Table of contents

See Chen et al., ibid. vol.22, no.18, p.2875 (1989). An unexplained discrepancy between energy positions of C²⁺, N³⁺, O⁴⁺ and Ne⁶⁺ 1s² 3l3l' resonances calculated within the same Feshbach formalism is clarified.

See Bachau et al., ibid., vol.23, no.7, p.L83-85 (1990). The energy positions of 1s²3l3l' resonances of C²⁺, O⁴⁺ and Ne⁶⁺ ions are recalculated using two-electron basis functions constructed from eigenstates of the one-electron model Hamiltonian. The results are in good agreement with the calculations of Macias et al. (1989) and Bachau et al.

The authors present a state-specific, complex eigenvalue approach to the calculation of molecular predissociation resonances. The theory employs correlated electronic wavefunctions and related coupling matrix elements computed on the real coordinate axis. Only the nuclear Gamow orbital along the fragmentation coordinate is complexified. Application to the predissociation of the nu =0, J=0 level of the A² Sigma ⁺ state of HeH has yielded a width of 2.9 cm^-1, in excellent agreement with the recently reported experimental value of 3.1+or-1.0 cm^-1.

Atomic collision experiments using low-energy positron beams have been actively pursued for more than two decades. With greater understanding of the basic physics governing the production of these beams, advances in technology have promoted studies beyond measurements of total collision cross sections. Such work forms the basis of this review and includes topics from elastic scattering, positronium formation and the associated production of positronium beams, excitation, ionisation and attachment.

A relationship between certain integrals of the product of two Laguerre polynomials is obtained. It is shown how this relationship, together with two known Hankel transforms, allows for an alternative derivation of some relations between matrix elements of r^k for hydrogenic ions. A further relation, involving matrix elements of logarithmic functions, is obtained in a similar manner.

It has been demonstrated by Gorceix et al. (1987, 1988) that the effective potentials for the interaction between the electrons, derived in the Feynman and Coulomb gauges, respectively, lead to different results, when applied in a many-body procedure, even when the orbitals are generated in a local potential. This apparent gauge dependence is due to the fact that the standard many-body procedures include only the reducible part of the multiphoton exchange, while the irreducible part does contribute to leading relativistic order in the Feynman gauge but not in the Coulomb gauge. When the irreducible part of the two-photon exchange (crossing photons) is taken into account, this gauge dependence is removed to leading relativistic order. The situation is expected to be the same in higher orders.

Extracule (centre of mass) and intracule (relative motion) pair densities are reported for the electronic ground states of the first-row hydrides, LiH, BeH, BH, CH, NH, OH and FH. For each type of two-particle coordinate and for each molecule, the radial, longitudinal and transverse densities are evaluated from self-consistent field molecular orbital wavefunctions constructed from 3-21G Gaussian basis sets. The radial intracule density I(r), the longitudinal intracule density L⁽ⁱ⁾(z) and the longitudinal extracule density L^(e)(Z) evolve systematically from double maxima functions in LiH to single maximum functions in FH, whereas the extracule radial density E(R), the transverse extracule density T⁽ⁱ⁾(b) and the transverse extracule density T^(e) (B) exhibit a single maximum in every case. These observations are explained in terms of the ionic character of the bond, the number of electron pairs, the probable orientations of the interelectronic vectors and the orientation of the vectors describing the centre of mass of an electron pair.

High-resolution laser-induced resonance fluorescence spectroscopy has been applied to investigate the isotope shift considering 19 lines in the isotope sequence ¹⁵⁴Gd-¹⁵⁸Gd, ¹⁶⁰Gd. From this experimental data the level isotope shifts of the ground term a ⁹D of 4f⁷5d6s² were extracted and show a strong J-dependence. With a parametric description, including also higher-order isotope shift effects, this J-dependence was analysed and the following crossed-second-order isotope shift parameters (in MHz) were determined: z_5d(160-158)=19.2 (1.0), z_5d(160-157)=32.2 (1.0), z_5d(160-156)=36.6 (1.5), z_5d(160-155)=47.1 (1.5), z_5d(160-154)=62.3 (2.0). The normalisation of these parameters with the corresponding mean square nuclear charge radii delta (r²) and the spin-orbit Slater integral zeta _5d was constant for Gd I:z_5d,_norm=z_5d /( delta (r²). zeta _5d)=6.0(0.6)*10^-6 fm^-2. This indicates a crossed-second-order effect of the spin-orbit interaction and the field shift operator. The magnetic and electric hyperfine structure splitting constants for the levels ⁹F_2-4 and ⁹D_2-6 of 4f ⁷5d6s6p for ¹⁵⁵Gd are also presented.

Transition assignments to all the satellites of L beta ₁ spectra reported in the literature have been presented using the HFS data on two-hole atomic state energies. The agreement between the calculated and measured energies and consideration of the relative probabilities of all the 2p^-13l^-1-3l^-13d^-1 transitions have been used as the basis for deciding the origin of a satellite. All four satellites observed in this region of the X-ray spectra of various elements are mainly emitted by 2p^-13d^-1-3d^-2 transitions.

The peak separation and peak intensity ratio of the allowed (4³D-2³P) and forbidden (4³F-2³P) components of the He I 4472 AA line were measured in a helium plasma with an electron density of 6*10¹⁶-8.9*10¹⁷ cm^-3. The observed peak separations agree fairly well with theoretical calculations at electron densities below 1.5*10¹⁷ cm^-3, but for electron densities over 1.5*10¹⁷ cm^-3 the deviation of the observed results from the calculated ones becomes larger with increasing electron density. The authors suggest that at electron densities over 1.5*10¹⁷ cm^-3 interactions with n=5 levels may contribute to the deviation from the calculated results.

Quantum mechanical and semiclassical formulae for the spatial distribution of the electrons photodetached from negative ions in a static electric field are derived. Sample calculations for the negative hydrogen ion are presented for photon energies both above and below the zero-field photodetachment threshold. Realistic experimental conditions for observation of photodetached electrons are discussed.

Anharmonic K_{lambda mu nu} parameters of the CH₄ molecule potential function are derived from the experimental rotational, tetrahedral splitting and resonance parameters of CH₄ and CH₂D₂. As an illustration of the correctness of the potential parameters obtained, the original spectroscopic constants of CH₄ and CH₂D₂ are theoretically reproduced. Calculations of the spectroscopic constants of CH₃D and CHD₃ molecules are also given for illustrations.

Quasibound electronic states of the CO²⁺ ion are investigated by means of photoion-photon-of-fluorescence coincidence (PIFCO) and ion kinetic energy spectroscopy (IKES) methods. Energies of the lowest four electronic states (³ Pi , ¹ Sigma ⁺), ³ Pi , ³ Sigma ⁺ and ¹ Pi are accurately measured at 41.25+or-0.05, 41.3+or-0.1, 41.8+or-0.1 and 43.8+or-0.1 eV respectively. The latter two states dissociate in less than 1 mu s, while the ³ Pi and ¹ Sigma ¹ states can survive for a few mu s. Furthermore, the lack of detected fluorescence is discussed and an upper bound is derived for the fluorescence efficiency of CO²⁺.

The O₂²⁺ ion has been studied by means of electron-impact-induced and photon-induced Auger electron spectroscopy and oxygen ion fragment spectroscopy of O₂. The oxygen ion kinetic energy spectrum was recorded by inverting the relevant potentials of an electron spectrometer for the detection of positive particles. The ⁴ Sigma ^- and ² Sigma ^- O 1s initial core hole states have been studied using monochromatised X-ray photoelectron spectroscopy. Potential energy curves for a number of electronic states of the O₂²⁺ dication have been calculated with the complete active space SCF (CASSCF) and multireference contracted CI (MRCCI) methods with a one-particle basis set of medium size ((8s, 6p, 2d)). An analysis of the O₂ Auger electron spectrum based on the computed potential curves of O₂²⁺ is presented. The autoionisation satellites are analysed and these lines correspond to molecular singly ionised final states. One line at 510.7 eV, however, is associated with an atomic-like transition. Two shake-up Auger satellites are identified by a comparison with a recent O 1s shake-up spectrum from O₂.

Balmer alpha emission resulting from the passage of 10-100 keV H₂ molecules through N₂, O₂, H₂O and CH₄ has been studied experimentally. For H₂O and CH₄ Doppler-shifted and unshifted H_alpha emissions were separately recorded. Cross sections have been determined for (i) the production of fast H(3s) atoms, (ii) total H_alpha emission from the fast projectile and (iii) H_alpha emission arising from dissociative excitation of H₂O and CH₄.

Coupled-channel calculations are reported for neutralisation in H⁺+H^- collisions and for ion pair production in H+H(1s) and H+H(2s) collisions for impact energies in a range from 0.15 to 50 keV, using a 23-state basis on each heavy particle. The results are compared with earlier calculations and with the available experimental measurements and, in particular, very good agreement is obtained with the most recent experimental data for neutralisation.

The paper reports on differential cross sections at keV energies for electron capture by various ions of charge q<9 colliding with helium atoms. Whereas the cross section for single capture into the principle quantum number n, the dominant process, is peaked near zero degrees, the cross section for capture into n+1, a less exothermic process, is peaked at large scattering angle. The process involves a population scheme taking place at small internuclear distance. This mechanism was predicted by the 'electron promotion model'.

Single- and double-electron loss cross sections of He, and single-electron capture and single-electron loss cross sections of He⁺ in collisions with H₂, O₂, CH₄, CO and CO₂ have been measured in the energy range 0.3-1.8 MeV. From the measured cross sections, charge-changing cross sections in collisions of He and He⁺ with C have been determined using the Bragg rule. Scaling is examined for electron capture cross sections.

Cross sections for charge transfer reactions of ground O⁺(⁴S) and metastable O⁺(²D, ²P) state ions with H₂ have been measured for reactant ions with 10 to 500 eV kinetic energies. Ground-state ion cross sections range from 0.5 to 0.9*10^-16 cm² and metastable-state ion cross sections are approximately constant at 10*10^-16 cm².

The authors present a new model for the calculation of the annihilation rate of ortho-positronium in gases. The measured positronium pick-off annihilation rate is identified with an expectation value over an ensemble of inhomogeneities of the fluid. A single density inhomogeneity is described with a predefined, suitably parametrised density profile and the expectation value is integrated over a reasonably selected volume in the parameter space. Comprehensive tests of the model are performed to determine the dependences on various model parameters and the results are compared with both experimental data and previously used theoretical models. Results for argon, xenon and methane are presented. The qualitative agreement with the experimental data is better throughout a wide density range than that obtained with previously used theoretical models.

Resonant Rayleigh-type optical mixing spectra for homogeneously and inhomogeneously broadened two-level systems are calculated. Differences in the forms of the spectra, such as symmetry, saturation and AC Stark splitting are presented. The observed differences can be used to discriminate whether the spectrum is homogeneously or inhomogeneously broadened.