Table of contents

Formulae for the calculation of expectation values of arbitrary powers of r for hydrogen-like wavefunctions are derived both in quantum mechanics and within the framework of the WKB approximation. The quantum to classical correspondence is then investigated in the limit n to infinity for finite n_r, as well as for finite l. For negative powers and finite l, an asymptotically remaining deviation of the semiclassical formulae from the quantum expressions is found. A modification in the eccentricity of the classical orbit is introduced which corrects the asymptotic behaviour of the WKB approximation.

The Hedin-Lundqvist (1971), Gunnarsson-Lundqvist-Wilkins (1974) and Ceperley-Alder (1980) local density approximations parametrized by Perdew and Zunger (1981) and Vosko, Wilk and Nusair (1980) are applied to calculate the first excitation energies of several atoms in the density functional theory for unequally weighted ensembles of Gross, Oliveira and Kohn (1988).

The Kr 4s-electron photoionization cross section as a function of the exciting-photon energy in the range between 30 eV and 90 eV was calculated using the configuration interaction (CI) technique in intermediate coupling. In the calculations the 4p spin-orbital interaction and corrections due to higher orders of perturbation theory (the so-called Coulomb interaction correlational decrease) were considered. Energies of Kr II states were calculated and agree with spectroscopic data within less than 10 meV. For some of the Kr II states new assignments were suggested on the basis of the largest component among the calculated CI wavefunctions.

The Volkov wavepacket is used to trace the evolution of a ground-state hydrogen atom in an ultra-intense field, where the peak electric field strength is larger than the mean Coulomb binding field. We find the probabilities for excitation and ionization by projecting the time-dependent wavefunction onto the field-free atomic states. The ground-state atom is drastically shaken up into highly excited Rydberg states and continuum states within a very short time. Results are given for linearly and circularly polarized oscillating fields and for static fields. The dominant mechanism is field excitation and emission as a result of the instantaneous binding potential being suppressed below all bound-state energy levels. Our results are quite different from those of several other authors, some of whom claim that bound-state stabilization occurs in the high-field limit. We see no stabilization and we comment on possible reasons for the diverse results.

Single photon double ionization of hydrogen chloride (HCl) and chlorine (Cl₂) has been studied in an electron-electron coincidence experiment. Threshold PhotoElectrons COincidence (TPEsCO) spectra have been obtained by using a penetrating field technique, in which only electrons with nearly zero kinetic energy (<20 meV) are detected. The high sensitivity and energy resolution attainable has allowed observation of several states of HCl²⁺ and Cl₂²⁺, excited essentially at their respective thresholds, including vibrational energy levels for some states. Accurate energies of the dicationic (doubly charged ion) features observed in the TPEsCO spectra are presented, and the possible mechanisms of formation are discussed.

The Hanle effect signal and the optical radio-frequency double-resonance signal on NO₂ may reveal an inversion effect under some particular conditions. However, the experimental results are not satisfactorily explained within the standard description of molecules. Recent experimental results suggest the overlap of Delta F=O transitions with Delta F=+or-1 transitions as a possible explanation. It is shown that this explanation fails to describe the experimental results for NO₂.

Using recoil-ion momentum spectroscopy (RIMS), the authors have measured differential cross sections for the production of highly charged Ne ions in collisions with 10 MeV F⁸⁺ as a function of the recoil-ion transverse momentum and charge state. In addition, the outgoing projectiles were charge-state analysed and detected in coincidence with the Ne recoil-ions. Thus different reaction channels like direct ionization, single- and double-electron capture by the projectile could be separated. The recoil-ion transverse momentum P_{perpendicular to} ^R was determined with an accuracy comparable to a projectile scattering angle resolution of 2.5*10^-5 rad. The experimental data are compared to results of classical many-particle (nCTMC) and semiclassical quantum-statistical calculations.

A simple semiclassical model is proposed for the calculation of the polarization potential at short range where the impacting electron penetrates the electron cloud of the target atom. The new method is applied to the scattering of electrons and positrons from He atoms at energies between 0.1 and 50 eV. The model is described in detail and is shown to produce a local effective potential which is computationally very easy to devise. Calculations for electron scattering are carried out for the elastic integral cross sections and for angular distributions of electrons at several collision energies and compared both with experiments and with more sophisticated calculations. The agreement of the present results with experiments is invariably very good. Further test calculations for the positron scattering processes also agree very well with the available experiments and with other theoretical predictions.

Electron impact excitation of the 3¹D state of helium is studied using the electron-photon polarization correlation technique. Four Stokes parameters are determined at an incident electron energy of 29.6 eV and for scattering angles in the range 40-120 degrees . The data are interpreted in terms of the shape and dynamics of the excited state produced over the range of kinematically defined collisions. The lack of a theoretical model to adequately describe low-energy D state excitation is exposed.

Electron-photon coincidence measurements are reported of the excitation of the 4s'[¹/₂]₁⁰ state of Ar at incident energies of 20 and 30 eV and scattering angles up to 80 degrees . All values of the rho ₀₀ parameter obtained are very small and not distinguishable from zero within experimental uncertainty. The level of agreement with available perturbative theories is quite good but significant discrepancies are noted at some angles particularly at the lower impact energy. Previous measurements from this laboratory of the 4s[³/₂]₁⁰ state are reanalyzed in terms of the ratio of the exchange to direct excitation cross sections.

The complex problem of a collisionally pumped Ne-like germanium laser is examined through several detailed models. The central model is EHYBRID; a 1 1/2 D fluid code which self consistently treats the plasma expansion with the atomic physics of the Ne-like ion for 124 excited levels through a collisional radiative treatment. The output of EHYBRID is used as data for ray-tracing and saturation codes which generate experimental observables. A detailed description of the models is given. The atomic physics is investigated through a three-level approximation, a steady state collisional radiative treatment and a time-dependent solution within the fluid model. The accurate calculation of the non-steady state ionization balance is identified as a key issue. Time resolved and time integrated output profiles are generated for various experimental configurations, and the effects of saturation and gain narrowing are examined. The agreement with experiment is excellent in virtually every respect.

A theoretical study of the propagation of steady-state video pulses in a double-beam-refractive one-axis crystal, consisting of non-resonant sigma -transitions, without using the slowly varying envelope and rotating wave approximations has been performed. On the basis of appropriate exact solutions a possibility of non-linear bound steady-state pulse of the ordinary and extraordinary waves is analysed.