Table of contents

The uniform semiclassical approximation is employed to calculate the first cross sections for two-electron photodetachment from K^- near threshold. Theory gives that the single photon total cross section is sigma ( gamma ,2e)=(66.7*10^-20) E^1.127 cm² where E is the excess photon energy in eV. This is a factor of approximately four greater than the available experimental result.

Differential elastic scattering cross sections of bare ions from hydrogen are calculated using the Eikonal approximation. The results satisfy a scaling relation involving the scattering angle, the ion charge and a factor related to the ion mass. A semi-classical explanation in terms of a distant collision hypothesis for small scattering angle is proposed. A unified picture of related scaling rules found in direct processes is discussed.

The production of H₂ neutral molecules in collisions between H₂⁺(X) and Na(3s,3p) has been investigated in the 0.25-2 keV energy range using time-of-flight spectroscopy. In the collision with the ground-state target the population of the H₂ states B ¹ Sigma _u⁺ and c ³ Pi _u^- has been observed. In the charge exchange with laser-excited sodium the states C ¹ Pi _u and E, F ¹ Sigma _g⁺ are found to be populated. The investigated electron transfer process is strongly forward scattered and the relative total cross sections sigma _3p/ sigma _3s for the production of H₂ bound states are found to vary between five and two in the explored energy range.

Doubly autoionizing states formed as a result of capture of the incident electron with simultaneous resonant excitation of a 2p electron in e-Mg⁺ collisions have been calculated using the R-matrix method. The author includes the most important sixteen autoionizing states arising from the 2p⁵3s², 2p⁵3s3p and 2p⁵3s3d configurations together with the ground 2p⁶3s ²S and excited 2p⁶3p ²P⁰ and 2p⁶3d ²D states in the close-coupling expansion. The calculated results compare well with the recent crossed-beam measurement.

Density functional theory calculations on Yb^- using the Hartree-Fock system and the analogous Dirac-Hartree-Fock methods predict that Yb^- is stable in the P^o_1/2 state. The electron affinity obtained after the correlation energy functionals used are calibrated against Ca^- and Lu is 21 mHartree (4327 meV). The calculations indicate that the P^o_3/2 state is unlikely to be bound due to spin-orbit coupling reducing its binding energy.

Negative ions of Yb have been observed for the first time using high-resolution heavy-ion accelerator mass spectrometry to remove the intense molecular interferences. The existence of stable negative ions of this element has been predicted recently.

The authors present a theoretical study of the electronic structure of molecular Rydberg states of several second row diatomic molecules which involve binding between atomic 2s and 2p valence orbitals.

A theoretical investigation is made of spectral lines of the hydrogen atom which interact with constant and microwave electric fields of different intensities and frequencies. It is shown that to obtain an adequate form of the spectral lines one must from the very beginning take into account the fine structure of the hydrogen levels.

Spectra of photoelectrons produced by threshold photodecay of a negative ion into a continuum having an autoionizing-like structure have been obtained. The basic feature of the spectra is that a considerable (in some cases major) portion of photoelectrons resides in the region far away from the threshold. This is due to the presence of a channel of ion photodecay from the excited state.

The Wannier theory not only gives the threshold laws for processes such as near-threshold double photoionization, but also predicts an angular correlation between the two escaping electrons. Within the frame of this theory and using an LS scheme the authors derive the triple, double differential and integral cross sections, for noble gases ions left in the ground or first excited states. The approach should allow a direct comparison with forthcoming experiments, as it uses the individual angles of the two electrons. In the case of unfavoured final states, which have a node when the angle between these electrons is 180', the triple differential cross sections depend critically on the angular correlation. The authors give their detailed structure in space for double ionization by both polarized and unpolarized photons. They also show that a particular choice of Euler angles allows a straightforward integration which leads to the double differential cross sections.

The Heitler-Ma equation is adapted so that it can deal with two-photon spontaneous emission and resonance fluorescence processes. The authors introduce two types of effective Hamiltonian, depending on whether all the intermediate states are non-resonant or one of them is resonant. The spectra for spontaneous emission, and for resonance fluorescence in both weak and strong excitation fields, are discussed.

Experimental results for the probability ratio R₃ during three-photon ionization of Ca, Sr and Ba atoms in circularly and linearly polarized fields are presented for the principal quantum number range n=4-13. Unlike the alkali atoms, in most cases the measured R₃ magnitudes are considerably less than the values estimated by the Klarsfield-Maquet factorial formula.

The authors have observed the production of Ba²⁺ from neutral Ba by intense picosecond laser pulses in the range 560-650 nm. Several new features have been observed, which were not observed with nanosecond pulses. Most of the strong resonances in Ba²⁺ production can be attributed to Ba⁺ transitions originating in the 6s_1/2 or 5d_j states, suggesting that the dominant process in double ionization in this intensity and wavelength regime is sequential removal of electrons. However, there are several resonances which cannot be attributed in this manner, evidence for possible alternative ionization mechanisms. Ba²⁺ production is strong throughout the region studied, at times comparable to that of Ba⁺. The authors have also looked for Ba²⁺ production in the range 280-325 nm, but no Ba²⁺ signal was observed in this part of the ultraviolet.

K X-ray absorption spectra of molecules and complexes containing low-Z atoms (CO, CO₂, OCS, ferrocene) are studied. Agreement of theoretical and experimental spectra on the photon energy scale is obtained if the electron scattering phaseshifts and amplitudes are calculated with the help of the spherical wave formalism and the non-local Hartree-Fock muffin-tin (MT) potential, constructed so as to provide the potential continuity on MT sphere surfaces. In the case of molecules and complexes the approach proposed enables one to determine MT zero position in relation to the absorption threshold without any fitting procedure.

The theory of ionization of highly excited atoms by neutral particles (atoms or molecules) is developed within the framework of the quasifree electron model. General formulae relating the ionization cross section with the differential cross section for electron-perturber scattering and with the momentum distribution function of the Rydberg electron in the initial atomic state, have been derived from the impulse approximation. These formulae are applicable for any arbitrary form of the amplitude of elastic or inelastic scattering of the slow electron by a perturbing particle. In the special case of the scattering length approximation, simple analytical expressions for the ionization cross sections and the rate constants have been obtained.

The theory of direct and associative ionization of Rydberg atoms in thermal collisions with neutral atomic particles has been developed. Some effective mechanisms of ionization due to the scattering of the perturbing atom on the core of the Rydberg atom have been investigated. These mechanisms of ionization correspond to the non-adiabatic energy exchange between the highly excited electron and inner electrons or nuclei of a quasimolecule. General equations obtained within the framework of perturbation theory and the quasiclassical approximation have been used to determine the rate constants of ionization for the various types of colliding atoms. In particular, the ionization processes of Rydberg rare-gas X(nl) atoms by ground-state atoms of the buffer Y(¹S₀) or the parent X(¹S₀) rare gas have been investigated in detail. The results of numerical calculations are presented for the Rydberg hydrogen and xenon atoms in collisions with He, Kr and Xe atoms.

Previous work on the double excitation of helium by protons below 500 keV seemed to prove the validity of the Born approximation. This conclusion is challenged by recent experimental evidence. The authors evaluate the sensitivity of the Born calculations to atomic parameters and in particular to electron correlation. They conclude that the previously observed agreement between experiment and the Born approximation was accidental.

Quasi-molecular (MO) radiation from single collisions of decelerated H-like sulphur ions onto Ar atoms were measured. Coincidences between the X-rays (detected at 90' and 20' with respect to the beam direction) and the scattered projectiles (resolved in both polar and azimuthal angles) allowed to determine the impact-parameter-dependent angular distribution of the MO radiation in both polar and azimuthal angles. The data, as a function of the MO X-ray energy, show a characteristic structure for interference between transitions taking place in the incoming and outgoing parts of the collision. Slight shifts are observed between the interference spectra obtained under emission angles of 20' and 90'. The effect of this anisotropy on the spectroscopy of the quasi-molecular 1s sigma -2p pi energy difference is discussed.

Cross sections for the J mixing (transfer of excitation between fine-structure sublevels) in the nD (n=10-14) states of caesium due to collisions with noble-gas atoms are measured by the integrated intensities method. It is found that the cross sections corresponding to the different noble-gas perturbers are of the same order of magnitude for lower values of the principal quantum number n but become increasingly different from each other with increasing n and for n=13 and 14 the difference approaches two orders of magnitude. Qualitatively, the n-dependence of the cross sections is similar to that obtained in the studies of the l-mixing collisions in the Rydberg states of the alkali-metal atoms. The results are compared with those predicted by the existing theoretical models. Good agreement is obtained for He and Ne perturbers with the results of calculations by Sirko et al. in which the Fermi pseudopotential and perturbation approach were employed.

The authors analyse the electron-spin depolarization of highly charged hydrogenic and alkali-like ions formed by single-electron capture in an electron-spin-polarized target. In general, ions are formed in highly excited states and are depolarized by the fine- and hyperfine-structure interaction as they cascade down to the ground state. The final polarization is determined by the state into which the electron is captured, the alignment of the excited state formed in the capture process, the nuclear spin, and the angle between the target electron-spin polarization vector and the ion-beam axis. They also calculate the polarization of the radiation emitted following polarized-electron capture. They point out that the circular and linear polarization of the emission line can be monitored to determine the polarization of the ions.

State selective cross sections for charge exchange collisions between O⁵⁺ and H have been calculated for ion energies ranging from 0.03 to 1155 eV amu^-1. Capture to the 4s, 4p and 4d levels of O⁴⁺ dominates over the whole energy region. At the highest energies the authors' results are in agreement with recent merged-beams experiments. However, between 1 and 10 eV amu^-1 there is a significant discrepancy between theory and experiment. The experimental cross section exhibits a pronounced minimum at about 2 eV amu^-1 while the calculated cross section shows a much less rapid variation. They have calculated the differential cross sections in the energy range 0.87 to 17.4 eV amu^-1 to investigate in detail the angular scattering properties of this system. They find evidence that the discrepancy may be explained as due to the limited angular acceptance of the experiment.

The coupled-channels optical (CCO) calculation of electron-atom scattering involves the solution of a set of coupled integral equations for a finite set of channels, with the effect of the remaining channels included by the optical potential. For discrete channels the calculation of the optical potential can be verified within the framework of the coupled-channels (CC) calculations: 54.4 eV electron scattering on hydrogen is calculated in a three-channel model with the n=2 channels either explicitly coupled or treated by different approximations to the optical potential. The same is done for the n=3 channels in a six-channel model. In both cases the authors find that the CCO model reproduces the corresponding CC calculation to a sufficient accuracy, which verifies internal consistency of their approach. They present the results of the 6CCO calculation at 54.4 eV and find them not to be significantly different from the 3CCO results recently reported by Bray et al. (1990), as expected.

Intermediate energy calculations of elastic, 1s-2s and 1s-2p electron scattering by atomic hydrogen based upon intermediate energy R-matrix theory have recently been reported. The authors present the resulting elastic, 1s-2s and 1s-2p differential cross sections and 1s-2p electron-photon correlation parameters. Agreement with experiment is good for elastic scattering over the entire intermediate energy regime. However discrepancies exist between their n=2 results and experiment particularly below 20 eV where their results lie below the experimental data at small and large angles. For scattering angles less than 60' their results agree with 1s-2p electron-photon correlation measurements but significant differences exist for large-angle scattering at 54.42 eV.

The intermediate energy R-matrix (IERM) theory, first introduced by Burke et al. (1987) has been applied to electron scattering by atomic hydrogen by Scholz et al. (1988) and Scott et al. (1989). In principle, the only approximation inherent in this approach relates to the treatment of the scattering wavefunction in the external R-matrix region. Here the couplings between the channels of interest and all other physically open channels are neglected. The author demonstrates that these couplings are small in this region of configuration space and hence that the IERM theory includes the physics which dominates intermediate energy scattering. The mechanism for the formation of pseudo-resonances in IERM calculations is also described along with the averaging procedure used to extract the physical T-matrix elements. The author shows that this procedure removes the unitarity of the S-matrix and therefore accounts for the flux lost into channels not explicitly included in the external region expansion of the scattering wavefunction. Calculations which explicitly include a large number of pseudo-channels are reported. The resulting T-matrices are found to be in agreement with the averaged results hence validating the T-matrix averaging procedure.

The author has previously derived an ionization threshold law on the basis of the Coulomb-dipole theory, but including only the contribution from the outer (semi-asymptotic) region of configuration space to the relevant matrix element. Here the author initiates a study of the (overwhelmingly dominant) inside contribution. The main part of this paper is devoted to a detailed analysis of the asymptotic form to which the inside function must be attached. The author returns to an older approach of deriving the ionization yield by extrapolating excitation cross sections of excited states (N) (because the asymptotic form there is known to be dipole configurations of degenerate (l) states for each N excited by the incoming electron) into the continuum. Here the author concentrates on the dipole matrix from which the dipole configurations are obtained; it is derived analytically; its eigenfunctions and eigenvalues are evaluated numerically to very high N (for several of the lower partial waves L). A key parameter emerges, J_C(N,L), which specifies the number of coherent, attractive, dipole states as a function of N and L.

The authors have used the results of 11-state and 19-state R-matrix calculations to obtain differential cross sections and electron-photon coincidence parameters lambda and mod chi mod for the excitations 1¹S to n^3,1P (n=2, 3 and 4) at energies 22, 24, 26.5 and 29.6 eV. Convergence studies on the five-state, 11-state and 19-state R-matrix calculations on lambda , mod chi mod and differential cross sections have been carried out. The calculated Stokes parameters and L_{perpendicular to} and gamma parameters are compared with experiments, and the n independence of these parameters and the differential cross sections as predicted by Csanak et al. is found to be not particularly strong at low energies.

Excitation cross sections for all fine-structure levels of Ar I 3p⁵4p, 3d and 5s configurations are calculated for the range of electron energies 16-100 eV in the distorted-wave Born approximation with exchange. Non-empirical intermediate-coupling wavefunctions are used and their quality is checked by calculating the dipole transition probabilities. The results are compared with the experimental data obtained by electrical and optical methods.

Collision strengths for electron impact excitation between the lowest 20 levels among the 1s²2s²2p², 2s2p³ and 2p⁴ configurations of Ca XV have been computed for collision energies of 15, 30, 45 and 60 Ryd in a Dirac-Fock approximation. Comparisons have been made with the R-matrix and distorted-wave collision strengths. Differences are discussed and the accuracy is assessed.

Dissociative attachment and vibrational excitation processes in low-energy electron collisions with methyl chloride are considered. A theory based on a combination of the resonance R-matrix approach and the non-local-complex-potential approach is applied for the calculation of the cross sections. For the input data of the theory the author uses Falcetta and Jordan's calculation of the adiabatic potential curve for the CH₃Cl^- anion. The calculated dissociative attachment cross sections averaged over the thermal vibrational distribution are about three orders of magnitude less than those obtained in a beam experiment of Chu et al. (1990). Possible reasons for this discrepancy are analysed. Vibrational excitation cross sections exhibit a sharp threshold peak and a broad resonance with the position corresponding to the vertical attachment energy. The author also investigates the sensitivity of the results to the input parameters.

A singular structure in the differential cross section for electron ejection by positron impact occurs when the positron and the ionized electron have low relative momentum in the final channel. This is the analogue of the process of 'electron capture to the continuum', well known in positive-ion impact ionization. Near the singularity a dip, due to interference between two double-binary scattering processes, is predicted if the scattering angle of positron and electron equals the critical angle of 45'. Dip and singularity coincide in the high energy limit.

The electronic structures of clusters of Li₇, Li₆Na and Li₆Mg with simple cubic geometry are calculated using spin density functional theory in the local approximation (LSDA). The results show that the shell model of clusters is indeed a good one, as the effect of inner electrons on clusters is very small in determining the physical properties of clusters and also show that there are some similar properties between the clusters of Li₇ and Li₆Na due to the odd number of electrons in both clusters and the closeness of the orbital energy of the highest occupied states of Na and Li atoms. However, the charge transfer and the distribution of the one unpaired electron between the central atom and the atoms surrounding it show different characteristics. The equilibrium lattice constants and binding energies and ionization potentials are compared with other calculations. Some are in very good agreement, some are quite different. For these a detailed discussion is given.