Table of contents

Hyperspherical adiabatic potential curves for the three-electron ions are calculated. Potential curves that support doubly excited states and triply excited states are analysed to interpret the possible existence and the nature, including the relative autoionization widths, of these states. We show that doubly excited states of can be classified using the same set of quantum numbers K, T and A that were used for classifying doubly excited states of two-electron atoms. We also show the relative energy levels of triply excited states exhibit a rotor structure.

The two-potential formula is used to find the change in the s-wave scattering length of a pair of colliding particles induced by a change in their interaction potential. It is illustrated by a calculation of the scattering length of hydrogen atoms interacting through the molecular state. The scattering length is also calculated with a recent potential.

Unexpected and previously unexplained behaviour showing appreciable intensity at low momenta in previously published binary (e, 2e) cross sections for ionization from atomic orbitals including Xe 4d, Zn 3d and Cd 4d is investigated. Similar effects have also been observed recently in binary (e, 2e) studies of metal hexacarbonyls for the least tightly bound molecular orbitals which are dominated by metal character. The finite cross sections at low momenta (i.e. non-nodal behaviour) are not predicted by plane-wave calculations for atomic targets, but can be accounted for using distorted-wave theory. These distortion effects at low momenta appear to be due to a high- effect and the even-parity nature of atomic orbitals.

Detailed theoretical studies of annihilation in low-energy positron - atom scattering reveal a very pronounced enhancement of the annihilation rate in the vicinity of the positronium formation threshold, providing qualitative confirmation of the predictions of a recent model in which the enhancement is attributed to virtual positronium formation. The present investigations relate specifically to atomic hydrogen but similar results have also been obtained for helium. From investigations of annihilation at energies above the positronium formation threshold, we conclude that no clear distinction can be made between direct annihilation and annihilation following positronium formation.

We have implemented a multi-channel quantum defect theory approach within the asymptotic R-matrix codes (STGF and STGBF) which treats all closed channels as open. Furthermore, making use of this approach, we have implemented an analytic preconvolution of undamped R-matrix photoionization and photorecombination cross sections. This removes any doubt whatsoever as to whether the resonances are fully resolved or not. We show that fully resolved undamped R-matrix photorecombination cross sections are in very close agreement with undamped perturbative cross sections. We demonstrate, with explicit examples, the large and widespread effect of radiation damping on low-lying photorecombination resonances for H-like through Ni-like ions. This refutes the recent claims made by Pradhan and Zhang and validates the original work of Robicheaux et al and Gorczyca and Badnell with regard to the importance of radiation damping.

The state-selective single-configurational N-electron spin-coupled formalism of Fletcher et al is presented in a multiconfigurational form and applied to the ground state and singly excited states of Be. The method retains the key benefits of the original formulation, particularly in the way that the state is specified by N integers, whilst allowing for the inclusion of angular correlation.

Relativistic semiclassical radial dipole matrix elements are derived analytically in the length and velocity forms using Klein-Gordon wavefunctions. In contrast to our previous works in which KGWKB matrix elements were derived only in the length form, neglecting the angular displacement of the relativistic trajectory, we have obtained these matrix elements in both length and velocity forms without the above-mentioned approximation. In the latter form, an effective velocity operator has been used. The present approaches lead to two new intermediate states which are discussed. Detailed numerical results obtained through a systematic study of lithium, sodium and copper isoelectronic sequences, starting with the nonrelativistic WKB approaches, enable us to draw inferences as to the improvements and also the limitations of the new relativistic formulae.

Since inner-shell photoionization cross sections in the region of purely inner-shell-to-inner-shell resonances are generally insensitive to the state of excitation or ionization of outer electrons, the most sensitive comparison between theory and experiment for photoionization cross sections of excited or ionized atoms is in the region of inner-shell-to-outer-shell resonance transitions. The experimental 3p absorption cross section of is compared with theory as an example.

We report a strong suppression of the three-photon ionization resonant with the 2s state of atomic deuterium at 243 nm by the introduction of a second colour (486 nm) coupling the 2s and 4p states. This coupling reduces the ionization probability by several orders of magnitude. The experiment is performed using fundamental and second harmonic frequencies of the same laser, with laser intensities up to a maximum value of and an atomic target density of the order of . The production of both ions and D(2s) atoms is measured as a function of the intensity of the fundamental beam (486 nm). The experimental data are compared with a theoretical calculation using essential states and taking into account the experimental laser bandwidths.

Using high-resolution laser photoelectron spectrometry we have determined absolute cross sections and the electron angular distribution parameter for one-photon ionization of metastable and atoms to the resolved ion states near threshold -244 nm). For comparison with the experimental data we have carried out many-electron calculations of the relevant partial cross sections and parameters over the photoelectron energy range (0-6 eV). We have used a configuration interaction method involving Pauli-Fock atomic orbitals (CIPF), designed to take into account the important electron correlation effects which are found to be essential for obtaining satisfactory agreement between the theoretical and experimental results. Including previously reported cross sections for metastable and atoms we present a discussion of the trends which are observed in the different energy-dependent cross sections as a function of the atomic number.

A very extensive compilation of Stark widths and shifts for more than 125 Ar II visible spectral lines is presented in this work. These atomic parameters have been measured in a pulsed discharge lamp by using different mixtures of pure argon or argon and helium. The electron density, which typically ranges from 0.2 to has been determined interferometrically and, in the case of pure argon plasmas, also spectroscopically from -Stark broadening. The Ar II excitation temperature (15 000-31 000 K) has been determined by Boltzmann-plot techniques from the intensity of Ar II lines. A detailed description of all the relevant points in this kind of measurement is given. Comparisons with most of the data published about this topic for Ar II are also included.

The dynamics of coherent elliptic states in time-dependent electric and magnetic fields has been studied and it is shown that a linear Stark state may be transformed adiabatically into a circular Zeeman state through a continuous range of coherent elliptic states provided the fields vary slowly and are not parallel. The orientation vectors of the elliptic states rotate relative to the fields during the adiabatic transformation except when the fields are orthogonal. The condition for adiabatic behaviour is established for an easily realizable choice of field variation.

The 3d photoabsorption spectra of Cu I-like Zn II, Ga III and Ge IV have been recorded in the 25-100 eV region using the dual laser plasma technique. The spectra are dominated by transitions of the type which have been identified by comparison with multiconfiguration Hartree-Fock calculations.

We present results of experiments studying the efficiency of high harmonic generation from a gas target using the TITANIA krypton fluoride laser at the Rutherford Appleton Laboratory. The variation of harmonic yield for the 7th to 13th harmonics (355-) is studied as a function of the backing pressure of a solenoid valve gas jet and of the axial position of the laser focus relative to the centre of the gas jet nozzle. Harmonic energies up to were produced in helium and neon targets from laser energies of approximately 200 mJ. This corresponds to absolute conversion efficiencies of up to .

We investigate multiphoton ionization (MPI) of at 532 nm excitation wavelength in an intensity range of -. We find strong fragmentation of the molecule with formation of , , , and as charged dissociation products. Photoelectron spectra show that the charged products originate in photodissociation of . Ionic dissociation starts after excitation of to the state and probably high vibrational levels of the bending mode in . Symmetric is significantly less probable than asymmetric dissociation. MPI of mainly populates ionic X (000) and (100) vibronic states below light intensity. At high intensity also B vibronic states seem to become directly populated with rising probability. All lowest-order MPI electron groups are accompanied by at least one above-threshold ionization group. A sensitive dependence of the photoelectron spectra on light intensity may point to bond-angle softening of low-lying X vibronic states of which is expected to be brought about by a favourable resonant one-photon coupling between the X and à electronic states.

A method, which filters the noise in measured generalized oscillator strengths (GOSs) by minimizing the with the cubic spline, has been developed and used to recover the smooth GOS function f. The function f is then extrapolated to (momentum transfer to obtain the optical oscillator strength (OOS). The method has been tested in the transition 1s-2p in a simulated e-H scattering at 500 eV. Also, OOSs for some transitions in Ar, Kr, Xe, and the vibronic bands of the b state of have been calculated and compared with those of other authors.

We present the results of quantum mechanical calculations of cross sections and hence rate coefficients for rovibrational transitions in ortho- and para-, induced by collisions with He. Rovibrational levels up to were included in the calculations, and rate coefficients are available for temperatures . Comparison is made with previous calculations of rate coefficients for pure rotational transitions within the vibrational ground state and with measurements of the rate coefficient for vibrational relaxation at both high and low temperatures. Agreement is found to be good.

We study the scattering of fast electrons by atoms in the presence of a strong laser field. Our method takes into account the `dressing' of the target states by including the laser-atom interaction to first order, while the laser-projectile interaction is treated to all orders. The interaction of the fast incident electron with the target atom is treated in the first-Born approximation. Detailed calculations are performed for the `elastic' scattering of 500 eV electrons by helium accompanied by the transfer of photons. We discuss the influence of the laser polarization on the differential cross sections as a function of the scattering angle, laser frequency and intensity.

We present both experimental and theoretical differential cross sections for the elastic scattering of electrons from copper atoms. The experimental measurements were performed for 40, 60, 80 and 100 eV. Theoretical calculations are presented for energies between 10 and 100 eV and compared with present and previous theoretical and experimental results. The present experimental measurements overlap with previous measurements at 60 and 100 eV and the overall agreement is good if previous measurements are renormalized. It is found that second-order distorted-wave calculations are in reasonable agreement with experiment only for energies of 40 eV and greater. Previous second-order distorted-wave studies for electron-alkali atom scattering found agreement to much lower energies, which implies that the inner d-shell for copper may play an important role for low-energy elastic scattering.