Table of contents

The authors report the observation of very-high-order odd harmonics of Nd:YAG laser radiation in rare gases at an intensity of about 10¹³ W cm^-2. Harmonic light as high as the 33rd harmonic in the XUV range (32.2 nm) is generated in argon. The key point is that the harmonic intensity falls slowly beyond the fifth harmonic as the order increases. Finally, a UV continuum, beginning at 350 nm and extending down towards the short wavelength region is apparent in xenon.

Data for the electron loss from 1 GeV H-, He- and Li-like Ni ions incident on thin solid carbon targets have been obtained. The targets were sufficiently thin to achieve single-collision conditions and hence permit the determination of the ionisation cross sections. Despite the high velocity ( beta approximately 0.2) of the H-like Ni ions the collision regime is such that neither the Bohr theory based on an impact parameter formalism nor the Born approximation, even to second order, are appropriate for treating the ionisation process. The cross sections measured for electron loss were found to be significantly lower than these theoretical estimates. The single-electron ionisation cross sections for the Li-like Ni ions showed a marked increase over those for the H-like or He-like ions consistent with a strong electron binding energy dependence.

An explanation is proposed for a long-standing major discrepancy between few-state close-coupling calculations and experiment for excitation of atoms in collision with 'heavy' targets like Ar, Kr and Xe. The discrepancy is associated with singly inelastic collisions in which the projectile is excited but the target remains in its initial state. The explanation of the difficulty lies with the interaction between the active electron in the projectile and the static field of the target. With increasing target size this interaction becomes very strong, with the result that couplings between all projectile states are important. Hence to restrict these couplings to those between a few eigenstates is a mistake. Calculations using the impulse approximation, which does not restrict the projectile-state space, are shown to be in relatively good agreement with the available experimental data for excitation of H(1s) to H(2s) and H(2p) in collision with Kr and Xe targets. This agreement demonstrates clearly the failure of the few-state close-coupling approximations and justifies the interpretation of this failure.

Charge exchange total cross sections are calculated for the H⁺+H(n=2) collision, in the range of relative velocities from 0.05 to 0.3 au, for all initial and final sublevels 2s, 2p₀ and 2p_+or-1 of the hydrogen atom. As a general trend, the largest contribution comes from the resonance reactions.

Polarised pseudostates representing the long-range polarisation potential are included in a new e-N₂ scattering calculation. The results remove an earlier disagreement in the total cross section between theory and experiment at energies below 1.5 eV and reduce the position and width of the ² Pi _g resonance.

Electron impact excitation of Ca II is investigated in six-state and three-state close-coupling (CC) approximations using accurate target wavefunctions for the ground state and the five lowest excited states. The results demonstrate the unanticipated enhancement of the cross section sigma (4s to 4d) over sigma (4s to 3d) in the energy range 3.3<or=E<or=100 eV, which is attributed to the virtual 'double-dipole' transition through the 4p ²P^o state. Cascade contribution to the resonance cross section is found to be almost consistent with experimental estimates (about 18% at 10 eV, which decreases to roughly 7% at 100 eV). Comparison of the authors' six-state and three-state CC cross sections with some existing theoretical and experimental data is effected.

The role of electron capture by protons from thermally excited impurity ions in modifying the transport of hydrogen in a fusion plasma is discussed. The rates for neutralisation are large, and it has been suggested that this new source of neutral hydrogen could explain anomalous transport. Sadly this is not the case.

Quantum dynamical theory of molecular Fermi resonance (v_a,2v_b) based on a model of two non-linearly coupled oscillators is developed. A time-dependent solution for the population of the vibrational modes in terms of elliptic functions is obtained. It is shown that the period of the intramolecular energy exchange is simply connected with the anharmonic potential constant. To substantiate the author's theoretical results subpicosecond pump and probe techniques and a new method of time-resolved spectroscopy without requiring ultrashort pulses may be used. The period of the energy exchange, the potential constant and the frequency difference of a Fermi doublet may be directly determined from the proposed experiments.

The Hartree-Fock method is applied to find the energy of the ground state of the helium atom enclosed in a spherical box with a finite potential wall. The model is used to describe the increase of the electronic kinetic energy of helium gas at high pressures. For pressures less than 25 kbar a small negative potential wall (<-0.1 Hartree) leads to significantly better agreement with experimental results than the widely used hard-box model. An approximate form of a new boundary perturbation method is applied to the enclosed helium atom and very good agreement with the SCF result is found up to pressures of the order of 1 Mbar.

A test is done by means of a trial density function in order to evaluate the quality of approximate solutions for the Thomas-Fermi model of atoms in a superstrong magnetic field. It is shown that the variational principle recently proposed is also valid for the case of finite atomic radius. Atomic binding energies are calculated and the results are compared with other theoretical results. In addition, the radii of atoms and ions are determined in an iterative way.

By means of saturated absorption spectroscopy in a discharge tube containing hydrogen isotopes, fine-structure intervals in the Balmer- alpha line of tritium have been measured, and for each of four components of that line the tritium-hydrogen and tritium-deuterium isotope shifts have been measured. The results agree with theory within the experimental limits of error, which are typically +or-2 MHz. The profiles of the components of Balmer- alpha have been analysed in detail and in particular a Stark-induced crossover signal has been revealed.

Correlations between photons emitted by an atom in a laser field and near a metal surface are studied. With polarisation-dependent detection it is feasible to select photons which are emitted in a specific transition between degenerate substates. Both the Einstein coefficient for spontaneous decay of a particular excited substate and its branching towards the various ground states depend on the distance between the atom and the surface. A combination of these notions to design a geometry for the correlated detection of polarised photons is employed, in order to predict a strong dependence of the correlation functions on the atom-surface distance. In general, an enhancement of the correlations between emitted photons due to the presence of the metal surface is found if the atom-surface distance is (roughly) less than 20% of the wavelength of the fluorescence radiation. In particular the correlations between circularly polarised photons with the same helicity are modified dramatically, and the correlation time tends to infinity if the atom approaches the surface. It is pointed out how the different photon correlations can be understood from a simple interpretation of transition diagrams.

The grid solution of the Schrodinger equation allows one to obtain highly accurate values of the binding energy E_B of low-lying states of the hydrogen atom in a homogeneous magnetic field B of arbitrary strength. The relative precision of E_B in the grid calculations presented was 10^-3-10^-6 for all the states and fields that appeared in the work by Rosner et al. For some states the precision reaches 10^-7-10^-8. The results of the calculations allows the author to fill all the gaps in the results of the work mentioned above for B>or=4.7*10² T and to improve the precision of a number of results in the intermediate field region. The computational method is described. The form of the results, set of states and field strengths are as in Rosner et al.

Potential curves for the Rydberg states of the LiHe diatomic have been calculated in the ab initio SCF and CEPA approximations (including electron correlation effects) up to the 5s state. The authors present results for potential wells and barriers, quantum defect functions, dissociation energies, vibration levels and oscillator strength functions. The last are used to predict the broadening of some emission and absorption lines of Li atoms under He gas pressure and well as a number of satellite structures in the Li absorption spectrum between 670 and 260 nm.

The transfer-ionisation Auger processes involving two active electrons in slow collisions of multiply charged ions with atoms are considered. The corresponding multichannel coupling problem is approximately reduced to interaction of three quasistationary states. The three-state S matrix is constructed by assuming a weak overlap of the two-state non-adiabatic transition regions and applying a matching procedure. Closed-form expressions for transition probabilities are given for each transfer-ionisation channel for two typical configurations of the initial, one-electron and two-electron capture potential curves.

State-selective capture in C³⁺-H collisions is investigated in the energy range 1-5000 eV amu^-1 using the molecular model, modified to include appropriate translation factors. The effect of translation in the dominant reaction channel (capture to the (2s3s)³S state of C²⁺) is of considerable importance for energies exceeding 50 V amu^-1. Calculated cross sections for this channel are in satisfactory agreement with experiment. An analysis of the experimental data concerning the singlet and the weak triplet channels is also presented. Some of the contradictions between the photon emission spectroscopy and the translational-energy spectroscopy data can be reconciled by the present calculations.

A theoretical investigation of electron capture processes leading to charge transfer in collisions of O⁴⁺ and Si⁴⁺ ions with atomic H is undertaken for the energy range 1-1000 eV amu^-1. An adiabatic molecular basis set, with the inclusion of appropriate translation factors, is used to describe the collision dynamics. It is found that the charge transfer cross sections are critically dependent on the nature of the ionic core at energies less than a few 100 eV amu^-1. A comparison with the experimental results on O⁴⁺-H reveals the authors' interest in the 100 eV amu^-1 energy range for testing the theoretical model and verifying the consistency of the experiments.

A theoretical investigation of charge transfer in collisions of Al³⁺ and Ti⁴⁺ ions with atomic H is carried out for the energy range 1-1000 eV amu^-1. An adiabatic molecular basis set, with the inclusion of appropriate translation factors, is used to describe the collision dynamics. In both the Al³⁺-H and Ti⁴⁺-H systems, the influence of rotational coupling proves to be considerable and depends quite critically on the correlation to the principal electron capture channel of the asymptotic energy splitting of the Sigma and Pi states.