Table of contents

We report for the first time the laser-induced dissociation and ionization spectra of a lithium dimer using a pulsed Nd:YAG laser-pumped dye laser in conjunction with a thermionic diode detector. The observed ionization peaks are identified as the atomic even-parity Rydberg states resulting from the absorption of two photons by the lithium dimer from the state to an excited intermediate molecular state and subsequent ionization. The atomic Rydberg series and have been identified. We have also observed a few members of the ns and nd series resulting from the excitation of the lithium dimer from the X ground state to the excited molecular states dissociating into a lithium atom in the 2s ground state and the other atom in the 2p excited state and subsequent excitation by the second photon from the same laser pulse.

Oscillator strengths have been calculated for the - , - and - transitions in Ca-like iron, cobalt and nickel (Fe VII, Co VIII, Ni IX). The pseudo-relativistic Hartree - Fock (HFR) method of Cowan and Griffin has been used for the calculations which include a considerable amount of valence and core - valence correlation. The inclusion of core - valence correlation leads to substantial changes in the gf values as predicted recently.

We report on radiative lifetime measurements of short-lived states in singly ionized platinum by excitation with pico-second laser pulses and subsequent time-resolved detection. The upper levels were populated from metastable ion levels in a laser-generated plasma using radiation in the UV/VUV spectral range. The following lifetimes were obtained: , and . By using relative intensities of the lines found in the literature, oscillator strengths were determined.

For a well selected case of resonant double photoionization in neon, two specific aspects are studied in detail by exploring the shapes of energy- and angle-resolved patterns of the triple-differential cross section (TDCS). One concerns the evolution of TDCS patterns for equal energies of the emitted electrons when crossing the resonance. Here similarities and differences for direct and indirect double photoionization are demonstrated. The other aspect concerns the on-resonance pattern where the photoelectron has the same kinetic energy as the autoionization electron. In contrast to former studies of resonance-affected double photoionization, it is shown here for the first time that in the whole angular range of opposite electron emission the TDCS is strongly affected by interference effects predicted by Vegh and Macek (1994).

An extended group function model has been applied to determine the interatomic potential for the state of . By using a nuclei-centred, contracted [12s, 9p, 5d,4f,3g/8s,6p,4d,3f] GTF basis plus a set of uncontracted bond-type functions, the following potential minimum parameters are obtained: and mHartree. On the basis of an error analysis it is concluded that the calculated binding energy is in error by no more than 0.4%. The accuracy of the potential is superior to previously determined potentials.

The mobility of ions in helium has been measured at 76.8 K and E/N values up to 15 Td, and at room temperature up to 300 Td. The experimental scatter is of the order of 0.1 - 0.2%. The experimental mobilities are compared with values calculated from a new and accurate ab initio potential (Røeggen et al 1996). The agreement between experimental and theoretical values is significantly better than in earlier studies of this ion - atom system, but some unexplained discrepancies of the order of 1% still exist.

We present a comprehensive theoretical study of the physical processes which govern inelastic transitions in slow collisions of antiprotons with hydrogenic ions. Two-centre - one-electron mechanisms are highlighted. Various channels for ionization and excitation are identified utilizing the theory of hidden crossings. New features of the quasimolecular potentials, caused by the negative charge of the antiproton, are described in detail, with particular attention to the topology of the electronic eigenenergy surface in the plane of complex internuclear distance. The ionization and excitation cross sections are calculated and compared with the results of other theories and recent experiments.

A non-relativistic description of the differential cross section (DCS) for Thomas double-scattering electron capture at asymptotically high velocity is developed within the third-order continuum distorted-wave (CDW) perturbation theory for transitions in proton - hydrogen collisions. It is shown that at the critical proton scattering angles, namely the forward peak, Thomas maximum, small angles and the interference minimum, the CDW series has converged at second order. Moreover, it is proven that the third-order correction makes no contribution to the velocity-dependent and behaviour of the Thomas double-scattering total cross sections at the leading angles. The Oppenheimer - Brinkman - Kramers (OBK) travelling atomic orbital plane wave theory is compared to the CDW approximation. It is observed that the second-order OBK (OBK2) theory has not converged. It remains an open question as to whether fourth-order terms or higher in the OBK approximation contribute to the various differential cross sections. It is concluded that the CDW model gives a superior description of the Thomas double-scattering mechanism than the OBK model.

Single-centred finite Hilbert basis set calculations, using bases with a substantial number of orbitals, have previously been shown to accurately reproduce excitation and electron removal cross sections for the single electron p + H(1s), p + H(n = 2), and systems. The present paper extends the method's range of application to the and systems. These cross sections are relevant to controlled nuclear fusion studies. We consider incident projectile energies ranging from to . Cross sections for , and , 2s and and excitation and for electron removal from the 1s, 2s, and 2p are presented and compared to other theoretical calculations. An interesting feature is exhibited by the and 4p excitation cross sections.

Ejected electrons from the doublet and quartet states of doubly excited ions, produced by 60 keV collisions have been measured with high resolution zero-degree electron spectroscopy. Only the doublet states were observed in collisions with gas. Both doublet and quartet states were observed when was used as the target. Two theoretical methods, perturbation theory or Z-expansion (MZ code) and multi-configurational Hartree - Fock (Cowan code) were used for identification of peaks observed in ejected-electron spectra. Contributions of correlation effects are discussed by comparison with other theoretical energies.

The three-body post-collisional interaction (PCI) between the scattered proton, recoil target ion and emitted electron has been investigated by electron spectrometry near the helium resonances, in the 20 - 100 keV energy range ( = 0.9 - 2 au). Particular attention has been paid to the PCI deformations of the Fano lineshapes when . Their angle and collision velocity dependences have been studied for the first time experimentally. A large variety of lineshapes has been observed, all of them being successfully described by a single formula. At the lowest proton velocities the rescattering effect (also called Coulomb two-path scattering) is seen.

Individual , , , and x-ray production cross sections for the elements Rh, Ag, Cd, Sb and I have been measured by protons in the energy range 1.6 to 5.2 MeV. The experimental results for x-ray production cross sections and cross section ratios are compared with the RPWBA - BC theoretical predictions and with the other available experimental data. The influence of using different databases for atomic parameters in the conversion of theoretical L-shell ionization cross sections to the L-shell x-ray production cross sections is discussed.

Energy-pooling collisions between low-lying excited states of Ba have been studied. We have used a CW dye laser to excite the level in Ba. Radiative decay and collisions with Ba or buffer-gas atoms (Ar, He) populate also the , and metastable levels. The spatial density distributions of these excited levels were measured by the absorption of lines from a Ba hollow-cathode lamp as a function of laser frequency. When the laser was on the exact atomic resonance we observed fluorescence from the , , , , , , and states populated by energy-pooling collisions of Ba(, ). By changing the buffer gas pressure the fluorescence intensity ratio between - and - transitions changes dramatically. This is found to be due to redistribution of population among metastable levels, both in magnitude and space. In addition, we have found that the and metastable levels are also populated when the laser is tuned to the blue from the 553.5 nm line under which conditions a pronounced red-shifted light cone (cone emission) is observed.

Cross sections are reported for backward scattering of electrons by CO for electron energies in the range 2 - 160 meV. At very low energies, scattering is dominated by rotationally inelastic events and a set of rotationally inelastic cross sections is presented, parametrized in terms of an energy-dependent effective dipole moment for CO. A new value for the scattering length of is also estimated.

The Faddeev approach is extended to study s-wave scattering in the resonance region just below the n = 2 threshold as well as between the n = 2 and n = 3 thresholds. Resonance positions and widths are obtained for the first two resonances and for the first resonance. The effect of finite mass of the proton on the resonances is studied for the first time. Cross sections of atomic transitions above the n = 2 threshold are also calculated.

A simple potential is proposed to describe low-energy s-wave scattering of electrons from closed-shell atoms. It has the correct structure at large and small distances from the nucleus and depends on the target polarizability and a dimensionless parameter. This dimensionless parameter is determined by extrapolation from the potential for the bound states in isoelectronic sequences. Apart from providing a satisfactory description of low-energy scattering of electrons from inert-gas atoms, the potential predicts a negative scattering length for electron scattering from Be, Mg, Zn, Cd, Hg, and zeros in the s-wave phase shift at a momentum k in the range 0.05 - 0.10 au.

Cross sections for positron scattering by atomic hydrogen are calculated in the energy range 0 - 110 eV using a 33-state approximation which employs the 1s, 2s and 2p eigenstates of both positronium and hydrogen together with 27 hydrogen pseudostates. Cross sections are given for elastic scattering, H(2s) and H(2p) excitation, Ps(1s), Ps(2s), Ps(2p) and total positronium formation, ionization, and total scattering. The degree of agreement with an earlier 18-state calculation of Kernoghan et al, with sophisticated single-centre approximations, and with the available experimental data, strongly suggests that the main cross sections for positron scattering off ground-state atomic hydrogen are now known to quite a high degree of accuracy. Problems with the total cross section highlighted by Kernoghan et al are now resolved by new experimental data of Stein et al which give excellent agreement with theory for both positron and electron scattering. Suggestions are made for further experimental investigations.

We present a combined experimental and theoretical study of ionization of the ground state of helium by 40 eV incident electrons. Absolute in-plane and out-of-plane measurements are provided for the case of 4 eV slow electron energy. These are found to be in good agreement with the convergent close-coupling (CCC) calculations. Three calculations are presented, in order to demonstrate convergence, which couple a total of 87, 99 and 103 states. These also simultaneously yield results for the equal energy sharing kinematics which are found to be in good agreement with the available relative coplanar symmetric measurements.

In this work, we present a theoretical study of elastic and inelastic electron - collisions in the low and intermediate incident energy range. More specifically, we report differential and integral cross sections for the elastic scattering in the 5 to 80 eV range as well as the excitation cross sections for the transitions leading to the lowest and states in the 10 to 100 eV range. The Born-closure Schwinger variational method was applied for the elastic scatterings whereas the distorted-wave method was used to study the electron impact excitation processes. The calculations were carried out using the fixed-nuclear static-exchange approximation at the equilibrium geometry of the ground-state . The comparison between the calculated results and the available experimental data in the literature is encouraging.

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Sequences of S, P and D resonances in positron - hydrogen scattering were shown to still exist below the n = 2 H excitation threshold in a coupled-state calculation employing a large coupling scheme. The positions of the resonances were found to shift slightly away from the threshold and their widths to broaden slightly, compared with those previously determined with smaller schemes. The energy ratios and width ratios of the successive resonances of a sequence still reasonably satisfy the predicted `scaling law'.

The radiative mechanism for lepton bound-state formation is investigated. In this process, two free leptons interact to form a bound state, with the emission of a photon. The cross sections for corresponding processes are derived, and some results given for the case of positronium formation in a free electron - positron collision.

The contributions of the atomic and field angular momentum in a non-relativistic dipole transition are calculated. For a spontaneous emission process the full quantum description is reconciled with the conventional view of angular momentum transfer of .

Low-Z Ne-like collisionally pumped x-ray lasers exhibit maximum gain at , a density at which the long relaxation time freezes the ionization on the hydrodynamic timescale. We demonstrate that the use of a long-wavelength driving laser (preferably matched to heat at the optimum lasing density) enables the creation of a high-, low-, long-density scale-length plasma within which the ionization is frozen in the Ne-like state. Such a plasma is ideally suited to collisionally pumped lasing. We calculate length-fitted gain coefficients of on the - 1 transition of Fe for prepulsed , (HF) irradiation, a four-fold increase over the gains calculated with a m driver. The calculations indicate that a driver may also be suitable for low-Z systems. We discuss the negative effects associated with long-wavelength driving and conclude that they are likely to be of little relevance, especially for the HF driven system.