Table of contents

A new analytical formula for the bound-state wavefunction of a target electron perturbed by an incident projectile ion is derived within the CDW approach. Unlike the CDW approximation of Cheshire, in addition we take into account a coupling between the electron - target and electron - projectile subsystems to the first order in a small parameter where R is internuclear separation. It is found that the derived wavefunction satisfies the Schrödinger equation up to terms of second order and, as a consequence, gives a better account of the interaction zone. The function can be used to investigate numerically the first-order coupling effects in processes of excitation and charge transfer of an atom by ion impact.

The electron affinity of lanthanum has been measured using laser photoelectron energy spectroscopy. This is the first electron affinity measurement for lanthanum and one of the first measurements of an electron affinity of a rare-earth series element. The electron affinity of lanthanum was measured to be . At least one bound excited state of was also observed in the photoelectron spectra, and the binding energy relative to the ground state of lanthanum was measured as . The present experimental measurements are compared to a recent calculation.

Excitation cross sections have been calculated using a close-coupling approach involving two-centre single and double excited states for H - H collisions in the energy range 1 - 100 keV. The present results are in agreement with one of the two independent sets of measurements for the excitation H(2s) cross sections. For H(2p) cross sections the discrepancies between previous calculations and the only set of existing measurements remain unresolved. The present calculations indicate that the cross sections have a strong dependence on the total electronic spin: at low energies the cross section is dominated by triplet states, whereas singlet states contribute mostly at intermediate energies.

Measurements of the orientation and alignment parameters have been performed on potassium atoms using a superelastic scattering technique. In these experiments a laser beam is tuned to the transition in potassium. Electrons are scattered from these excited atoms and those electrons that gain energy in the collision are detected. These superelastic electrons carry information about the orientation and alignment parameters which describe the scattering process. Orientation and alignment parameters have been measured at an electron energy of 10 eV referred to the ground state and over an angular range from to . These parameters are in excellent agreement with those predicted using the convergent close-coupling theory.

Recent theoretical results by Das and Dhar (1998 J. Phys. B: At. Mol. Opt. Phys. 31 2355) suggest a significant role of three-body effects in relativistic collisions. We point out a number of problematic assumptions in this work, and show that the measurements discussed in this work can be described quantitatively without reference to such effects.

The lowest D-wave autodissociating resonance in positronium - hydrogen scattering is calculated using the method of complex-coordinate rotation. The resonance energy and width are determined by employing extensive Hylleraas-type wavefunctions with up to 3437 terms. Comparisons are made with other results in the literature.

The efficiency of the conversion from laser energy into aluminium K-shell line energy has been measured for different interaction regimes of a sub-picosecond laser pulse with controlled preformed plasmas. We compare the cases of thermal plasmas for which transient ionization effects are dominant to non-thermal plasmas in which both transient and non-Maxwellian effects are present. In a first experiment, a low-intensity green laser pulse (400 fs, ) is normally incident on a preformed plasma previously generated by a similar laser pulse. In a second experiment a high-intensity p-polarized green pulse (400 fs, ) is sent with a large incidence angle on the preformed plasma. We have observed in these experiments that the conversion efficiency (He- or Li-like lines from aluminium) scales as , where is the duration of the x-ray emission measured at the full width half maximum, and that this scaling was not affected by non-Maxwellian effects.

We solve the Gross-Pitaevskii equation to model the behaviour of a weakly-interacting Bose condensate. Solutions are presented for the eigenstates in one- and two-dimensional harmonic traps. We include the effect of gravity and coupling to a second condensate to simulate an output coupler for Bose condensed atoms, and find that the output pulse shape is in qualitative agreement with experiment. We also model flow under gravity through a constriction, demonstrating excitation and separation of eigenmodes in the condensate.

When a monochromatic laser resonantly excites a multi-level atom the optical Bloch equations for the reduced atomic density matrix, which describe the evolution of such a system, may exhibit generalized damping terms. We justify the origin of these damping terms within the radiative cascade description of resonance fluorescence in the dressed-atom picture and in the spin-orbit coupling picture. We describe the connection between generalized damping terms and quantum interference in the process of spontaneous emission.

It is shown that the application of the high-frequency Floquet theory to an atom interacting with a circularly polarized field is equivalent to a treatment of the free atom with special boundary conditions. With these boundary conditions, it is possible to define other equivalent formulations. Some consequences of this flexibility are illustrated for the hydrogen atom.

The energies and wavefunctions of the , and states of beryllium are calculated with a model-potential method by using selected B-spline basis functions in a configuration-interaction (CI) scheme. Our length results of the oscillator strengths for electric dipole transitions and the energies, which are set to be zero at the ground state of Be III, are compared with the experimental and other theoretical results. The agreement showed that our method is capable of giving accurate results for various states of beryllium efficiently.

We have obtained some exact solutions for a solvable model for the hydrogen molecular ion where the Coulomb interaction between the electron and the nuclei is replaced by a harmonic potential. We have also obtained the solutions to this model within the ordinary Born-Oppenheimer (BO) approximation for the electronic and nuclear degrees of freedom. The model shows some of the qualitative features of the real system and allows for a detailed study of non-adiabatic effects on the electronic structure through the calculation of the first-order corrections to the BO approximation, resulting from the nuclear kinetic energy and the vibronic coupling. The results are of interest in the context of non-adiabatic treatment of molecules and for the description of molecules in external magnetic fields.

Excitation rate coefficients for transitions among the energetically lowest 20 fine-structure levels belonging to the , , 3s3p, 3s3d, 3s4s and 3s4p configurations of Al II are presented, over a wide electron temperature range of 5000 to 100 000 K. In order to calculate the collision strengths, the configuration interaction wavefunctions have been used in the latest RMATRX1 program of Berrington K A, Eissner W B and Norrington P H 1995 Comput. Phys. Commun. 92 290. All partial waves with have been included, and a contribution of L > 20 has been added to ensure the convergence of collision strengths for all transitions.

Three-step resonance ionization mass spectroscopy has been performed to observe the even-parity Rydberg series converging to four states of (, , and ). Using low-angular-momentum states as the second-step intermediate levels, simple and unperturbed series structures have been obtained. From an analysis of these structures, the first ionization potential of atomic Gd has been determined with improved accuracy as . In addition, on the basis of the isotope shifts of the Rydberg states the isotope shift (-) of the ionization potential of has been found.

Measurements of L-shell x-rays and Rutherford backscattering in some rare-earth elements produced by 2 MeV ions were used to study the dependence of individual L-subshell ionization cross sections on atomic number. The results were compared with the theoretical predictions of the ECPSSR theory. The discrepancies observed were reduced but not completely overcome by the inclusion of modifications to the theory, known as ECPSSR-IS and ECPSSR-IS-UA, which favour vacancy transfer from the -subshell to the - and -subshells. The theoretical models used for the comparison with experiment could successfully predict the functional dependence of -subshell ionization cross section on atomic number, but not the functional parameters.

We examine the ionization of highly excited hydrogen atoms by a linearly polarized microwave field quantum mechanically and classically. We investigate the role of classical phase-space resonances on the process of ionization and quantum mechanical ionization rates.

Very intense hydrogen Lyman- light emission is observed from neon gas near atmospheric pressure containing small admixtures (per mil) of hydrogen when this gas mixture is excited by ionizing particle beams. A DC beam of 15 keV electrons or a pulsed beam of 100 MeV ions were used in different experiments for excitation. A collisional energy transfer rate constant from neon to of has been measured using time-resolved optical spectroscopy on the Lyman- line. Conversion efficiencies of particle beam power into Lyman- light of the order of 10% have been observed. No other significant radiation was emitted in the entire VUV, UV and visible spectral region. In particular, no other hydrogen lines are observed under these conditions. The selective excitation of the H(2p) level is interpreted as arising from a resonant energy transfer between excimers and hydrogen molecules.

We have carried out a theoretical study for the lowest doubly excited Feshbach and resonance states of below the threshold under the influence of external parallel electric and magnetic fields. The method of complex-coordinate rotation is used. At first, when only the magnetic field is turned on, both the and states are shifted upward by the diamagnetic term, with the latter state being shifted faster than the former, resulting in the accidental degeneracy for these two states at a magnetic-field strength of about . With such a magnetic-field strength being fixed, and with the external electric field then turned on, the linear Stark effect for the and states is observed. Field effects on the resonance positions and widths are discussed.

Using a 110 fs, 800 nm Ti:sapphire laser, we have collected angle-resolved, high-resolution (<25 meV) photoelectron kinetic energy spectra of xenon for multiple intensities in the range -. For various ATI peaks, we present angular distributions, which show complex and varied structures not previously reported or predicted. For each intensity, abrupt variations in the structures of the angular distributions coincide energetically with the onset of the plateau region in the photoelectron spectra.

The cross sections for direct and total dissociative and non-dissociative ionization of the nitrogen molecule by positron impact have been measured for projectile energies from threshold to 2000 eV. The results are compared with corresponding data for impact of electrons, protons and antiprotons. The comprehensive model which during recent years has been developed to explain the behaviour of the single- and multiple-ionization cross sections of atoms by charged particle impact was found to also apply for this case of a molecular target. In particular, the interference and factorization models which have been found to explain important features of the cross sections for double ionization of atoms apply also to the dissociative ionization of nitrogen molecules.

We have calculated the angular distribution of photons emitted after dielectronic recombination of highly charged uranium ions and obtained differential cross sections for photon emission in resonant transfer and excitation (RTE) in collisions of with a graphite target using the impulse approximation. The results are compared with experimental data.

A method to evaluate integral cross sections (ICSs) from experimental differential cross sections has been developed. Expressions to calculate the contribution to the ICSs from near-zero scattering angles have also been derived for both the dipole-allowed and dipole-forbidden transitions. For the Ar - transitions our method gives a better agreement with the experimental data while for the He - transition our calculated ICSs are in very good agreement with other theoretical curves over a wide range of impact energies.

A crossed-beam experiment has been performed to measure differential elastic and vibrationally inelastic cross sections for electron scattering from ethane in the energy range E = 0.3-10 eV. In the present paper, the results for elastic scattering are reported. The energy and angular dependence of the cross sections is determined. Absolute values of the differential cross sections (DCS) are obtained by using -He scattering as a reference system. By use of phase-shift analysis (PSA) and modified effective-range theory (MERT), the measured DCS are extrapolated into the experimentally inaccessible regions. Integral and momentum transfer cross sections are determined which can be compared with the results of electron transmission experiments and swarm experiments. The most important features in the elastic scattering cross section of ethane are a Ramsauer-Townsend minimum below 1 eV and a broad resonance structure at about 7.5 eV.

Spin-resolved triple differential cross sections have been measured for ionization of xenon atoms by polarized electrons for energies between 40 and . Significant spin up-down asymmetries in the cross sections have been obtained at all energies. These are dominated by the interplay of orbital orientation, fine-structure interaction and exchange of the colliding electrons (`fine-structure effect'), whereas the influence of relativistic orbitals and of the continuum spin-orbit interaction is less important. The results are compared with recent DWBA calculations that include exchange with the electrons of the residual ion (exchange distortion), and satisfactory agreement is found. In particular, the discrepancies between theory and experiment at and symmetric energy sharing are resolved.

The impulse approximation is considered as a point of departure for a consistent derivation of the weak-field soft-photon approximation as well as for a generalization of the Kroll-Watson theory for electron-atom scattering in arbitrarily polarized laser light. General validity conditions and corresponding order parameters are identified with careful distinction between allowed and classically forbidden regions. As a simple test of the theories we consider electron scattering on the zero-range potential in a circularly polarized laser field in which case exact results are available. It is found that the impulse approximation generally represents the exact results very accurately while the peaked impulse approximation, in accord with the work of Kroll and Watson, is restricted to the classically allowed regime. The weak-field soft-photon approximation compares favourably with the exact result in allowed as well as classically forbidden regions in the case of one-photon exchange but becomes less accurate for multiphoton processes as the strength of the vector potential is increased.