Table of contents

A very rapid dissociation of core doubly excited states in a molecule has been observed for the first time. The channel is identified in the Auger electron spectra of H₂S where the resonating structures, at higher kinetic energy than the normal Auger LVV electron lines, display non-dispersive kinetic energies relative to photon energy above the S 2p ionization limits.

The near-threshold double photoionization of an inert gas atom is considered in the case when the residual ion remains in the p⁴³P^e state. The extended Wannier ridge model is used to calculate the electron angular correlation in ³P⁰ and ³D⁰ partial waves in the two-electron continuum. The interference between the related contributions to the triple differential cross section is analysed. The results are compared with recent experimental data for the Kr atom.

We have performed calculations of the triply differential cross section (TDCS) for double photoionization of helium. Our calculations are based on two different ab initio methods, one where the final continuum state is represented by a product of three Coulomb wavefunctions, with the dipole matrix element evaluated directly, the other where the final state is represented by a product of two screened Coulomb wavefunctions, which is substituted into a flux formula that in principle yields exact results. We compare the results of the two calculations with each other and also with two different sets of experimental data, for various geometries, for various ratios of the energies of the two photoelectrons, and for various photon energies such that the total energy of the photoelectrons ranges from 0.6-20 eV above threshold. The theoretical results for the shapes of the TDCS are in good agreement with the experimental data and also with each other, although the calculated absolute values of the TDCS differ. We comment on various features of the angular distributions.

We present a model to describe certain aspects of the multiphoton multielectron dissociative ionization of simple molecules in intense laser fields. We obtain fragment ion dissociation energies and appearance intensities that are in good agreement with experiment.

An intersecting beam technique has been used to measure total cross sections for resonant charge transfer in ³He²⁺-⁴He⁺ collisions at barycentric energies between 4 and 200 keV. Our measurements, obtained using coincident detection of the reaction products, are in good agreement with theoretical predictions. Furthermore, they confirm a scaling law by Reinhold and Falcon (1988) for symmetric resonant charge transfer in one-electron systems at intermediate energies.

The angular dependence of electrons superelastically scattered from metastable (2³S) helium has been measured for the first time at incident electron energies of 10 and 30 eV. For comparison purposes these distributions, which cover the angular range from 35 degrees to 125 degrees , have been normalized to the convergent close coupling calculation of Bray et al. (1994). We also compare the normalized angular distributions to first-order many-body theory and R-matrix calculations.

Excitation of linear triatomics, CO₂ and CS₂, has been studied in 1.8 keV collisions with monoenergetic projectile ions H₂⁺, N₂⁺, O₂⁺ and CO₂⁺ by means of energy loss spectrometry. The dependence of the excitation cross section upon the quantal description of projectile ions is probed. The excitation cross section is largest in the case of collisions with H₂⁺ ions, in conformity with wavefunction overlap computations of the overlap integrals of the highest occupied molecular orbitals of the colliding systems.

Discussion of the parity violation and the break of the time-reversal invariance in diatomic molecules has continued for a number of years. Experiments on the TlF molecule gave one of the most stringent limits on the electric-dipole moment of the proton and on the T-violating nuclear forces. At present, a new generation of experiments with paramagnetic diatomic molecules is underway. These experiments are aimed mainly at the search for the electric-dipole moment of the electron. In this topical review we examine theoretical aspects of parity non-conservation in diatomic molecules. We focus on molecular theory leaving aside the nuclear part of the problem. In this approach the nucleus is characterized by the number of P-odd and P,T-odd moments, namely anapole moment, Schiff moment and magnetic quadrupole moment. Molecular theory has to link these moments to the experimentally measured quantities, such as frequency shifts, etc. The other possible sources of the parity non-conservation in molecules are the electron-nuclear neutral current interactions and the electric-dipole moment of the electron. They are also discussed in this review.

The results of Hartree-Fock-Dirac numerical calculations are given for the magnetic dipole and electric quadrupole hyperfine constants for the K and L_I-III shells for all Z from Z=10-100. A comparison is made with results obtained with simple analytic formulae for the case of hydrogen-like ions with a simple screening and nuclear finite-size corrections taken into account. Formulae are given for the atomic inner-shell hyperfine structure in the form suitable to calculate the hyperfine shifts of atomic states, excited in different processes, such as electron capture or an internal conversion.

Previous work has suggested that non-analyticity of the time-dependent coupling between two states can lead to spurious non-physical contributions to transition probability. We demonstrate that this does not occur for the exponential pulses in the Demkov (1963) and Nikitin (1970) models, nor for the case of a rectangular pulse. Analysis in terms of adiabaticity suggests that the specific properties of the transition probabilities in these models at large detuning are due to pulse shape effects.

We have studied excitation of the ¹S^e(2s²), ¹P^o(2s2p) and ¹D^e(2p²) resonant states of helium by proton impact as a function of projectile scattering angle in the energy range 50-500 keV. Our results are compared with recent experimental measurements of Schulz and co-workers (1995). The ratio of double excitation to single excitation has also been evaluated as a function of the projectile scattering angle. We show that this ratio is representative of the momentum transfer to the electron in the processes involved.

Cross sections (differential and integral) for the electron-impact vibrational excitation of H₂O are calculated at the collision energies 6-50 eV. Excitation of the three fundamental modes of vibration, (100), (010) and (001), is considered separately. The calculation is based on the rotationally sudden and a vibrationally close-coupling method taking an ab initio electrostatic potential. The effects of electron exchange and target polarization are taken into account approximately. The resulting cross sections are compared with the experimental data available. A comparison is also made with the Born cross section, which is often cited in experimental papers. Qualitatively, the present differential cross sections well reproduce the experimental data, particularly the angular dependence of the cross section. Quantitatively, on the other hand, there are some disagreements between the present calculation and the experimental data.

A combination of Floquet theory and the complex coordinate method is used to establish a compact time independent expression for the generation of harmonics by a diatomic molecule. Only the nuclear motion is considered as a source for the spectra. In the assumption that a single Floquet state describes the system, only odd harmonics are obtained. An application to H₂⁺ is presented. Correlation between the multiphoton nature of the absorption process and the appearance of higher harmonics is observed.

The problem of skin depth and Fresnel reflectivity is addressed for dense strongly coupled plasmas (SCP). Frequencies are considered which are less than or close to the electron plasma frequency both for classical and degenerate SCP. For classical SCP the calculations go beyond the Born approximation for the dynamic collision frequency. The correlation effects are treated through the dynamic local field corrections, whose role turns out to be essential at high values of plasma coupling parameter. Preliminary data for degenerate dense plasmas are compared with the results obtained for classical SCP. The effect of a radiation 'cut-off' near the plasma frequency turns out to be less pronounced in classical SCP due to the more collision-dominated nature of this plasma, in comparison with weakly non-ideal or strongly degenerate plasmas. Available related experimental and molecular dynamics data are presented and analysed for comparison with the developed theory.