Table of contents

New experimental spectra are reported for doubly excited resonances in the photoabsorption spectrum of Sr I. These were obtained by the use of a coherent four-wave mixing source and a thermionic diode detector. The results are compared with predicted spectra calculated from the eigenchannel R-matrix method. To within experimental accuracy, very good agreement between the two is obtained, but there are some discrepancies of detail which are pointed out.

Elastic electron scattering from a modified dimer Yukawa potential is studied. The Schrodinger equation is separable for this potential in prolate ellipsoidal coordinates. The authors find that for repulsive interaction, electrons are predominantly scattered into a direction perpendicular to the dimer axis. For attractive interaction, electrons are focused into the direction of the dimer axis.

As part of their continuing study of the influence of the environment on the properties of atoms and molecules, the authors calculate the ground-state energies of three two-electron atoms and ions-H^-, He and Li⁺-constrained at the centre of a hard spherical cavity. The calculations are performed using the techniques of quantum Monte Carlo (QMC); the particular QMC algorithm employed is unique in that the associated timestep error is quadratic, rather than linear, and thus yields highly accurate energies, a typical error bound being +or-0.0003 Hartree. This accuracy exceeds that previously obtained in studies employing self-consistent field Hartree-Fock and configuration interaction methods.

The resonance wavefunctions of two-electron ionization ladder (TEIL) states can be identified a priori and computed to a high degree of accuracy from the solution of the complex eigenvalue Schrodinger equation. Depending on the form of the trial functions and on the concomitant non-Hermitian matrices, energies, energy shifts and partial widths can be obtained. The authors present the first predictions for the partial widths of the ¹P⁰ TEIL states in H^-, for n=3, 4 and 5, from calculations which include interchannel coupling. As before for the ¹S symmetry, their analysis of the decay distribution shows that most of the width goes to the nearest hydrogenic threshold, while within each threshold there is strong mixing of the angular momentum channels. As n increases, the lifetime increases while it is given with increasing accuracy by the decoupled width (i.e. golden rule) to the nearest hydrogenic threshold.

In the framework of a unified approach, based on the representation of Stark broadening of spectral lines as an effect of both static and dynamic polarization of the level, a universal description of ionic forbidden line spectral intensity is obtained which includes static and impact limits. The broadening is treated in terms of non-resonant Raman scattering of the Fermi equivalent photons which simulate the electric fields of perturbing particles. The interrelation between this Raman scattering and correlation theory is traced. It is shown that the intensity of forbidden lines due to electronic impacts in hot plasmas with multicharged ions may compete with the intensity due to ionic static action.

High resolution spectroscopy is performed on a highly excited diamagnetic manifold of the hydrogen atom perturbed by a weak electric field parallel to the magnetic field. A two-step excitation is realized via the 2p level, using two single-mode VUV and UV pulsed lasers. Simultaneous crossings are observed at some peculiar values of the electric field. Positions and intensities of the spectral lines are found to be in close agreement with the calculated predictions taking into account the mixing in the intermediate state.

In the hydrogenic approximation a scaling relation for K-shell photoionization cross sections of atoms is given. Expressions for the continuum dipole strength sums have also been derived.

The Auger spectra of photoionized argon have been studied around the threshold for ejection of a 2p electron. Above threshold the normal Auger line shape is distorted due to post-collision interaction. The line shape is well described using existing theories. Below threshold two high-resolution spectra have been recorded of resonantly excited inner-shell states. In these spectra the multiplet structure of the final state is partly resolved. Shake up of the excited electron appears to be an important exit channel. The authors find evidence that orbital and spin angular momentum of the excited electron are conserved during the shake up and shake down processes under the influence of the Auger decay.

A method is presented for using scattering calculations performed with the R-matrix method to compute transition dipole moments between deeply bound and diffuse (Rydberg) states of molecules. The method is adapted from Seaton's atomic method. Results are presented for transitions of molecular hydrogen from the ground X ¹ Sigma _g⁺ to all the ¹ Sigma _u⁺ and ¹ Pi _u states with principal quantum number less than or equal to 5 as a function of internuclear separation (1.0<or=R<or=3.0 a₀). Comparisons are made with previous theoretical calculations, which do not extend beyond states with n=3.

The authors study the formulation of atomic scattering in the impact parameter approximation. They first note that the asymptotic conditions for the case of Coulomb interactions are not defined unequivocally within the impact parameter approximation. They then show that any ambiguity can be waived when the impact parameter approximation is derived as the eikonal limit of the quantum equations. Finally, in showing the uniqueness of a minimal form of asymptotic conditions, they provide a proof of the celebrated prescription presented by Cheshire (1964).

Electron excitation by multicharged ions colliding with hydrogen targets is calculated with the prior eikonal impulse approximation. Cross sections for excitation to n=2 and 3 by impact of antiprotons (p), protons (p), He²⁺, Be⁴⁺, and O⁶⁺, are reported as well as the density matrix elements, multipole expansion parameters, dipole moments and perihelion velocities. The theory gives a good account of the available experiments and compares well with other sophisticated theoretical methods in the intermediate energy range (25>or=E/Z_P>or=200 keV).

The semiclassical coupled channel impact parameter model in conjunction with the independent electron model and a target centred atomic orbital expansion has been applied to calculate the L_{gamma 2,3} X-ray production cross sections for ¹H, ⁷Li, ¹⁴N, and ²⁸Si ions on Yb with projectile energy range 0.5-3.0 MeV amu^-1. In comparison to previous L_I subshell ionization calculations the work shows improvement in the agreement between theory and experimental data.

Single and double electron transfer from He into the ground state ion O⁶⁺(1s²)¹S₀ and the metastable ion O⁶⁺(1s2s)³S₁ have been studied in collisions at v=0.39 v₀. The single capture takes place dominantly into the n=3 shell for both ions; for O⁶⁺(1s2s)³S₁ the transfer excitation ending in O⁵⁺(1s2p3l) represents of the order of 10% of the core conserving capture. The single capture main features are similar for both ion configurations. The double capture into the ground state and the metastable ion is discussed, assuming that its main features (energy gain and thus binding energy) are also similar.

Differential cross sections are calculated for electrons elastically scattered from krypton atoms at energies from 0.8 eV to 150 eV by a continuum-state relativistic Dirac-Fock method. The polarization of the target is approximated by a model potential with dipole and quadrupole terms. All electrons are treated equivalently, thus ensuring orthogonality between the continuum wavefunction and the bound target orbitals. The exchange connected with the electrostatic and polarization interactions is calculated exactly for both large and small components of the continuum wavefunction. The exchange part of the polarization interaction is found to decrease the d-wave phaseshifts by as much as 25%.

A recently proposed ab initio method for calculation of shape resonance, which is based on the Feshbach formalism, is applied to low-energy electron scattering of the HCl molecule. A metastable ² Sigma ⁺ state of the HCl^- anion is assumed to be responsible for the structures, which have been found experimentally in the excitation cross section. In the authors' approach resonances are described by a metastable anion, the so-called discrete component. This anion is obtained by a stabilization procedure invoking an increase of the electron-nuclei interaction. The use of a configuration interaction procedure for the calculation of the resonant state provides for a description of correlation and polarization effects between target and projectile. The construction of the metastable ² Sigma ⁺ state is discussed in detail as well as the influence of the stabilization on the wavefunction of the molecule and the anion.

A variational R-matrix calculation has been performed for electron scattering by the HCl molecule. Rotational non-adiabatic effects have been included using a modified frame transformation method, in which the body-frame vibronic R-matrix constructed from a variationally computed fixed-nuclei electronic R-matrix, is frame-transformed at the R-matrix boundary. In the authors' implementation Born closure can be used effectively for the final calculation of cross sections. The validity of the first Born approximation (FBA) for different classes of rotational transitions is discussed. The influence of the different parts of the asymptotic potential on near-threshold cross sections in the vibrational elastic channel is shown; the dipole moment was found to be responsible for the threshold peak in this channel.

The derivation of the threshold law for ionization of an atom by electron impact is reduced to calculation of the probability of the system survival in the vicinity of the Wannier ridge. The approach is applicable for a small but finite energy excess above the threshold. This reveals the energy range where the Wannier law applies as well as the nature and the character of the deviations outside this region.

The electron impact excitation of the (2p²)¹D and (2s2p)¹P autoionizing states of helium and their subsequent radiationless decay are studied by observation of ejected electrons. The study is carried out in the incident electron energy range 70-200 eV and for ejected electron scattering angles of 40-130'. The ejected electron spectra are analysed using the Shore-Balashov parametrization. The resulting parameters are compared with other experiments and theory where possible.

Cross sections for the scattering of electrons by mercury in the region from 0 to 8 eV have been calculated using the Dirac R-matrix method. In these calculations the five lowest levels of Hg are included. Cross sections converged to within 5% are obtained with sixteen (N+1)-particle collision symmetries (partial waves). The authors have computed the profiles of, and identified, the resonances 6s²6p and 6s6p² in the cross sections. The oscillator strengths for radiative transitions between the five levels have also been calculated. Their results are compared with previous calculations and experiments.

Cross sections for the scattering of electrons by lead in the region from 0 to 4 eV have been calculated using the Dirac R-matrix method. In these calculations the five lowest levels of Pb are included. Cross sections converged to within 5% are obtained with sixteen (N+1)-particle collision symmetries (partial waves). The authors have computed the profiles of, and identified, the resonances 6s²6p³ in the cross sections. Their results are compared with previous calculations.

Values of cross sections for the production of positive ions resulting from electron collisions with NH₃ at impact energies ranging from threshold to 1 keV have been measured using a crossed-beams apparatus. These cross sections have been normalized by using the relative flow technique and the previously determined cross sections of He and Ne. The measured values are compared with previous data available in the literature. The threshold energy for the production of each ion has been measured. Total ionization cross sections as a function of electron impact energies have been deduced from the measured partial ionization cross sections.

Total cross sections have been measured for electron scattering on Kr, O₂ and CO in the 5-300 eV energy range utilizing a linear attenuation technique. The present results are found to be in good agreement with other experimental data for Kr and CO but deviate significantly from some of the previous measurements in the case of O₂.

Absolute, apparent cross sections for 295-410 nm band emission from naphthalene have been measured over an electron-impact energy range from threshold to 300 eV. Numerous processes, including electron exchange, are observed to contribute to the excitation in the near threshold region. At higher energies direct excitation through optically allowed channels is preferred. The electron-impact-induced fluorescence was found to be unpolarized (<0.5%) in the near-threshold region but showed small negative polarizations of a few per cent at impact energies greater than 25 eV. The radiation decay occurs to excited vibrational levels of the ground electronic state which is expected to lead to strong IR emission.