Table of contents

The authors present theoretical calculations for the angular anisotropy parameter beta of the resonant Auger decays Kr 3d^-1_5/25p (J=1) to 4p^-25p and Xe 4d^-1_5/26p(J=1) to 5p^-26p. The transition amplitudes are obtained within a model treating the outer electron as a spectator. Its influence on the final states is taken into account via intermediate coupling MCDF calculations. This model reproduces the unusually large negative beta values observed in the considered transitions. Predictions are given for transitions not yet investigated experimentally.

Double photoionization experiments on Xe (. . .5p⁶) using a helium discharge lamp have been performed in a plane perpendicular to the unpolarized photon beam. Of special concern is the case where the Xe²⁺ ion is left in its excited (. . .5p⁴¹S^e) state. Angular measurements performed with equal energies of the two electrons clearly show that the cross section is nearly zero when they are emitted in opposite directions. This is consistent with the properties of two electrons of ¹P⁰ symmetry. These measurements give direct information on the correlation which originates in the three-body interaction between the ion and the two electrons.

New data on the photoabsorption spectra of LiH, NaK and KH are reported involving ground state X ¹ Sigma and excited state A ¹ Sigma near the dissociation limits. Rotational and vibrational analysis of the observed bands is presented: LiH X ¹ Sigma V=1 to ¹ Sigma (16<or=V<or=26)NaH X ¹ Sigma V=0/V=1 to A ¹ Sigma (13<or=V<or=22)/(12<or=V<or=25) KH X ¹ Sigma V=0/V=1 to A ¹ Sigma (24<or=V<or=34)/(27<or=V<or=38). In each molecule, the highest observed vibrational level lies very close to the dissociation limit and it corresponds to about 99% of the expected bands for the A ¹ Sigma state potential energy curve.

The so-called binary-encounter electron peak resulting from collisions between quasifree target electrons and energetic projectile ions is studied. The shift in energy of the binary-encounter electron peak relative to the energy of scattered free electrons at rest is measured as a function of scattering angle in collisions between 1 and 1.5 MeV amu^-1 C^q+ (q=4, 5 and 6) and H₂ and He. The measured shifts are in good agreement with theoretical results based on the continuum distorted wave eikonal initial state approximation. Both theory and data show an increase of the shifts with projectile charge state and scattering angle. For observation angles larger than 30', the experimental shifts rise faster than theory.

Formation and subsequent decay of very highly charged states of CO molecular ions has been studied in 97 MeV collisions between F⁸⁺ and CO using a crossed beams apparatus by measuring the energy distributions of carbon and oxygen ions. The kinetic energies released (KER) upon dissociation of CO^q+ (q=2-10) ions have been investigated by means of large scale, ab initio configuration interaction molecular orbital calculations. Results indicate that dissociation via non-Coulombic potential energy curves of CO^q+ ions yields lower values of KER than would be expected from purely Coulombic considerations due to the significance of electronic charge density distributions in the internuclear region.

The authors report measurements of high resolution electron scattering from vibrationally excited CO₂ at an incident electron energy of 4 eV and scattering angle of 30'. Differential cross sections for elastic, inelastic and superelastic scattering are presented for the ground state and (01¹0) vibrational mode.

The recent development of the S-matrix adiabatic QED theory of the two-electron highly charged ions is reviewed. The evaluation of the energy levels up to the second-order corrections due to the interaction between the electrons is considered. The applications of this approach to the calculation of transition probabilities, natural linewidth and natural lineshape are also discussed.

Analytical expressions of atomic shellwise electron densities have been constructed. The authors' approach was based on the superposition of Yukawa potentials for the atomic electrostatic potential. The electron density distribution was then established by the Poisson equation. Parameters characterizing the shellwise electrostatic potential and the corresponding electron density distribution were obtained by a fitting procedure utilizing the Hartree-Fock-Slater (HFS) data. These parameters were determined by the atomic number and electron occupation numbers in various states. The accuracy of constructed electron density distributions was tested by comparing their various moments with the corresponding results obtained from other methods. Applications of these atomic shellwise electron densities were made for calculations of the elastic scattering cross section and the mean excitation energy.

The authors considers the introduction of orthogonality conditions in frozen-core Hartree-Fock calculations using basis-set expansions in which the orbitals are constrained to be orthogonal to core orbitals of the same symmetry. They find that such a constraint usually leads to a non-Hermitian eigenvalue problem. However, they show that for arbitrary core orbitals the eigenvalues are all real and that the required eigenvectors are orthogonal. The result is not restricted to Hartree-Fock calculations but also applies, for example, to model-potential calculations in which the orbitals for some reason should be orthogonal to a set of suitable core orbitals. If the core orbitals are chosen to be eigenfunctions of the Hamiltonian then the resulting matrix is Hermitian and has zero eigenvalues corresponding to the core orbitals. The authors also show how the problem may be transformed into a Hermitian matrix eigenvalue problem which is convenient for computational purposes.

The asymptotic behaviour of the charge density distribution for an electron bound by the Coulomb potential in the ground state and interacting with the vacuum electromagnetic field is calculated perturbatively. The radiative correction to the unperturbed exponential behaviour at large r limit has the form r e^{-2m alpha r}, with a coefficient proportional to the radiative energy shift. It is shown that the same asymptotic correction of the charge density distribution can be derived within the Welton model.

The calculation of the one-electron finite nuclear size corrections to the energy levels of multicharged ions by the known nuclear charge distribution moments (r²) and (r⁴) using the effective radius method is considered. Approximative formulae which allow calculation of the one-electron nuclear size corrections at intervals Z=1-100 (Z is the nuclear charge) with a relative error approximately=0.2% using the known moments (r²) and (r⁴) are proposed.

Photoionization cross sections, oscillator strengths and energy levels of the silicon-like ions, Si⁰, S²⁺, Ar⁴ and Ca⁶⁺ are calculated in the close coupling approximation using the R-matrix method. A large number of bound states with n<or=10 have been considered and oscillator strengths for transitions among all these states and photoionization cross sections of all the bound states are obtained. Partial photoionization cross sections of the ground state into various excited states of the residual ion are also obtained for each ion. Detailed comparisons have been made for the calculated energies, oscillator strengths and photoionization cross sections with available theoretical and experimental values.

The authors report upon advanced measurements of absolute photoionization cross sections from a laser-excited Ba I state. Progress has been achieved by improving the thermionic diode technique. They now obtain better resolved spectra with an excellent signal-to-noise ratio. Absolute cross sections are given from the threshold of the 6s continuum at 417 nm down to 370 nm. For the prominent resonances they have fitted Beutler-Fano profiles the parameters of which are presented. Comparison of their data with recent publications, experimental as well as theoretical, show satisfactory to excellent agreement.

An extensive study of the fragmentation patterns of photoionization from 4d_5/2 ionization in xenon observed in coincidence with the subsequent N₅-O₂₃O₂₃¹S₀ Auger electron is given. It is based on the two-step model for photoionization and Auger decay and on a complete set of experimental data for the photoionization process at 94.5 eV photon energy together with simple and energy-independent factors from the selected Auger transition. Fragmentation patterns are discussed for different kinds of light polarization, for energy- and angle-resolved analysis of both coincident electrons and finally for the case of complete differentiation of the two with respect to their energies, angles and spin directions.

The inner electron in the two-electron atom is excited in the presence of a high Rydberg electron with large orbital momentum. In the initial state the electrons are assumed to be non-correlated. In the final state the Rydberg electron moves in the field of the excited core with degenerate levels. This leads to the appearance of electron-electron correlations. The most important part of them is taken into account in the approximation where only the dipole term is retained in the multipole expansion of the electron-electron interaction. The positions and strengths of the lines in the excitation spectra are calculated. The factorization of the transition matrix element typical to the ICE (independent core excitation) picture is shown to be retained. The outer (Rydberg) electron transition is described by the overlap (shake-off) radial matrix element. These are not essentially influenced by the long range interaction with the core.

The authors present an analytical model of strong-field photoionization by a short-pulse laser of an electron bound in a short-range potential. They confirm its predictions by numerical evaluation of the electron's space- and time-dependent wavefunction. They show that wavepacket spreading alone makes an important contribution to the photoionization process for times shorter than a specific critical time T* that is directly proportional to the electric field strength of the laser. They refer to this regime as the spreading regime. They find that stabilization occurs for sufficiently strong fields, and that the characteristics of stabilization can also be observed in the spatial distribution of the final wavefunction as well as in the time-dependent ground state probability. Secondly, by studying the detailed nature of the electron wavefunction during both abrupt and non-abrupt switching-off of the laser pulse, they have identified a function that can be conveniently evaluated during a laser pulse, but which accurately provides the value of bound state probability at the end of a smooth switch-off.

A sequential model of multiple vacancy production according to which the vacancies are created in successive collisions of a fast ion with electrons belonging to a definite shell of an atom, with different single electron ionization probabilities, is discussed. A simple formula for the probability distribution of the multiple vacancy production is derived in the framework of this model which must be valid for sufficiently highly charged ions. Comparison of this formula with experiments shows that the sequential model allows discrepancies between the widely used binomial distribution and these experiments to be explained without invoking any fitted parameters.

In the energy range of a few keV the orientation and alignment in electron capture by proton impact from polarized Na*(3p) atoms is studied in a semiclassical molecular model. The adiabatic basis functions, obtained using a model potential method, are modified by a common electron translation factor. The orientation effect is interpreted in terms of a three-state molecular model at large impact parameters 10 au(b(20 au even though the detailed study of the total cross sections requires at least 14 states of positive symmetry.

Eikonal wave Born calculations for the relativistic (e,2e) experiments carried out recently in Tubingen are presented. It is shown that even for large projectile energies the triply differential cross section (TDCS) is very sensitive to the value of atomic field. The idea is put forward that the cross section peak angle shift in the case of symmetric (binary) (e,2e) experiments can be used for direct determination of a mean atomic field value.

The authors extend the formalism introduced by Klar and Schlecht (1976) and more completely developed by Rau and Greene (1982, 1983) to explore the implications of the Wannier threshold theory for the spin dependence of electron impact ionization of atomic hydrogen. By analysing the properties of the contributions to the singlet and triplet cross sections from the multiplicity of (L, S pi ) final states and by combining the results of the investigations of spin-asymmetry measurements of Guo et al. (1990, 1993) and the earlier investigations of spin-averaged cross section measurements of McGowan and Clarke (1968), the authors are able to provide insight into the role played by higher order terms and the possible influence of collisional dynamics. The authors further show that the critical energy above threshold at which the contributions from higher order terms become visible is substantially lower for the singlet and triplet partial cross sections than it is for the spin-averaged cross section.

The pronounced threshold peaks found in cross sections for electron-impact excitation of the vibrational levels of HF have been studied using an R-matrix method, in which electron exchange, correlation effects and polarization are included. Vibration is modelled inside the R-matrix sphere using the energy-modified adiabatic phase matrix method (EMAP). Rotation of the target molecule is treated outside the R-matrix sphere, using the frame transformation method. The external rovibronic close-coupling equations are solved by analytic methods. Analysis of the multichannel threshold structures indicates that the peaks found for rovibrational excitation of nu =1 can be interpreted as virtual states while those for nu =2 correspond to nuclear-excited Feshbach resonances. These two different results correspond simply to a change of sign of a small parameter that is determined by the threshold analysis.

The authors present simple estimates of the binding energies of clusters of rare-gas atoms and excitation energies of low-lying states in trimers, which are obtained by formally writing the Schrodinger equation in d dimensions and expanding its solution in power of 1/d. Comparison with Monte Carlo calculations for Lennard-Jones clusters Ne_n, Ar_n, Kr_n, Xe_n n=3-7 and 13 shows good agreement, and suggests that the authors' formulae may be used to estimate the energy of rare-gas atoms interacting through relativistic potentials.