Table of contents

The angular anisotropy of Auger electrons from cascade Auger transitions induced by photoexcitation of the 3d^-1_5/25p resonance in Kr is reported. In contrast to the previously reported measurements for Ne and Ar, a large anisotropy is observed for the first-step spectator transitions 3d^-1_5/25p → 4s^-14p^-15p and 3d^-1_5/25p → 4s^-25p while the second-step participator transitions are almost isotropic. These findings are qualitatively explained by consideration of the alignment transfer via the first-step spectator Auger transitions.

New data for double, triple and quadruple ionization of Xe at impact energies up to 400 eV are presented. The contribution to the total triple ionization cross section from ionization with positronium formation, below the threshold for direct ionization, is small (less than 1.6 ± 0.2% of the triple ionization cross section maximum). By interpolating the present data for quadruple ionization, the relative contribution from quadruple ionization with positronium formation below the threshold for direct quadruple ionization, is indistinguishable from zero. These findings are in qualitative agreement with the model recently proposed by Humberston and Van Reeth.

We have measured the partial cross sections and angle-resolved energy dependence of the electrons emitted over the energy range of the triply-excited 2p^3 2P hollow lithium resonance. We have also calculated the β-behaviour using the R-matrix methodology. Experimental and theoretical results are in good agreement and show deep minima in the 1s2p ^1,3P ionic channels.

Cross sections for inner-shell and outer-shell excitations of lithium atoms in collisions with fast charged projectiles have been calculated. For these excitation processes, the relative importance of the first- and second-order mechanisms, the dependence of the cross sections on the sign of the projectile charge, as well as the effect of electron correlations in the initial and final state have been investigated. The calculated results are compared with experimental data. Correlations are found to be essential in explaining the large ratio between cross sections for an excitation to the 1s(2s2p ³P) ²P_a and the 1s(2s2p ¹P) ²P_b states.

The e, 2e reaction of lithium is studied as a four-electron system, whose initial and final singlet and triplet spin states produce a variety of exchange terms whose importance in the determination of the transition matrix element of the ionization process is studied systematically over a very large domain of incidence energy (50 eV to 5 keV) by applying in the small and intermediate energy domain a distorted-wave procedure concerning all continuum electrons. The comparison with experimental and recent theoretical results shows that the use of an all-electron procedure improves the agreement with the experimental results.

Electronic structure calculations for several ³Σ^- states of OH⁺ are reported. These states are important for the charge-exchange process O + H⁺↔O⁺ + H. Diabatic potential curves and their couplings were determined using the methodology of Pacher et al and of Domcke and co-workers, which is based on CAS-MCSCF-CI calculations followed by block diagonalization. The dimension of the matrix representation of the diabatic Hamiltonian is determined by N_α, the number of interacting states included in the block diagonalization. Suitable values for N_α depend on the internuclear separation, and several alternatives are compared. Further analysis of the calculations indicates that the excited oxygen (2p³3s ³S) level participates in charge exchange through a two-step mechanism not previously recognized: Stark mixing of oxygen levels by a distant proton leads to an admixture of 2p⁴ and 2p³3s configurations, and the diffuse 3s orbital then provides the dominant coupling with the O⁺ + H state.

We present a comparative ab initio study of the direct double photoionization process in Ca and He. Strong similarities are found, but core effects become observable as the energy increases, leading to more correlated patterns in Ca than in He, as well as to the appearance of weak secondary structures in Ca, which call for further studies. A consistency check of two measurements by Ross et al of the resonant double photoionization cross section of Ca is also performed successfully.

Breit-Pauli results for energy levels, lifetimes and some transition data are reported for all levels up to 2s2p(³P)3s ²P_J^o of the B-like spectrum for 5⩽Z⩽14. For all but the lowest members of the sequence, these include the 2s²2p, 2s2p², 2p³, 2s²3s, 2s²3p, and 2s²3d and 2s2p3s configurations, though only the 2s2p(³P)3s terms of the latter. A simultaneous optimization scheme was applied so that a radial basis could be determined for a set of terms that mix in the Breit-Pauli approximation. Convergence of the LS line strength is used as a factor in estimating accuracy as well as the agreement of energy levels and their splitting between theory and experiment. The results are evaluated by comparison with other theoretical results and experiment for transition rates.

The matrix elements of the singular terms r_ij^-3 and r_i^-3 are evaluated for three-electron systems in Hylleraas coordinates. These terms are parts of QED corrections of orders α⁵mc² and (µ/M)α⁵mc² respectively.

A detailed investigation of the relation between five dynamical parameters (partial photoionization cross section σ, angular asymmetry parameter β and three spin polarization parameters A,α,ξ) and five corresponding theoretical values (three dipole matrix elements and two phase shift differences) describing photoionization from p_3/2,d_3/2 and d_5/2 subshells of rare-gas atoms reveals a systematic ambiguity preventing the extraction of a unique set of five theoretical values from the experimental data which constitute a `complete experiment'. Xe(5p_3/2)^-1 photoionization serves as an example to illustrate the problem using a numerical procedure as well as a mathematical analysis. A new equation is presented which connects four dynamical parameters. From these facts it follows that a complete experiment on closed-shell atoms with the extraction of five theoretical values is, in principle, impossible, if the photoelectron properties are measured exclusively.

The alignment of Kr II 4p⁴5p (EJ) states after the Auger decay of the Kr I 3d⁹_5/25p (J = 1) resonance was measured using photon-induced fluorescence spectroscopy (PIFS) and the alignment parameters A₂₀ were calculated within the stepwise decay model. Using the same wavefunctions, the angular distribution parameters of the Auger electrons were also calculated. Good agreement with the measured A₂₀ value and with published experimental and calculated values for β_Aug is obtained.

A quasi-classical three-step model for high-harmonic generation is presented. The results for the harmonic emission rate obtained using this model agree very well with the exact results. For an elliptically polarized laser field a staircase of plateaux is observed. Both the heights and the cut-off positions of these plateaux are explained. An analytical formula for the cut-off law is derived and applied to determine the cut-off of the harmonic spectrum for each of the observed plateaux. We also present the electron orbits, obtained as solutions of the classical equation of motion with the complex time.

We applied the method known as fermion molecular dynamics to the description of an isolated argon atom interacting with a short pulse of intense, long-wavelength laser radiation, for both linear and circular polarization. We discuss the results of these calculations, insofar as they bear on the mechanisms leading to the production of high-ionic-charge states during laser-atom interaction.

A numerical method is described for the evaluation of the regular and irregular basis functions used in quantum defect theory for arbitrary potentials (generalized quantum defect theory). The method is related to Milne's phase-amplitude method and yields phase and amplitude parameters which vary smoothly with energy and radius. It also permits the evaluation of these parameters for strongly closed channels, in the absence of a classically accessible radial range.

We address the validity and practicality of using multi-channel quantum defect theory (MQDT) to treat electron-ion collision problems for cases where some channel solutions propagate with extremely negative, classically forbidden, energies. We discuss theoretically and demonstrate numerically, within an unrealistically sensitive model-problem study, how MQDT results can be obtained which are in agreement with those obtained from the more standard non-MQDT method at all energies.

A self-similar collision model for the motion of radiating particles in a perturbed gas is suggested. The influence of effects of self-similar collisions on the shape of a spectral line in the Doppler regime is considered in the framework of classical Fourier integral theory. In the limiting case when the self-similar character of collisions is not taken into account the self-similar collision model becomes identical to the Galatry diffusion model. The general formula for the distribution function of the radiating particles in the self-similar model and a new correlation function in the impact approximation are derived. It is shown that the self-similar collision mechanism in the Doppler regime leads to additional spectral line narrowing.

Relativistic distorted-wave calculations are reported for the differential cross section and a complete set of S, T and U parameters for electron impact excitation of thallium from its ground state 6p ²P_1/2 to the fine-structure state 6p ²P_3/2 as well as the 7s ²S_1/2 state, and of lead for the fine-structure transitions ³P₀→³P₁, ³P₂ and ¹D₂. Our results for the asymmetry parameter S_A are compared with the only available experimental measurements from the Münster group and the agreement is very satisfactory.

We develop a theoretical approach to characterize the dynamics of a plasma surface irradiated by a high-intensity electromagnetic wave. The method is based on the analysis of the harmonic content in the reflection of a probe field, which is aimed at the plasma simultaneously with the strong field. In particular, a explicit formula for the angles of reflection of these harmonics is derived, showing a particular distribution directly related to the frequency of oscillation of the vacuum-plasma interface. Also, a system of equations for the reflected field amplitudes is formulated, and solved numerically in some particular cases. The experimental implementation of this scheme would provide a direct test of the so-called moving mirror models, which presently provide a basis for the understanding of the intense field-plasma interaction.

The mean-field properties of trapped, two-dimensional interacting Bose gases of finite size are surveyed numerically. The coupled nonlinear equations of the Gross-Pitaevskii (GP) equation and the Hartree-Fock equation for the condensate and the thermal component, respectively, are calculated by the hybrid method, in which the GP equation and the low-lying excitations less than cut-off energy are calculated quantum mechanically using the basis representation, while the high-lying states are obtained within the local density approximation. Consistent solutions are obtained for the whole range of temperatures including the critical temperature by determining the thermodynamic chemical potential µ from the condition that the total atomic number must be the desired value, instead of identifying µ with the eigenvalue of the GP equation. Comparisons with the results of the semiclassical two-fluid model are presented.

Excitation of the 3 ³D state of helium by electrons with an energy of 370 meV above threshold is studied experimentally using the electron-photon polarization correlation method and theoretically using the convergent close-coupling (CCC) and R-matrix with pseudo-states (RMPS) methods. Stokes parameters and associated coherence parameters are presented. Despite difficulties associated with a sharp resonance in the cross section at this energy, generally good agreement is obtained between experiment and the calculations. At small scattering angles the results show a violation of the propensity rule for angular momentum transfer. CCC and RMPS calculations have also been performed at 22 and 24 eV for the 2 ¹P state and are compared with earlier angular correlation data.

Positronium-hydrogen elastic scattering is investigated using the six-state full close coupling approximation employing the basis set Ps(1s,2s,2p) + H(1s,2s,2p). S-wave singlet and triplet scattering and also the corresponding scattering length and effective range are compared with the corresponding predictions of Drachman and Houston and the Belfast group. We also report phase shifts up to l = 2 and the integrated elastic cross sections. Present results indicate that the effect of target excitation as well as the effect of excitations of both the atoms are significant in the energy region considered. We also report the angle integrated Ps(1s) + H(1s) → Ps(2p) + H(1s) and Ps(1s) + H(1s) → Ps(1s) + H(2p) excitation cross sections.

Low-energy positron-helium scattering is investigated using three different one-electron models of helium in order to find out if such a model is capable of yielding accurate results for elastic scattering, positronium formation and positron-electron annihilation. Comparisons are made with the accurate results obtained from ab initio variational calculations of Van Reeth and Humberston (Van Reeth P and Humberston J W 1999 J. Phys. B: At. Mol. Opt. Phys.32 3651). The most elaborate helium model used here gives accurate values for the elastic scattering phase shifts throughout most of the energy range up to the positronium formation threshold. However, near-threshold resonances associated with this model have substantial effects on all the partial wave contributions to the positronium formation cross section that have been investigated. The other two helium models yield rather less accurate elastic scattering phase shifts but the positronium formation cross sections are in good qualitative, and even reasonably good quantitative, agreement with the accurate results. None of the models yields very accurate results for the annihilation cross section.

The high-order harmonic generation process due to the interaction of a multi-well quantum system with an intense laser field is examined. A plateau extension up to a photon energy of I_p + 8U_p and the generation of attosecond pulses are demonstrated.

The dynamics of atomic hydrogen placed in a static electric field and illuminated by elliptically polarized microwaves is studied in the range of small field amplitudes where perturbation calculations are applicable. For a general configuration of the fields any generalized time-reversal symmetry is broken and, as the classical dynamics is chaotic, the level statistics obeys the random matrices prediction of a Gaussian unitary ensemble.