electron beam ion trap (EBIT) is presented. The scheme is applied to thorium, tantalum and oxygen ions using a specific set of EBIT operating parameters. The results of the calculation are compared to experiment and previous calculations. Rates for the individual atomic processes in EBIT, in particular, single and multiple charge exchange processes are discussed. The time evolution of the ion temperatures for both thorium and its coolant oxygen are also given.

Bose–Einstein condensates are studied in a potential of finite depth which supports both bound and quasi-bound states. This potential, which is harmonic for small radii and decays as a Gaussian for large radii, models experimentally relevant optical traps. The nonlinearity, which is proportional to both the number of atoms and the interaction strength, can transform bound states into quasi-bound ones. The latter have a finite lifetime due to tunnelling through the barriers at the borders of the well. We predict the lifetime and stability properties for repulsive and attractive condensates in one, two and three dimensions, for both the ground state and excited soliton and vortex states. We show, via a combination of the variational and WKB approximations, that macroscopic quantum tunnelling in such systems can be observed on time scales of 10 ms to 10 s.

The exponential representation is used to construct highly accurate wavefunctions for the triplet states in various two-electron helium-like ions. It is shown that the exponential variational expansion in relative coordinates (r₃₂, r₃₁ and r₂₁) provides a very high accuracy for the triplet 2³S(L = 0) states in light two-electron ions. The developed methods are used to determine the highly accurate non-relativistic energies and other bound state properties for the 2³S(L = 0) state in a number of He-like two-electron ions Li⁺, Be²⁺, B³⁺, C⁴⁺, N⁵⁺, O⁶⁺, F⁷⁺ and Ne⁸⁺. To represent the computed energies of these ions the Q⁻¹ expansion is applied. The asymptotic form of the 2³S(L = 0) state wavefunctions at large electron–nuclear distances for the He-like ions is briefly discussed. We also consider the hyperfine structure splitting in the 2³S(L = 0) state of the helium-like ions with non-zero nuclear spin. For each of the considered two-electron ions one can determine the isotopic shifts by using our approach based on the derived interpolation formula. The lowest order QED corrections are also determined for the triplet states in all mentioned two-electron ions.

We perform a systematic analysis of how nonsequential double ionization in intense, linearly polarized laser fields is influenced by the initial states in which both electrons are bound and by the residual ionic potential. We assume that the second electron is released by electron-impact ionization of the first electron with its parent ion, using an S-matrix approach. We work within the strong-field approximation and compute differential electron-momentum distributions using saddle-point methods. Specifically, we consider electrons in 1s, 2p, 3p and localized states, which are released by either a contact- or a Coulomb-type interaction. We also perform an adequate treatment of the bound-state singularity which is present in this framework. We show that the momentum distributions are very sensitive with respect to spatially extended or localized wavefunctions, but are not considerably influenced by their shapes. Furthermore, the modifications performed in order to overcome the bound-state singularity do not significantly alter the momentum distributions, apart from a minor suppression in the region of small momenta. The only radical changes occur if one employs effective form factors, which take into account the presence of the residual ion upon rescattering. If the ionic potential is of contact type, it outweighs the spreading caused by a long-range electron–electron interaction or by spatially extended bound states. This leads to momentum distributions which exhibit a very good agreement with the existing experiments.

A determination of the angular distributions, β, and relative partial photoionization cross sections, σ, of the valence satellite lines of atomic scandium in the region of the 3p → nd, ms excitations has been carried out using electron spectrometry in conjunction with monochromatized synchrotron radiation. This work is complementary to our earlier study of the partial cross sections of the Sc valence satellites but now includes angular distribution measurements of the most intense satellite lines. A correct theoretical description of the experimental satellite σ and β values presented herein should help in rectifying present discrepancies between theory and experiment for the 3d and 4s mainline σ and β values.