Table of contents

Positions and widths of the '2p⁵3s²' excited terms for Na and Mg are calculated using the close-coupling method, i.e. by analysing the resonant behaviour of the scattering element from the LS coupling Na⁺+e^-(Mg²⁺+e^-) collision. These terms are described in a multiconfigurational scheme.

The angular distribution of photoelectrons with defined spin polarisation ejected by polarised or unpolarised light from oriented diatomic or linear polyatomic molecules is derived in the dipole approximation. An analysis of its geometrical part, which is separated from the dynamical one, permits the author to draw definite conclusions without calculation of matrix elements. Connections with particular cases already studied are discussed.

The authors report vibrational branching ratios for resonant photoionisation of N₂ leading to the X² Sigma _g⁺ state of N₂⁺. Their theoretical values are obtained from an accurate solution of the adiabatic-nuclei frozen-core Hartree-Fock model of molecular photoionisation. In contrast to other theoretical results the present results are in very good agreement with experimental measurements. Differences between the present and previous calculations are discussed.

The differential cross sections of particle scattering accompanied by n-photon exchanges are derived and compared, through an amplification factor, to those calculated when a single-mode laser assists the collision. A strong averaging is found, even in the two-mode case, with an increase (a decrease) of the probabilities of the processes less (more) favoured in the single-mode case, depending on the parameters of the scattering process. As in this problem the laser field is treated exactly, the information obtained sheds light on the known discrepancy between theory and experiment in multiphoton ionisation of atoms by a multimode laser in that it is shown that perturbative approaches are inadequate to treat problems where many photons are exchanged.

The influence of both the finite band width of the laser pump and the effect of the frequency shift of the molecular vibration in the electronic transition on the intensity correlation function gamma ^2,2( tau ) of the resonance fluorescence light from a single two-atom molecule are studied. In the case of small vibrational damping a double-exponential asymptotic behaviour of gamma ^2,2( tau ) is found, which is independent of the bandwidth of the exciting laser light.

An extended version of the so-called two-particle-hole Tamm-Dancoff approximation (2ph-TDA) scheme for the one-particle Green's function is presented. Symmetry reduced 2ph-TDA equations for applications to atoms are derived. With the aid of this method theoretical ionisation spectra for the two outermost shells of the alkaline-earth elements Be, Mg and Ca are calculated. The results are compared with experimental data and with other theoretical results. The energies of the outer-shell main ionic states are very accurately reproduced by the extended 2ph-TDA calculations. Pronounced final (ionic) correlation effects are found for the Ca M-shell ionisation. In particular the Ca 3s ionisation spectrum shows strong mixing of the 3s hole with 3p⁴3d4s² configurations.

For highly excited states of atomic systems, and for electron-atom collisions at energies below the threshold for collisional ionisation, one can define a value r₀ of r such that, to a good approximation, there is only one electron in the region r>r₀. This is referred to as the asymptotic region. The wavefunction for an electron in the asymptotic region is a solution of a system of coupled differential equations with multipole potentials. If the dipole length operator is used to calculate matrix elements for radiative transitions, the contributions from the asymptotic region are slowly convergent or, for free-free transitions, only conditionally convergent. It is shown, as a generalisation of earlier work by Peach (1965), that the dipole length integrals in the asymptotic region can be expressed in terms of dipole acceleration integrals, which are more rapidly convergent, together with surface terms evaluated at r=r₀.

Auger rates and radiative transition probabilities are calculated for highly excited single K-vacancy terms of boron II. The analysis of the resonant behaviour of the S scattering matrix of the boron III+e^- theoretical collision gives both the width and the energy of each excited term. Energies, widths, radiative transition probabilities and fluorescence yields are shown.

The generator coordinate method is applied to the calculations of ground and low-lying energy levels of H and He in an electric field. A modified version of the generator coordinate method for metastable states is formulated. Polarisabilities and dipole moments of ground and low-lying excited states of H and He are calculated.

Absorption and emission profiles of the 4²S-5²P transition (4044/47 AA) in the K-Ne couple have been measured by a laser-induced fluorescence technique and compared with predictions based on a new model potential and quasi-static calculations. A blue satellite has been observed very close (<or=1 AA) to the position predicted by the potential. The observed intensities of the blue and red wings are about one-half and twice the calculated ones, respectively.

The broadening of the Cs(6S-7P) resonance lines by hydrogen molecules is investigated by a high-resolution laser technique. In the range of hydrogen pressures (20-200 Torr), the line width is determined by comparing the recorded profile of sensitised fluorescence with calculated profiles corresponding to various Lorentzian widths. The broadening constant is gamma /N=(33+or-3)*10^-20 cm^-1/cm^-3.

In real atoms, levels connected by a dipole transition contain a number of degenerate Zeeman states. Thus in practice, the phenomena of optical pumping and collisional transfer amongst states severely restrict the application of a simple two-state model. There are a number of cases, however, where the macroscopic dipole induced in an atomic gas has the same general form as predicted by the two-state model, and these are presented here. The effects of optical pumping and collisional transfer, and the implications for the description of optical bistability, are discussed. In particular, ground-state optical pumping can, under certain conditions of low perturber density, considerably reduce the power necessary to achieve bistability.

A theoretical and experimental study of resonantly enhanced two-photon absorption in ²³⁸U vapour excited by two continuous wave dye lasers is presented. Good quantitative agreement between theory and experiment is found. In particular the central prediction of the theory, that antiparallel laser beams of modest intensity can pump an appreciable fraction of the Maxwell velocity distribution, has been checked directly by measuring the spectral width of the fluorescence from the two-photon excited level.

Differential cross sections for Compton scattering of 145 keV gamma rays from K-shell electrons of tin and molybdenum at scattering angles in the range 30 to 150 degrees . Measurements are made employing NaI(Tl) detectors and a coincidence set up with resolving time approximately=30 ns. The experimental results are compared with the available theoretical data. The total cross section is estimated to be about 0.43 sigma _F for tin and 0.41 sigma _F for molybdenum.

Vibrationally resolved photoelectron angular distributions have been calculated for the 5 sigma photoionisation channel of CO using the multiple-scattering method. Vibrational branching ratios and vibrationally unresolved integrated cross sections and photoelectron angular distributions are also reported and compared with available measurements. Both angular distributions and branching ratios exhibit striking non-Franck-Condon behaviour caused primarily by the f-wave shape resonance in the sigma photoionisation continuum. Significant discrepancies between theory and experiment exist for the weaker v_f=2,3 vibrational levels and interaction with nearby two-electron excitation is proposed as a likely cause.

Efficient laser photodecomposition of dimers is obtained for the wavelength 758 nm. Photodissociation of dimers dominates the process for a low laser intensity whereas three-photon ionisation of dimers prevails for laser intensities higher than 0.4 MW cm^-2. The physical processes involved in the photodecomposition are explained tentatively. Application is made to the preparation of a Cs atomic beam where the laser photodecomposition technique combined with the usual superheating system permits the authors to reduce the residual Cs₂ components by more than three orders of magnitude.

For classical trajectory Monte-Carlo calculations the authors propose the use of the Wigner function for the initial distribution of the coordinates and momenta of the electron instead of the microcanonical phase space distribution. For physical reasons they restrict the Wigner function in energy space. The allowed energy interval is determined by a cut-off parameter E_max which is fitted to the experimental data. The calculated total cross sections for ionisation and charge transfer in H⁺-H collisions reproduce the experimental data in the range of incident proton energies E_lab=25-200 keV. In addition they present the probabilities of ionisation and charge transfer as functions of the impact parameter and angular distributions of the ionised electron.

The energy dependence of the cross section for simultaneous excitation of both collision partners to the 3³D state in He-He collisions has been measured at incident energies between 0.9 and 3.5 keV. At an incident energy of 1750 eV this cross section reaches a maximum of about 5.5% of the cross section for excitation of one atom to the 3³D state, whatever the final state of the other atom. Further, at 1500 eV ratios have been measured of the cross sections for simultaneous excitation of one atom to the 3³D state and the other to different n>or=3 triplet states. From these measurements it can be concluded that the 3³D state is only for about 7% simultaneously excited with n>or=3 triplet states, which means that in simultaneous excitation with one atom in the 3³D state there is a more than 90% probability to find the other atom in the 2³S or 2³P state.

Collisional ionisation of the rubidium 9S state with other rubidium atoms is presented for the first time. The formation of atomic and molecular ions is analysed using a mass spectrometer. The associative ionisation cross section deduced from the experimental results (7.4*10^-15 cm²) is in good agreement with the calculations of Janev. Some information on the possible mechanisms of production of the atomic ion Rb⁺ is also reported.

Symmetric resonant charge transfer in atomic collisions in the presence of a laser field is investigated within the impact parameter method and the atomic two-state approximation. The laser field is treated classically, in the dipole approximation, with the electron-field interaction taken in the gauge of the electric field. For the formal structure, the derived total cross section shows a considerable departure from the field-free result. In addition to the usual two-body cross section, cross sections averaged over the scattering plane orientations and/or over the last field initial phase are also derived. Within the adopted model the problem is solved exactly. The derived cross sections are analysed for different geometries and in the weak-field limit. Numerical evaluation of the exact total cross section is performed for the cases H⁺+H to H+H⁺ and Rb⁺+Rb to Rb+Rb⁺.

Nineteen atomic basis functions are included in the close-coupling calculation for electron capture from ground-state atomic hydrogen by an alpha particle. The energy range covered is from 6.25 to 400 keV of the alpha particle impact. The calculated total capture cross sections are in fairly good agreement with recent experimental data.

Cross sections for single electron capture by Li³⁺ and Li²⁺ ions, sigma _3,2 and sigma _2,1, in collision in H₂ molecules and He atoms have been measured in the energy range 8-38 keV. The cross sections obtained are consistent with published experimental results at higher energies.

A method of implementing the Schwinger variational expression in electron-atom and electron-molecule scattering calculations with polarisation included via an ab initio optical potential is described. The Schwinger expression is also used to obtain approximate molecular Hartree-Fock continuum orbitals at arbitrary energies which are needed to calculate the optical potential above the first electron excitation threshold. Explicit closed-form expressions of integrals required for a Gaussian orbital basis are given. The method is applied to low-energy electron-helium scattering with a second-order optical potential.