Table of contents

Energy levels and optical oscillator strengths for transitions in Fe XV are calculated using the 14 configurations , 3p4d and 3p4f in Hibbert's CIV3 program. Agreement between the present calculated relative energy levels and measurement as well as the most recent calculation is generally good. Our results also confirm the relative position of the level which previously had been located only tentatively, but raise some concerns regarding the choice between the recent 6- and 13-configuration calculations.

The energies of the closely spaced Ca 2p thresholds have been determined by crossing an atomic beam of calcium with the monochromatized synchrotron radiation from the high-resolution undulator beamline SX 700 - BW3 at HASYLAB and analysing the kinetic energy of the outgoing photoelectrons with a high-resolution SCIENTA SES 200 electron analyser. The experimental results are compared to the predictions of different theoretical approaches and the strong influence of electron correlation in the core ionized state is demonstrated.

The dissociative recombinations of , , and were studied with an extremely low-temperature electron beam of the order of 1 meV produced by a superconducting electron cooler at the storage ring TARN II. The dissociative recombination spectra show a fine new structure and a clear isotope dependence in the low-energy region. Those features are well reproduced by the theoretical calculations based on a multichannel quantum-defect theory including rotational motion and discretized dissociative states. According to the theory, the newly observed structure is evidence for a new indirect mechanism where the intermediate states are not vibrationally excited Rydberg states but virtual dissociative states.

Ab initio electron scattering calculations using the R-matrix approach have been performed for within a three-state valence configuration-interaction model (VCI). The lowest three electronic target states (, and the ) of this molecular nitrogen cation are included in the close-coupling method, with each state being represented by a valence CI approximation. From a detailed analysis of the resonance structure found in our work for the symmetries we find four prominent Rydberg series of the type , , , and a interloper resonance. This interloper molecular resonance associated with the B state of is seen to cause distortions of the resulting resonance spectra. A comparison of our total cross sections for the X - B transition shows excellent agreement with the available experimental data.

Calculation for the electronic excitation of the ground state of to and states by positronium- (Ps) atom impact has been carried out using the first Born approximation considering discrete Ps excitations up to n = 6 and Ps ionization in the final state. To include the effect of electron exchange, we propose an alternative approximation scheme in the light of the Rudge approach, which takes into account the composite nature of the Ps-atom projectile.

Caesium atoms in a magnetic trap have a higher loss rate from intra-trap collisions than rubidium under comparable conditions. We have found that this loss from inelastic collisions can be suppressed by periodic optical pumping of the atoms back into the most strongly trapped magnetic state , although this reclamation of the strayed atoms gives rise to some heating of the sample. This observation shows that the dominant loss mechanism in the magnetic bias field regime investigated is from collisions which change the magnetic sublevel (quantum number ) and not the hyperfine level (F quantum number).

A formula for estimating spectral widths of narrow or moderately broad isolated shape resonances is established, by simple arguments based on the three-turning point WKB theory. This formula enables one to handle all resonances of the potential at one go, by only using as input real-energy wavefunctions obtained from a standard Hamiltonian-matrix diagonalization. For its derivation, the ratio between the probability amplitude computed near the equilibrium position and that defined asymptotically, both referring to maxima of the (un-normalized) resonance wavefunction, is of major importance. The performance of the formula is demonstrated on the unimolecular dissociation of the ground electronic state of the exotic . Comparison is made with results from a complex-energy propagation and a previous WKB analysis.

The ground state of the molecule is modelled in the unrestricted Dirac-Fock approximation. Molecular orbitals are taken to be linear combinations of kinetically balanced four-component spherical Gaussian spinors. The nuclei are assumed to be spherical distributions of charge, the radial dependences of which are taken to be Gaussian. Uncontracted basis sets with 27s, 27p, 12d and 8f exponents on the Yb centre and 15s and 10p exponents on the F centre yield an equilibrium bond length of , a dissociation energy of and a dipole polarizability of ; the experimental values of these quantities are, respectively, , or , and . The matrix element of the operator , the interaction between an electronic electric dipole moment and the molecular electric field, is estimated to be atomic units within this approximation.

On the basis of the Moiseyev-Hirschfelder generalization of the complex coordinate method, a universal energy-independent complex absorbing potential (CAP), is derived. It is proven that the universal CAP consists of flux and diffusion-type operators. When a smooth exterior scaling is used, the CAP gets non-zero values in the region where the interaction potential vanishes. An illustrative numerical example is given where narrow and broad, isolated and overlapping resonances were all calculated with more than nine digits of accuracy.

The position of atomic states with an outer electron of large angular momentum l are analysed using a Sturmian expansion of the Coulomb Green's function. The method is applied here to low doubly excited states of alkaline-earth atoms, namely in Ca, in Sr and in Ba. The figures turn out to be as accurate as those from R-matrix theory and may be even better, for example, in strontium. A critical discussion of measured positions is given, completing previously uncertain assignments. New positions for large n are tabulated as well as for previously unreported total angular momentum values. Local series interactions can be analysed and are shown to be weak between two series with an outer g electron.

Using an ion-photon merged-beam apparatus, photoion yield spectra of and from the 4d photoionization of were measured as a function of photon energy in the range 110-160 eV. The measured spectra look very similar to those from a neutral Eu target. Both and yield spectra, a broad `giant resonance' appears at 140 eV with some preceding small peaks at 131.5, 132.8 and 134.7 eV. The summit of the `giant resonance' in the spectrum is split into two peaks. This is not observed in neutral Eu photoionization.

L-shell x-ray production cross sections by proton and ion impact on Hg and Tl (of great importance from the biological point of view, owing to their moderate toxicity properties) have been measured in the energy ranges and , respectively. L-subshell ionization cross sections have been extracted and arranged, together with the few existing data from other laboratories, in an analysis aimed at testing the performance of various theoretical approximations in the description of the ionization process. The disagreement observed at low values of the reduced velocity parameter between the data and the predictions of the ECPSSR model is reduced significantly by using a united-atom approximation in the treatment of the binding correction, suggested by our group (ECPSSR-UA). The remaining discrepancies, mostly for the -subshell, are strongly reduced by replacing the screened hydrogen-like wavefunctions with the more realistic Dirac-Hartree-Slater ones. In the case of ions, especially for the -subshell, a more refined Coulomb deflection correction and/or more sophisticated coupled-channels calculations are also needed.

A new channel for recombination of free electrons on multicharged ions with a complex core is under investigation. The channel is connected with a dynamic polarization of an ion's core by the colliding electron which results in radiation of the core and capture of the electron. This new channel (called polarization recombination, PlR) is estimated in the frame of a statistical (Thomas-Fermi) model of the complex ion. It is shown that the contribution of PlR to the total recombination rates may be comparable to or exceed (by more than an order of magnitude) the standard contribution of radiative recombination.

The angular dependence of the UV/VIS and VUV fluorescence was measured to determine the alignment of Xe II and Xe III ionic states populated via the decay of the Xe I autoionization resonance through (multi-step) spectator-Auger or Auger-shake transitions. The anisotropy of the fluorescence radiation and the alignment parameter were also determined for a decay cascade involving electron as well as fluorescence transitions starting from the Xe I resonance and ending in the Xe II ground state .

We use multichannel resonant scattering theory for an extension of the conventional theory of autoionization resonances in photoabsorption. It incorporates autoionization induced by resonance fluorescence. As a result we obtain a modified Fano profile formula. The result shows that the new mechanism has in certain cases a strong influence on the shape of the resonances.

A general theory is developed and numerical results are presented for the angular distribution of molecular Auger electrons emitted after photoabsorption. The angular distribution reflects the dynamical properties of both processes: the photoabsorption and the Auger emission. We will analyse in detail the difference between ionization and excitation by arbitrarily polarized photons. We will show that for resonant Auger transitions the primary photoexcitation can be treated analytically. Based on this result an expression for the asymmetry parameter is developed, which determines the Auger angular distribution. Numerical results for the parameter are presented for several resonant Auger transitions of HF.

The ionization dependences of triatomic molecular ions exposed to ultra-short intense laser fields on the bond extension, the laser frequency, intensity and pulse envelope have been investigated numerically. We find that the ionization of is greatly enhanced when one of the two bonds stretches into a critical bond length range -6 bohr for the laser intensity . Moreover, the enhanced ionization is laser-frequency dependent, namely, the critical bond length shifts toward a small value for the short-wavelength excitation. In addition, the critical bond length is found to be variable with the laser intensity. For low-frequency excitations, the enhanced ionization (EI) effect can be explained by the field-induced over-the-barrier model. However, the EI effect is attributed to the resonance-enhanced multiphoton ionization mechanism for high-frequency (248 nm) excitation.

Exact numerical solutions of the time-dependent Schrödinger equation have been performed in order to calculate ionization rates in intense short laser pulses for linear three-dimensional and linear one-dimensional . All molecular ions exhibit unusually large ionization rates at large critical interfragment distances . Expressions are derived for based on quasistatic models of field-induced suppression of electron Coulomb barriers in the molecules. The predicted agree satisfactorily with the exact numerical results. In the symmetric case, laser-induced electron localization can also enhance ionization. The present numerical results show that large or enhanced ionization rates exceeding those of nonionized fragments will dominate for laser pulses which are sufficiently long to allow dissociative ionization fragments to reach such critical distances.

Absolute differential and total cross sections for single-electron loss and capture were measured from ions on He in the energy range of 1.0-5.0 keV. The reduced differential cross sections ( versus ) are shown to scale reasonably well for each of the two processes studied. For single-electron loss, our cross sections are found to be of the order of magnitude of . An extrapolation of the high-energy results to low energies shows good agreement with the present data. The total cross section for single-electron capture is compared with other available measurements. These results give a general shape of the whole curve of single-electron capture cross sections for the system over a wide range of energy.

We have studied binary-encounter electron ejection from thin solid foils (C, Al, Cu, Au) with highly charged swift heavy ions at both experimentally and theoretically. The theory, based on the electron impact approximation describes the shape and the angular dependence well for very thin targets only (for example C less than 260 Å). For thicker targets, solid state effects (in particular, electron transport) influence the shape of the binary-encounter electron peak initially given by the target atom Compton profile. Such effects have to be taken into account in atomic collision experiments even if single-collision conditions for the primary interaction (e.g. ionization) are fulfilled. Not only may the electronic structure be different for free atoms and atoms bound in solids, but also the observable spectra of, for example, electrons may be different from the primary ones due to transport effects in condensed matter.

A detailed semiclassical study of the orbital angular momentum and energy transfer processes induced by collisions of Rydberg atoms with atomic particles is reported. Basic equations of the unitarized perturbation theory for the probabilities and cross sections of the inelastic transitions are derived using the impact-parameter method combined with the JWKB approximation for the wavefunctions of highly excited states. Analytical formulae are given for the quasi-elastic l-mixing cross sections and rate constants averaged over the Maxwellian velocity distribution. Unlike the well known formulae of the Born and impulse approximations they are valid for both the weak- and close-coupling regions and may be used at high, intermediate and low enough n. The contributions of the short- and long-range interaction between the Rydberg electron and perturbing atom are explicitly expressed as functions of the principal quantum number and the inelasticity parameter of the process. Theory is applied for calculations of quenching processes in thermal collisions of the Rydberg , and atoms with the ground-state atom and for the comparison with experiment. It is demonstrated that the orbital angular momentum mixing processes are appreciably dependent on the energy transferred to the Rydberg atom even at small quantum defect of the initial nl-state. This work points out that the l-mixing collisions with large values of the transition energy defect behave essentially like inelastic n,l-changing processes and cannot be described within the framework of available theoretical models for the orbital angular momentum transfer. The observed effect, of a strong drop of the l-mixing cross sections in the collisions as compared with the quasi-elastic process involving the -state, is quantitatively explained using the developed theory.

A Monte Carlo simulation of radiative transfer in an atomic helium beam is carried out to investigate the effects of radiation trapping on electron-atom collision experiments. By comparing the simulated and measured radiation intensity distributions the pressure at the collision site can be determined, while such information has been hitherto otherwise inaccessible.

Simultaneous excitation-ionization of helium to the ionic state by electron impact is studied experimentally and theoretically. Electron-photon angular correlations measured at an incident electron energy of 200 eV and electron scattering angles between and are in reasonable agreement with theoretical predictions. The double-differential cross section shows structure due to interference effects arising from autoionization states decaying into this channel. The emission cross section from threshold to 300 eV is also reported and compared with previous data.

Cionga et al have employed the Kramers-Henneberger gauge when performing Floquet close-coupling calculations of electron-hydrogen scattering. It is shown that with their basis the effect of the laser on the target electron is described very inaccurately: their model assumes that the electron attached to the proton oscillates freely in the laser field, whereas it is actually tightly bound for the laser intensities and frequencies considered. Such an effect could well explain the apparent breakdown they observed of the Kroll-Watson low-frequency approximation.

The long-range asymptotic behaviour of the interaction potential is of crucial significance for low-energy laser-assisted electron-atom scattering for emission/absorption of one or more photons, especially for the small scattering angles; this is not satisfied by the ansatz of the comment by Rabadán and co-workers, equation (9).