Table of contents

Ejected electrons from the triplet states of doubly excited O⁴⁺ ions, produced by 60 keV O⁶⁺+O₂ collisions, have been measured with high resolution zero-degree electron spectroscopy. The triplet states of doubly excited configurations are selectively produced through double electron transfer processes with the O₂ molecular target under single collision conditions. Electrons ejected from the 1s²2pnl and 1s²3lnl' configurations are observed over a wide energy range. Ejected-electron spectra from the O⁴⁺(1s²3l3l') are compared with theoretical calculations. The calculation of energy levels of 1s²3l3l' states has been carried out by the Z-expansion method, where Z is the nuclear charge. Correlation, relativistic and radiative effects are taken into account. Electrons from triplet states are identified by comparison with the calculations and the 'singlet state' spectrum produced by O⁶⁺+He collisions.

Experimental and theoretical studies of excitation of the 3³D state of helium by 40 eV electrons are reported. Experimentally, electron-photon polarization correlations are measured in the scattering plane defined by the incident and scattered electron momenta. These are combined with previously reported linear and circular polarization measurements perpendicular to the scattering plane. After allowing for the effects of fine structure depolarization, parameters describing the shape and dynamics of the excited state produced in the collision are determined. Theoretically, the measured polarizations and excited state parameters are predicted using the convergent close coupling (CCC) theory. Excellent agreement is obtained between the experimental and theoretical results.

In this letter, we use the results of a 15-state R-matrix calculation to search for resonances in e^--He⁺ scattering with positions lying below and converging on to the n=3 threshold of He⁺. The resonant positions and widths are computed and compared with previous calculations and experiments. The excitation function of the 1s-2s transition is obtained and the resonance structures are examined and compared with the calculation of Bray et al. (1993) and the measurements of Dolder and Peart (1973, 1976).

Electron impact excitation rates for the three transitions 2s2p ³P^o-2p²³P, 2s2p ³P^o-2s3d ³D and 2s2p ³P^o-2p3s ³P^o in Ne VII are evaluated using the multichannel R-matrix method. Twenty-six target eigenstates consisting of the six n=2 states with configurations 2s², 2s2p and 2p², and the twenty n=3 states with configurations 2s3s, 2s3p, 2s3d, 2p3s, 2p3p and 2p3d, are included in the expansion of the total wavefunction. Comparisons are made with experimental data of Lang (1983) for which differences of up to a factor of 5 were found in comparison with earlier theoretical excitation rate values. The rates produced by the present sophisticated calculation are however in excellent agreement with these previous theoretical results, the greatest disparity being less than 20%. Therefore discrepancies of between 60% and a factor of 4 still remain between theory and experiment for all three transitions considered.

We show that the expansion coefficients appearing in the general expression for a coupled atomic wavefunction satisfying Brillouin's theorem must satisfy a matrix equation. In some circumstances this matrix equation is underdetermined, so that the coefficients of the Brillouin expansion are not unique. This is illustrated by considering some examples.

Simple formulae for calculation of the hyperfine structure of hydrogen-like ions taking into account the relativistic and nuclear effects are presented. These formulae are used for the calculation of the hyperfine splitting of the ground state of the ions that are candidates for the experiments at GSI.

A closed analytical expression is obtained for the dynamic polarizability averaged over the angular quantum numbers of Rydberg atoms.

Radiative decay rates for the 2s²2p ²P⁰-2s2p²⁴P intersystem transitions at 233 nm in C⁺ and at 175 nm in N²⁺ have been calculated taking configuration interaction (CI) and relativistic effects into account with the help of the computer code SUPERSTRUCTURE. The present physical model yields satisfactory agreement between the calculated probabilities and recent measurements obtained by recording the time dependence of the photon intensities emitted from the metastable levels in the quartet system 2s2p²⁴P. In particular, the large discrepancies between the new experimental data and previous theoretical results have been considerably reduced in the case of N III.

The a ³P₀-y ³D₁⁰ Ti I transition at 592.2 nm was investigated by using a hollow-cathode discharge. Sub-Doppler resolution was obtained by combining optogalvanic detection with saturation spectroscopy and the non-linear Hanle effect. The upper state Lande factor was also determined by Zeeman intermodulated spectroscopy.

The collisional broadening and shift of the ¹¹⁴Cd intercombination line lambda =326.1 nm (5 ¹S₀-5 ³P₁) perturbed by He and Ne have been investigated using a pressure-scanned Fabry-Perot interferometer. The values of pressure broadening and shift coefficients determined from the experiment compared with those calculated by Czuchaj and Sienkiewicz (1987) and with values resulting from the Lennard-Jones potential.

We attempt to interpret the relative line intensities in the soft X-ray spectrum of Al III ions emitted from a Penning discharge, on the assumption that the 2p⁵3s² and 2p⁵(3s3p ³P) autoionizing levels are excited predominantly by inner-shell ionization from Al II ground and metastable levels, respectively. The plasma has been spectroscopically diagnosed and the Al III soft X-ray spectrum predicted using a collisional radiative model incorporating the experimental plasma parameters and theoretical predictions for the properties of the autoionizing levels. The model includes collisional inner-shell ionization from Al II levels by fast electrons, inelastic collisions with cold Maxwellian electrons and radiative and autoionization decays. The comparison between the experimental and the predicted relative line intensities seems to confirm that most of the 2p⁵(3s3p ³P) Al III levels have large radiative branching ratios. This conclusion is supported also by a comparison between Al III and Mg II spectra obtained in similar excitation conditions. Several 2p⁶3p ²P-2p⁵(3s3p ³P)²P, ²D Al III transitions having radiative yields close to one and large gain cross sections, may be of interest for soft X-ray lasing.

The dynamics of autoionizing Rydberg electron wavepackets in molecular hydrogen are calculated using a multichannel quantum-defect theory (MQDT) approach. The results of the calculation illustrate how the rotation and vibration of the molecular ion core influence the radial dynamics of the Rydberg wavepacket.

Experimental data for the Sherman function of xenon at collision energies between 50 and 150 eV are presented. The main idea of this investigation was to check the reliability of the experimental techniques by using two different and independent methods: the measurement of the polarization after scattering of unpolarized electrons and the measurement of the intensity asymmetry of polarized electrons scattered through the same scattering angle to the left and to the right. The agreement of the data from the two approaches is good. The results are compared with latest theories.

Calculations have been carried out for the scattering of slow electrons from acetylene molecules in the gas phase, using the fixed-nuclei (FN) dynamical approximation and computing elastic cross sections only, integral and differential. Ab initio results at the exact static-exchange (ESE) level have been supplemented by a model local potential describing short-range and long-range polarization forces. The target wavefunction was obtained at the SCE level from a multicentre expansion over Gaussian orbitals (GTOs). Both the static and exchange interactions, V_st and V_ex, were obtained from a discrete basis by using multicentre orbitals and the separable exchange approximation. The final results are compared with available experiments and a detailed search for the 2.6 eV shape resonance is discussed. The very-low-energy computed cross sections also reveal the presence of a Ramsauer-Townsend (RT) minimum.

Semiclassical S-matrix theory is applied to electron-hydrogen scattering near the fragmentation threshold E=0. The relation between resonant scattering (classically chaotic motion for E<0) and ionization (E>0) is worked out with emphasis on the qualitative behaviour of the different types of orbit.

The change in the polarization vector perpendicular to the scattering plane is examined for collisional excitation of mercury atoms by electrons at a collision energy of 40 eV. The three measured polarization parameters for excitation of the states 6 ¹P₁ and 6 ³P₁ give information about the influence of spin-orbit and exchange interaction on the excitation process. The results, which are quite different for the two excited states, are compared with the latest theoretical data.

We present a new theory for calculating dissociative recombination (and dissociative attachment) cross sections applicable for systems with or without curve crossings. The theory uses the R-matrix method to build up N+1 electron 'curves' in an inner region which can be coupled in a full non-adiabatic treatment. We report the first ab initio determination of the low-energy dissociative recombination cross section for ⁴HeH⁺. The cross section is found to be in good agreement with measurements, now disputed, made in a merged beam experiment.