Table of contents

New results are presented for the photoionization of Fe⁺ in the close-coupling approximation using the R-matrix method. Cross sections are reported for the ⁶D symmetry, including the ground state 3d⁶4s(⁶D). The effect of photoionization of the 4s and the 3d shells is investigated through a series of calculations and it is shown that the latter makes the dominant contribution. The final 21-state expansion for the core ion Fe²⁺ includes all coupled states from the 3d⁶, 3d⁵4s and 3d⁵4p configurations. Photoionization of bound states along a Rydberg series is studied and the influence of strong photoexcitation-of-core type of resonances is pointed out. Partial cross sections for photoionization into individual final states of the core ion are also obtained. The new ground state cross section is up to two orders of magnitude higher than recently reported values.

The authors present results of calculations of two-photon ionization of the ground and (1s2s)¹S states of He by linearly polarized light in the region of the autoionizing resonances below the n=2 and n=3 excitation thresholds of He⁺.

The authors review recent advances in the coherent excitation during atomic collisions. Special attention is paid to alignment and orientation of the excited charge cloud and to the collision-induced dipole moment. The authors focus their discussion on one- and two-electron collision complexes like H⁺-H and (H-He)⁺ which provide the benchmark systems for such studies. The theoretical concepts to describe the excitation process utilize the quasi-molecular picture at small velocities and use coupled-channel techniques at medium velocities; they are supplemented by intuitive models to interpret the investigated coherence parameters.

The exchange energy is calculated for several atoms in the exact exchange-only density functional theory using the Levy-Perdew relation. The spin orbitals, the total energy and orbital energies are compared with the Hartree-Fock spin orbitals, total energy and orbital energies.

Experiments with an electron cyclotron resonance ion source have been employed in a hunt for the elusive CH²⁺ ion. The authors' experiments do not find evidence for the existence of stable CH²⁺ ions. Different levels of ab initio molecular orbital theory have been employed to obtain potential energy curves for CH²⁺. Although SCF calculations show a small minimum in the potential, post-Hartree-Fock computations indicate CH²⁺ states to be repulsive.

The radial couplings between the ^3,1 Sigma ⁺ adiabatic states dissociating into Li(2s, 2p, 3s, 3p, 3d, 4s and 4p)+H and Li⁺+H^- are determined from accurate diabatic data. The results are validated by a comparison with previous ab initio calculations. It is shown that the peaks of the radial coupling can be interpreted from the diabatic curves. An over-repulsiveness of the valence diabatic states as compared to the Rydberg diabatic states generates, at short internuclear distances, a series of crossings between neutral diabatic states which are associated with intense peaks of the radial coupling both for the singlet and triplet Sigma states.

Three-photon perturbative photoionization cross sections from the ground state of beryllium have been calculated employing B-spline basis functions. Two-photon ionization cross sections are also calculated and compared to other L² basis set calculations. Various approximations for the 1s² core are discussed, calculations are performed in velocity and length gauges including electron correlations.

The fluorescence spectra of a two level atom interacting with amplitude modulated intense fields are presented. The spectra are multipeaked and can be in general described as a superposition of triplets. The incoherent spectra for the fully amplitude modulated field and those under conditions of parametric resonance are predicted to have a width that depends on the modulation intensity. This feature is different from the known intensity-independent widths of the peaks in the Mollow spectrum. The transient spectra for an atom placed in the dressed state of a partially amplitude modulated field are found to show variation in the peak height. In some cases, one of the peaks of each triplet is suppressed while in another situation, all the peaks lying to one side of the spectral centre are absent.

The effects of changes in coordination number on K beta /K alpha X-ray intensity ratio have been studied experimentally. The K beta /K alpha X-ray intensity ratios for both pure Cr, Mn and Cu and their various chemical compounds with tetrahedral and octahedral symmetry are investigated using a Ge(Li) detector. The measured results indicate that the K beta /K alpha ratios for compounds with tetrahedral symmetry are in general larger than those with octahedral symmetry. The obtained K beta /K alpha X-ray intensity ratios are comparable with both previous published experimental and theoretical results.

The momentum transfer cross sections and the diffusion coefficients of ground state and metastable magnesium atoms in He (300-3000 K) and Ne (300-2800 K) are calculated. The measured diffusion coefficient of ground state Mg in He, D=340+or-27 cm² s^-1 for 1 Torr, which is determined at 300 K in this work, as well as all other experimental results at higher temperatures agree well with the calculations.

State-selective differential cross sections for double-electron capture by Ar²⁺ ions from Ar have been studied experimentally at laboratory impact energies of 2 and 4 keV and scattering angles between 0' and 2' using a time-of-flight spectroscopy technique. For small angles (0'-0.5'), the spectra indicated that the predominant process observed is due to capture into the Ar(3p⁶¹S) ground state. However, as the scattering angle is increased, other channels due to capture into the excited states of Ar successively start to contribute. These measurements show that the differential cross sections for double-electron capture are strongly peaked in the forward direction and display only weak oscillatory structure, in contrast to the marked structure generally expected when resonant electron capture can occur.

Differential cross sections for vibrationally elastic scattering of electrons from N₂ have been measured at three energies using the relative flow technique and normalizing to the cross sections in helium. At 1.5 eV the authors' results generally support the observations of Brennan et al. (1992), who find substantially larger cross sections than previously reported by Shyn and Carignan (1980) and by Sohn et al. (1986). At 0.55 eV, the authors' results are also larger than those of Sohn et al. but in good agreement with the calculations of Morrison et al. (1987). A measurement is also reported at the second peak of the ² Pi _g temporary negative ion state, and several difficulties in making comparisons with theory and experiment are elucidated.

Absolute elastic differential cross sections are presented for electron scattering from nitrous oxide (N₂O) for incident energies between 5 eV and 80 eV. Integral and momentum transfer cross sections have also been determined by extrapolation of the differential cross section data to 0' and 180'. Particular attention has been paid to the measurement of differential cross sections below 10 eV as in this region only one earlier measurement at 5 eV has been reported. Previous works investigating vibrational excitation have shown evidence for a negative ion resonance near 8 eV, however no manifestation of this resonance has been found in the elastic scattering channel in this work. A comparison is also made between the differential cross sections of N₂O and CO₂ as previously it has been found that the cross sections of isoelectronic molecules (i.e. N₂ and CO) exhibit very similar shapes and values.

Coincidence measurements are reported for the circular polarization of Lyman- alpha radiation resulting from the decay of 2²P_j states excited by electron impact at an incident energy of 54.4 eV in a range of scattering angles from 5'<or= theta _e<or=40'. The data were obtained using a recently developed efficient polarization analyser for this spectral line and the statistical significance of the data is much better than for previously reported measurements. The data are analysed to yield values for the coherence parameter P⁺ and show that spin exchange interactions play a significant, if unexpected, role in the dynamic range investigated.

Absolute cross sections for double ionization of Ba⁺ by energy-resolved electrons have been measured from below threshold to 120 eV. The results are similar to those of Hirayama et al. (1987) except that considerable step-like structure was resolved in the ionization function for electron energies between 80 and 120 eV. The origin of this structure is discussed and it probably arises from excitation of a 4d electron followed, either by double autoionization, or by auto-double ionization.

Two sets of close coupling calculations have been carried out on the Cray-2 and the Cray Y-MP using the R-matrix method: (i) a 38-term calculation in LS coupling and (ii) a 41-level fine structure calculation in the Breit-Pauli approximation. The first set includes the quartet and sextet terms dominated by the configurations 3d⁶4s, 3d⁷ and 3d⁶4p and collision strengths are calculated for all 703 transitions in LS coupling. The second set of calculations is carried out using the Breit-Pauli version of the R-matrix method and includes a number of the important fine structure levels from the quartet and the sextet multiplets and 820 corresponding transitions. Detailed autoionization structures are obtained in both sets of collision strengths, and a significant enhancement is seen in the effective collision strengths for a number of transitions due to the resonances: for example, an enhancement of factors of 1.4, 2.5 and 1.15 respectively for transitions from the ⁶D ground term to the lowest ⁴F, ⁴D and ⁴P terms.

The author studies the inelastic scattering, accompanied by the transfer of L photons, of fast electrons by the CO molecule (in the ¹ Sigma ⁺ state) in the presence of an infra-red laser beam taken in the dipole approximation. Both the laser-projectile and the laser-molecule interactions are treated non-perturbatively while the electron-molecule interaction is treated within the first Born approximation. The utility of the author's formalism is illustrated by computing the cross sections for vibration-rotation transitions, vj to v'j', in the CO molecule due to electron impact for the absorption of L photons. The author discusses also the influence of various laser parameters (polarization, frequency and intensity) which strongly affect the dynamics of the collision. The results are also compared with those for the field-free case.

Triple differential cross sections (TDCS) for the asymmetric geometry have been calculated in a positron-H-atom collision process at intermediate and high incident positron energies. Since the velocity of the ejected electron is small compared with the outgoing positron, direct ionization may be considered to be the only mechanism for ionization. In other words, the probability of ionization through the process of positronium formations to the continuum may be neglected. The TDCS have been computed for different incident positron energies as well as ejected electron energies with different fixed scattering angles of the outgoing positron. In the absence of experimental data, present results are compared with other theoretical predictions and good agreement is noted.