Table of contents

The wavefunctions of a hydrogenic system confined by a potential barrier are sought as an expansion in the eigenfunctions of a free hydrogen atom in prolate spheroidal coordinates. The ground and first excited states are calculated for a positive magnitude of the barrier. It is shown that the terms of the atom near a medium that reduces the Coulomb interaction are similar to those at a hard wall.

Transitions between doubly excited levels in Ne-like ions have been identified for the first time. By means of the beam-foil method eight lines have been found as combinations between quintet levels of the 2p⁴3s3p and 3s3d configurations of S VII and Cl III. The interpretations are supported by theoretical calculations and by comparisons with observations of doubly excited quartet level in Na-like S VI and Cl VII.

A new method for plasma electron density determination is discussed. It depends on the condition of a 'thermal zone' for Rydberg series of autoionizing levels and does not require a precise knowledge of atomic data. The method is applied to the analysis of He-like Al XII spectra obtained from a low-inductance vacuum spark.

The non-resonant interaction of an intense nanosecond CO₂ laser pulse with Hg₂, N₂, and H₂ shows that the molecules are tunnel ionized as if they were atoms with the same ionization potentials as the molecules. Moreover, dissociation of the neutral molecule appears to be much less probable even though the dissociation energies are lower than the ionization potentials.

A calculation of simultaneous ionization and excitation to the n=2 state of helium by electron impact is presented. This process requires three electrons to change their state, but can nevertheless be realized by a first-order theory using correlated wavefunctions. A triply differential cross section for an incident energy of 645 eV is computed and compared with a recent measurement.

The status of the knowledge on the mechanisms of multiple captures by a highly charged ion colliding with an atom is reviewed. The various and complementary techniques are first described, including energy gain spectroscopy, electron and photon spectroscopy with a particular emphasis on the rapidly developing techniques combining these spectroscopies in coincidence experiments. The double capture processes are simple enough to allow theoretical calculations and a detailed analysis of capture mechanisms such as the relative role of the electron-nucleus interaction and the electron-electron interaction. This is not yet the case for multiple captures, which imply a very large number of channels. However the simple classical overbarrier model, in its extended form, is able to account for the gross features of multiple capture processes such as the number of electrons stripped from the target.

A fully relativistic analysis of the 3p spectrum of Ca I based on ab initio single and multiconfigurational Dirac-Fock calculations is presented. The calculations are compared with recent high-resolution photoabsorption data obtained with synchrotron radiation, and with earlier non-relativistic analyses based both on semi-empirical multichannel quantum defect theory (MQDT) and on single-configuration Hartree-Fock (HF) calculations.

Some general aspects concerning dianions are discussed. The small LiF₃^2- and related XY₃^2- free dianions are predicted to be stable long-lived species. Quantitative results are given and the findings are illuminated by drawing a connection with crystals.

Results of energy level calculations using a B-spline basis set are reported. The truncated-diagonalization method has been extended to include a correction for the presence of the open channels. It is also shown how to determine autoionization widths within this basis. The approach is demonstrated by calculating these properties for singlet terms belonging to the 3l3l' and 3l4l' configurations in the doubly-excited N⁵⁺ ion. It is shown that for the calculation of energy level values the B-spline method can compete with the most accurate methods available. The accuracy in the determination of the autoionization widths appears to be limited by the independent-channel approximation.

Many-body perturbation theory has been applied to calculate the partial and total photoionization cross sections for neutral sulphur from the first ionization threshold at 10.36 eV to 60 eV. The excitations from the ground state S(3s²3p⁴³P), leaving the residual S⁺ ions in the lowest five LS states (3s²3p³⁴S,²D,²P and 3s3p⁴⁴p,²p), are considered. The results are presented in both the dipole length and velocity forms. The threshold behaviour of the partial photoionization cross sections are found to be dominated by the overlapping autoionizing resonances converging to S⁺(3s²3p³⁴S,²D,²P) and S⁺(3s3p⁴⁴P,²P) limits. The excited single-particle states are calculated in an effective single-particle potential which includes, in an approximate way, the effects of the interaction among many final channels. The results are compared with the previous photoionization studies of sulphur.

Using monochromatized synchrotron radiation a sample of thin samarium and titanium foils was excited, and the induced Sm L X-rays and Ti K X-rays were detected by a Si(Li) detector. When the primary photon energy is tuned across the Sm L₂ and L₁ edges, the intensities of the Sm L X-ray lines normalized to the Ti K alpha intensity exhibit abrupt changes. From these intensity changes the Coster-Kronig factors were determined: f₂₃=0.14+or-0.03, f₁₃=0.18+or-0.03 and f₁₂=0.19+or-0.03. The value of f₁₃ is substantially smaller than predicted by theory and extrapolated from heavier elements. Furthermore, from the total intensities of all lines originating from the same Sm L subshell the ratios of the fluorescence yields omega ₁/ omega ₃=0.48+or-0.07 and omega ₂/₃₌1.05+or-0.05 were obtained.

The authors present a study of the Si K-edge absorption spectra of the Si(X)₄ (X=Cl,F,CH₃) gas-phase molecules. Both the near-edge and the EXAFS (extended X-ray absorption fine structures) regions are analysed. These simple molecules have been chosen to test multiple-scattering theory in the case of open structures with no second neighbours. Hedin-Lundqvist complex self-energy (1969) has been employed for phaseshift generation. Data analysis demonstrates the presence of strong multiple-scattering effects in the near-edge region. Near-edge structures are interpreted in terms of two-body and three-body contributions. A fitting procedure taking account of the presence of a KL double-electron excitation has been adopted to investigate the EXAFS region. Determination of distances using such an ab initio procedure is shown to be in agreement with diffraction data.

An experimental study of the depopulation and final-state distribution of Na 36s and 30p Rydberg states under Na⁺ bombardment in the presence of electric fields up to 50 V cm^-1 has been carried out. Incident energies E from 171 to 967 eV span the range from 0.6 to. 1.4 times the mean orbital speed of the excited electron in the initial state. Absolute depopulation cross sections of order 10^-8 cm², believed accurate within 25%, are reported as functions of field and energy. sigma _36s is proportional to E^-1, but sigma _30p is nearly independent of energy. The distribution of final states was measured by selective field ionization. At weak fields for which the initial state is well separated from the nearest Stark manifold, final states are broadly distributed across the nearest manifolds. But for stronger fields that shift the initial state close to a Stark manifold the final states are strongly concentrated among the energetically closest manifold sublevels.

Electronic emission from CH and OH radicals, CH⁺, CO⁺ and N₂⁺ ions and atomic hydrogen lines have been observed during collisions of CHO⁺ and H⁺ ions with CH₄ and N₂ molecules in the laboratory kinetic energy range 100-900 eV. Emission cross sections of the excited CH radicals, N₂⁺ ions and H_beta transition of H atoms have been measured in the above energy range.

The halfway house phase-integral variational continuum distorted wave (PIVCDW) theory of non-relativistic Thomas double scattering electron capture is developed within the distorted wave perturbation approximation and has the distinct advantage of satisfying all known short and long range boundary conditions. Closed analytical results are obtained for both first-order (PIVCDW1) and second-order (PIVCDW2) amplitudes of this theory for transitions from an initially excited n'l'm' state to a final nlm hydrogenic one-electron state. Both the first- and second-order amplitudes contain the correct single and double scattering form respectively at asymptotically high velocities. Moreover the PIVCDW2 approximation is used to derive simple analytic formulae for the double differential cross section (DDCS) for electron capture to the continuum.

Photon emission in the wavelength intervals 17-24 AA and 100-200 AA following single and double electron capture in collisions of O⁶⁺+H₂ at 3.8 keV amu^-1 has been analysed. The overall single electron capture features on both ion cores are similar and show dominant population of n=4 on O⁶⁺(1s²)¹S₀ and O⁶⁺(1s2s)³S₁. At variance to the capture from He, double 'correlated' capture on O⁶⁺(1s²)¹S₀ appears as a minor channel.

It is the first time that beyond the Born approximation general expressions are obtained describing the angular distribution and polarization of the dipole bremsstrahlung radiation formed during intermediate energy particle-atom scattering with the 'atomic' bremsstrahlung mechanism taken into account. The procedure is presented for the calculation of the infinite sums of the partial cross sections corresponding to different partial waves of the projectile.

Angular correlation measurements have been made of helium (e,2e) scattering in the perpendicular plane, defined as the plane normal to the direction of the incident electron beam, at excess energies of 1, 2, 4 and 6 eV. The simultaneous analysis of these measurements with the coplanar measurements of Selles et al. (1987-90) gives the value 74+or-7' for the Wannier parameter theta ₀ that governs the widths of the angular correlation functions. It is also concluded that the upper limit of the excess energy for which the Wannier model is valid in helium is approximately 1.5 eV for coplanar scattering, but appears to be higher for scattering in the perpendicular plane.

A completely relativistic distorted-wave calculation of the excitation of mercury from the ground 6 ¹S₀ state to the 6 ¹P₁ and 6 ³P_0,1,2 states by electron impact has been carried out. A complete set of cross sections, spin polarization and orientation and alignment parameters are presented at 40 eV. Detailed comparisons of the present results with existing theoretical calculations and with the extensive experimental measurements of the spin asymmetry parameter and other experimental data for this atom are presented where available.

Total cross sections have been measured for electron scattering on H₂ and N₂ in the 4-300 eV energy range utilizing a linear attenuation technique. The present results are compared with existing experimental data of other groups in this energy range where good agreement with other experimental data is found.

The authors report measurements of absolute differential cross sections for the electron impact excitation of the nu _1,3 vibrational modes of NH₃ at incident energies of 5, 7.5 and 15 eV. Previous experiments have demonstrated the influence of a broad shape resonance (E symmetry) on the vibrational excitation cross section but have differed significantly as to the position and width of this feature. Consequently, the authors have also measured the energy dependence of the nu _1,3 excitation cross section in the energy range 5-10 eV at a scattering angle of 90'. This revealed a resonance centred at about 7.3 eV with a width of approximately 3.5 eV, the latter being in good agreement with the measurement of Cvejanovic et al. The present differential cross section at 7.5 eV also confirms the dominant l=2 character of the resonant partial wave.