Table of contents

A method used to obtain the first state-selected cross sections for the near-threshold photoionization of argon from an analysis of threshold electron spectra is described. The yield of photoelectrons in the wings of threshold peaks is normalized by division with the efficiency function of the penetrating field threshold electron analyser, convoluted with a Gaussian photon beam profile. Although very non-uniform within the first 150 meV, the analyser efficiency function was found to have a particularly simple analytic form suitable for further numerical treatment. We have discussed the measured ratios of the two components of the lowest Ar⁺ 3p⁵²P state in relation to the spectral repulsion theory of Fano and Cooper (1965, 1968). The potential of the technique is illustrated on the 3p⁴ (³P)3d ⁴F_3/2 ion state cross section, showing enormous resonant peaks. The limitation in the general applicability of the method is also discussed.

We have measured the branching ratios for the decay of multiply excited Rydberg ions produced in slow I¹⁰⁺-Ne, Ar, Kr and Xe collisions by using the coincidence method. The multi-Rydberg ions are stabilized through radiative or Auger decay. We report target dependence of the branching ratios after two to five electron transfers.

Presents an outline of the main observational results to date, and the future prospects for spectroscopic studies of solar X-ray and UV emission in the Russian (former Soviet Union) space programme. The investigations of solar XUV line spectra carried out at the P.N. Lebedev Physical Institute of the Russian Academy of Sciences are stressed. A short description of the coronal project, which aims to study the complex nature of the Sun, is also given.

Doubly excited intrashell states near the lower edge of n² manifolds are studied. The interelectronic repulsion is estimated within the two-electron dipole approximation. When diagonalizing the two-electron Hamiltonian operator one obtains an analytical dependence of wavefunctions on quantum numbers and the nuclear charge Z. The three first terms of the Z^-1 expansion of energies are found as explicit functions of quantum numbers. The comparison with accurate data demonstrates the applicability of the expressions obtained for reliable estimates of the properties of helium-like positive ions and the neutral atom He.

The reliability of current estimates of free-free absorption coefficients of the negative helium ion are assessed. Results by sophisticated versions of the multiconfiguration-self-consistent field, close-coupling acid polarized orbital methods are shown to have converged to within a few per cent of each other for wavelengths greater than 1 mu m over the temperature range 1400 K to 25200 K. A table of coefficients giving the best current estimates of He^- continuous absorption are presented.

The spectral shape of a light-induced drift (LID) signal in alkali-noble gas mixtures has been investigated. Realistic calculations of the velocity dependence of the transport cross sections in different quantum states have been made with the help of the semiempirical potentials of Pascale and Vandeplanque (1974). The anomalous LID spectral profile for the Li-Ne system has been obtained theoretically.

The K partial and total fluorescence yields of lanthanum, in the presence of a spectator hole in one of the three L subshells, are discussed. It is shown that the K partial fluorescence yield can change considerably in the presence of the L spectator hole. Dirac-Fock wavefunctions have been used for the calculation of the dipole radiative transition rates. A statistical weighting semi-empirical formula for the calculation of the relative K radiative rates is proposed, which permits the evaluation of the relative K radiative rates in the presence of spectator hole(s), for middle and high Z elements, with high accuracy. Finally the lifetime of a K hole in a KL hole atomic configuration is calculated and is compared with the lifetime of a K hole in a single K hole atomic configuration. It is shown that the lifetime of the K hole is affected by the presence of the L hole and it depends on the particular L subshell.

A two-step model is suggested to explain relative high harmonic efficiency in different atoms and molecules in the multiphoton regime. We discuss the correlation of the high harmonic yield and the static polarizability for systems with nearly the same ionization potential. The comparison of the theoretical estimations with experimental data shows a good agreement.

Inelastic atomic collisions constitute loss mechanisms for neutral atom traps, and the study of trap-loss rates may provide detailed information on the long-range interaction between atoms and on the dynamics of very slow collisions. Gallagher and Pritchard (1989) have given a simple two-state model for such collisions which includes a radiative transition or a single curve crossing at short range representing, for example, fine-structure-changing collisions. In order to better describe the more complicated case found in alkali metals, we have extended this model to include several excited hyperfine structure levels with curve crossings at long range. We find that collisions in a laser field that are capable of expelling atoms from the trap can be suppressed in some laser frequency regions and enhanced in others, in comparison with the predictions of the simple two-state model. Using this model to represent radiative escape via the 2_u potential of alkali dimer molecules we are able to greatly improve agreement between theory and experiment, including new measurements on ⁸⁵Rb reported here.

Proton-induced differential and total alignment is reviewed and some calculations presented. A density matrix formalism is employed as the theoretical framework and a more efficient expansion into state multipoles used. The collision T-matrix elements entering the expression for the alignment tensor are calculated in the semiclassical approximation to first order. A coordinate space formulation including classical hyperbolic trajectories for the projectile path is adopted here, to take proper account of deflection. Relativistic Hartree-Fock orbital wavefunctions for bound and continuum electronic states in the partial-wave expansion cover both screening and relativistic effects in the atom in a unifying scheme. The continuum orbitals are calculated iteratively until the frozen core V^N-1 potential converges.

Within a semiclassical overbarrier model for non-destructive, long-range charge exchange, the population dynamics of target and projectile levels, charge-state evolution and electron emission during the scattering of 50 keV N⁵⁺ ions on C₆₀ are simulated. The model describes the transient formation of hollow projectiles due to resonant electron capture and Auger relaxation of the multiply excited projectiles. Estimates for total cross sections in charge-state-changing collisions and the deflection functions for the dynamical Coulomb trajectory are given. Evidence for a strong enhancement of the angle-differential scattering cross section is found and related to the dielectric response of the target.

We reply to a paper by Kernoghan et al. entitled 'On structures seen in the Ps(1s)+H(1s,2p) approximation to positron scattering by atomic hydrogen' (see ibid., vol.27, p.L211 (1994)).

We have examined elastic and inelastic scattering of electrons from potassium. For the case of elastic scattering, we have compared potential scattering theory and perturbation theory. If one wishes to use the potential scattering approach, it is shown that both the non-local effects of polarization and absorption must be properly modelled to get accurate elastic differential cross sections. The second-order distorted wave results For elastic scattering and excitation of the (4p)²P and (5p)²P states are compared with existing experimental data and a very recent close coupling calculation for energies between 7 and 100 eV.

We consider the generation of harmonics during the scattering of laser radiation from a metal surface. The metal will be described by Sommerfeld's free-electron model and we artificially assume the metal to have an infinitely large work function. Contrary to what has been observed for a real metal, in our model the evaluated harmonic rates show the formation of a plateau, if plotted as a function of the harmonic order n. Apparently, the depth of the potential well has some influence on the plateau-formation.

In our topical review we made the point that the early work of Goeppert-Mayer (1941) on orbital collapse had remained neglected until the late 1960s when the advent of fast computers made it possible to calculate the effect more easily, and we cited the work of Griffin et al (1969) on orbital collapse.

It has since been brought to our attention that there is a slightly earlier paper by Rau and Fano (1968) which does not actually calculate any radial wavefunctions, but studies the influence of the centrifugal term on the radial potential and draws attention to its effect on the long periods of the Periodic Table. It is based on the numerical calculations of Herman and Skillman (1963).