Table of contents

The top level of a three-level atom is populated by resonant two-photon optical pumping from the bottom level by a field intense enough to produce dynamic Stark splitting. The frequencies and intensities are calculated for the one-photon cascade emission to the ground state via a nearly resonant intermediate level. Two strong (shifted) doublets near the atomic frequencies, and a weaker triplet centred at the laser frequency are predicted.

A two-state model for non-adiabatic transitions with Gaussian-type interactions is considered. The model describes the transitions in a diatomic collision system near the turning point, the excitation of a two-state atom by a Gaussian laser pulse, and some other physical situations. Several limiting cases of the model are discussed.

As a preliminary part of an investigation of the proton/antiproton interactions with matter, semiclassical impact parameter cross sections for direct excitation and ionisation of atomic hydrogen by proton impact have been computed in a wide energy range from 30 keV to 5 MeV. The coupled-state calculations used a 51-state two-centre atomic orbital expansion. The computed cross sections are in excellent agreement with available experimental data. Calculations of energy loss for protons moving in a gas of atomic hydrogen are also reported.

Optical studies of the total Li I 2²S-2²P light polarisation in Li⁺-He collisions seem to indicate that near 1 keV the dominant asymptotic quasimolecular channel is a Sigma state. Differential studies, however, show primarily Pi population. Using a simple model it is pointed out how a consistent picture may be obtained by taking trajectory bending into account.

Very accurate results for the elastic P-wave phaseshifts below the positronium formation threshold in positron-hydrogen atom collisions are obtained using only four terms in a discrete basis set expansion in the Schwinger variational principle. Stationary results are also presented for higher partial waves up to l=6 for the scattering of electrons and positrons from the static potential in the incident momentum range 0.1-1.0 au.

Coupled integral equations for a relativistic two-fermion system of masses m and M are derived variationally within the Hamiltonian formalism of quantum electrodynamics, using an improved ansatz that is sensitive to all terms in the Hamiltonian. The equations are solved approximately to determine the eigenvalues and eigenfunctions, at arbitrary coupling, for various states of the two-particle system.

The authors study the organisation of the periodic orbits of a hydrogen atom in a uniform magnetic field. The orbits can be mapped one-to-one onto a ternary symbolic tree. The results indicate that the field-free hydrogen atom has a purely chaotic limit for positive energies.

The authors have developed an L² analogue of R-matrix theory to calculate the bound and scattering states of helium using large Sturmian basis sets. The first application is to the lowest several doubly excited ¹P resonances in the synchrotron photoionisation of the helium ground state. To facilitate comparison with experiment, the resonance shape parameters introduced by Fano were redefined in terms of Siegert boundary conditions to allow their ab initio calculation without fitting. Excellent agreement with recent highly precise experiments is obtained.

Photodetachment cross sections for Na^-, Ag^-, I^- and Si^- negative ions, calculated using the random phase approximation with exchange are presented. The cross sections for outer and nearby inner subshells are strongly influenced by intrashell and intershell electron correlations. It is found that in some particular cases correlations radically change the photodetachment cross sections.

The authors advance and examine a new mechanism which strongly influences the photoionisation processes above the 4d threshold in Xe and Ba. It is shown that the elastic and inelastic scattering of the inner-shell photoelectron by the outer-shell electrons determines the rate of single and double ionisation, decomposing the cross section of the inner-shell photoabsorption into these two channels. The results of these calculations are in good agreement with experimental data.

Relative ion-yield spectra for Rbⁿ⁺ (n=1-4) and Srⁿ⁺ (n=1-4) produced by photoionisation have been determined using a combination of a time-of-flight mass spectrometry and monochromatised synchrotron radiation crossed with an atomic beam. The measurement was made in the energy region from the vicinity of the 3d ionisation threshold to the higher-energy tail of the 3d- in f shape resonance: 110 to 280 eV for Rb and 130 to 280 eV for Sr. The main products in this energy region are triply charged ions for both Rb and Sr. A strong 3d shape resonance peak is observed in the yield curves of Rb²⁺, Rb³⁺ and Sr³⁺. The yield curve of Rb⁴⁺ has its onset around the lowest photoelectron shake-off (4d^-15s^-1) state. Although the multiply charged ions are produced mainly by stepwise autoionisation from the 3d hole states, 3d ionisation with simultaneous ionisation of another electron (i.e. the photoelectron shake-off process) is found to have considerable probability. The photoelectron shake-off probability for Rb is estimated from the present data to be about 0.2 in the higher-energy region of this experiment.

Conventional light-induced drift is based on purely elastic velocity-changing collisions. The authors discuss qualitatively the potentialities of light-induced drift originating from inelastic velocity-changing collisions, which may occur in a molecular buffer gas. In such collisions a large fraction of the electronic excitation energy of the absorbing particle may be transformed into kinetic energy thus affecting light-induced drift. The authors report experiments on light-induced drift of Na in various molecular buffer gases and compare these with their predictions.

The authors calculated the spectrum of highly excited H₂ molecules in a laboratory strength magnetic field (B=4.7 T) including both the linear and the quadratic Zeeman interactions in the range E=-300 to -20 cm^-1. The lowest major feature of the field-free photoabsorption spectrum of ground-state H₂ results from the interaction between two Rydberg series corresponding to the H₂⁺ core in its N=0 and N=2 rotational states (these are usually termed the np₀ and np₂ series respectively). In the presence of the field, the character of the spectrum differs dramatically depending on the degree of interaction between neighbouring N=0 and N=2 lines. The quadratic Zeeman interaction is also known to be associated with chaotic behaviour in the hydrogen atom.

The cross sections for simultaneous excitation and ionisation in helium following impact of fast electrons, protons and C^q+ (q=3-6) ions have been investigated by measuring the vacuum-ultraviolet radiation from transitions in HE II (np-1s) in the wavelength region 220-315 AA. The cross sections obtained with 500-2000 eV electrons are found to be larger by a factor of 1.4-2.3 than those obtained with protons of the same velocity (0.92-3.76 MeV). The difference is discussed in terms of an interference between two excitation-ionisation mechanisms.

The relative importance of electron detachment and excitation processes in 0.5-15 keV F^--He,Ne collisions has been studied by electron and optical spectrometry. The dominant excitation process populates the F (¹D)nl states. For both collision systems, the population of doubly excited F^- states is rather weak. By means of an electron-photon coincidence technique, low-energy electrons recently reported to be emitted from doubly excited F^- states have been shown to originate from the decay of the F^-(¹D)3s²¹D level to the F (³P)3p ²P and ²D states. The branching ratio for the depopulation of this F^- level has been determined experimentally and compared with theoretical calculations.

Collision-induced dissociative excitation processes involving N₂⁺ and CO⁺ ions and HgBr₂ molecules have been studied in the kinetic energy range 1-1000 eV (laboratory frame). Emission spectra from the B ² Sigma -X ² Sigma ) transition of the HgBr radical and atomic mercury lines from various highly excited energy levels have been observed. Emission cross sections of the most intense band (0-22) of the HgBr(B ² Sigma -X ² Sigma ) transition at 502 nm have been measured at different kinetic energies of the projectile ions.

The Coulomb-Born approximation has been formulated for the capture of an electron, initially bound to a nucleus of nuclear charge Z_T, by a bare nucleus of charge Z_p in the high-energy region. The earlier Coulomb-Born formulations for electron capture in asymmetric collisions are derivable from the present formulation. The simple replacement of the outgoing and incoming scattering wavefunctions in the Coulomb-Born approximation by their asymptotic forms is shown to reproduce the recently revived first-order Born approximation for charge exchange with Coulomb boundary conditions. A semiclassical version of the Coulomb-Born approximation is also presented in which its relation to the distorted-wave solution of the two-state coupled equations is established. The Coulomb-Born approximation is illustrated by its application to the computation of total cross sections for charge exchange in H⁺+Ne collisions. An independent-particle model is employed with a one-electron orbital for the target.

The energy and angular distributions of pairs of fragment N⁺ ions produced in H⁺-N₂ collisions have been investigated within the energy range 5-25 keV. Six dissociation channels have been observed and the precursor electronic states of N₂²⁺ tentatively identified. Total cross sections for the formation of three of these states have been separately determined. Angular distributions of the fragment ion pairs exhibit strong energy-dependent non-dipolar anisotropies, characteristic of momentum transfer and short predissociation lifetimes.

Cross sections for one-electron capture by Al²⁺ ions in both atomic and molecular hydrogen have been measured and calculated at energies within the range 10-2000 eV amu^-1. Measurements are based on the use of a tungsten tube furnace to provide a target of highly dissociated hydrogen. Translational energy spectroscopy used with a specially developed recoil source of metallic ions is used to show that ground state Al⁺ ion production is predominant. The calculations, which are based on a model potential description of the orbitals, predict cross sections which are somewhat larger than the measured values.

The factorisation approximation in electron-atom scattering usually consists of eliminating the exchange contribution to the scattering by writing the scattering cross section as the product of the square of a direct scattering target form factor times the Mott cross section for electron-electron scattering. It is shown that a factorisation approximation is only possible for scattering angles less than 30' and incident energies approximately 10 times the binding energies of the electron involved in the excitation process if accuracies of better than 5% are desired. In a correct factorised result the equivalent term to the Mott cross section must be replaced by one that does not contain a term for pure exchange scattering.

The ability of the current effective core potentials of quantum chemistry to account for the scattering properties of molecules is studied in the test cases of e-HCl and e-HBr elastic scattering. Unexpected problems show up when using most recently published potentials. A single effective potential is able to compete with the all-electron methods in terms of accuracy. The associated reduction of computer time makes it highly desirable to overcome the few difficulties outlined in the present work.

The interaction mechanism between low-energy electrons and atoms is examined in detail. Electron interaction within a few meV of the energy of an excited state is studied. High-resolution data showing total ionisation cross sections of low-energy electrons with Xe atoms are reported. Interactions appear as small perturbations superimposed on the ionisation continuum. The observed perturbations are compared with the theoretical predictions for negative-ion resonances, excitation of neutral states and direct ionisation. These interactions are reported in the energy range between the states ²P_(3/2) and ²P_(1/2) of Xe⁺ and their spectroscopic configurations are given. These data do not show any obvious post-collision interaction shifts that are reported in other channels.

Electron impact excitation has been used to study high-lying states of mercury that exist in the energy range 8.4 to 12.2 eV. Excitation of several 5d¹⁰6snl and 5d⁹6s²(²D₅2,3/2/)6p states has been observed. The decay of negative-ion resonances to the states lying below the Hg⁺(²S₁2/) ionisation potential has been studied. More information regarding the recently discovered higher-lying metastable state has been obtained and its lifetime has been measured to be 380+or-90 mu s. Above the Hg⁺(²S₁2/) ionisation potential the effects of post-collision interaction in the decay of 5d⁹6s²(²D₃2/)6p autoionising states have been studied.

Absolute elastic differential cross sections for electron collisions with silane (SiH₄) have been determined for incident electron energies of 1.8, 2.15, 2.65, 3, 4, 5, 7.5, 10, 15, 20, 40 and 100 eV over a scattering angular range of 20-130'. The measured angular distributions have been found to correspond quite well with the results of approximate theories employing either a multiple scattering formalism or an optical potential. Angular and energy distributions as well as vibrational excitation functions obtained with electrons of about 1.5-7 eV reveal the existence of a Si-H sigma * shape resonance with a maximum at around 1.8-2.2 eV in the t₂ component of the scattered electron state. As predicted by a selection rule the same resonance is found in vibrational excitation functions obtained with electrons in this energy range. A multiple scattering calculation has been used to explain details of the resonance structure.

Conservation of ortho-positronium in oxygen has been investigated by the high-resolution 2 gamma angular correlation method. The oxygen pressure was low enough to suppress possible conversion of para-positronium to ortho-positronium. Silica aerogel was used to form a sufficient number of positronium atoms in a restricted region necessary for the high-resolution angular correlation measurements. The existence of an inelastic conversion process has been reconfirmed. The cross sections for the elastic and inelastic conversion processes have been estimated to be (1.0+or-0.3)*10^-19 cm² and some 10^-17 cm², respectively.

A new technique for synchronising short and long laser pulses is presented. A Q-switched oscillator which employs mode selection etalons was pumped below its threshold for oscillation and used as a regenerative amplifier. In this oscillator, a 3-5 ns pulse was produced by injecting a small fraction of a 30 ps mode-locked pulse into the regenerative amplifier. To produce the long pulses, a double resonant reflector plus extra 60% reflectivity mirror output coupler was found to be most suitable to obtain single-mode operation of the regenerative amplifier. Time jitter between the short (injected) and the long pulse was negligible for a wide range of injected and pumping energies.

The interaction of linearly polarised ultrashort laser pulses with optically active devices such as half- or multi-order wave plates is considered. It is shown that, for pulses having coherence lengths comparable to their centre wavelength, such devices do not act in the usual manner and a method of measuring the duration of the pulses is proposed. It is pointed out that radiation of sufficiently broad bandwidth is capable of exhibiting a polarisation state not possible for quasi-monochromatic radiation.

The physical basis of electron-ion recombination in tenuous plasmas as a result of electron collisions is reviewed. The theory is developed from the optically thin collisional-radiative model by a continuous representation of the discrete state. It is shown that for a collision-dominated cascade this leads to a Fokker-Planck equation for the distribution among the excited states, whose analytic solution is derived. In the radiation-dominated limit an equivalent equation is derived, and the solution obtained. The physical basis of these results is explored. The results are in good agreement with direct numerical evaluation of the collisional-radiative equations, where the effect of collision-rate approximations is explored. It is found that the conventional criterion for the local thermodynamic equilibrium limit in such systems may need modification.

Precise electron density (n_e approximately=10¹⁸ cm^-3) and temperature (T approximately=1 eV) profiles have been obtained in high-pressure (p approximately=10 bar) pulsed arcs produced in noble-gas filled flashlamps. These quasi-stationary (1 ms) dense plasmas show a flat profile over 2/3 of their diameter and are highly ionised ( alpha >60%). Such experimental situations allow reliable measurements of the electrical (DC) conductivity and comparison with theoretical works. The experimental values are significantly lower than Spitzer's (1962). Recalling the main features of the line profiles, which lead to the conclusion that the plasma becomes less collisional, the authors propose that scattering by oscillating microfields originating in plasma waves may explain the high electron-ion collision frequency (v)_ei.