Table of contents

A simple extension of the classical over-barrier model for electron capture by highly ionised slow projectiles is proposed to describe the angular momentum distribution of the captured electron. The model predicts the average angular momentum and, for sufficiently narrow/distributions, the most probable l value. The authors find reasonable agreement with experimental data of Dijkkamp et al. (1984).

Measurements of cross sections for charge transfer between H^- ions and unexcited B²⁺ and C³⁺ ions are presented. As the relative speed of the reactants is increased from 10⁵ m s^-1 to 8*10⁵ m s^-1 the cross section for B²⁺-H^- collisions falls from 6*10^-14 cm² to 2.3*10^-14 cm² (+or-10%). For C³⁺-H^- interactions the corresponding cross sections are 8.2+or-10^-14 cm² and 3.15*10^-14 cm² (+or-10%). When plotted against relative speed the results for B²⁺-H^- are similar to earlier measurements for charge transfer between ³He²⁺ and H^-. Comparisons are also made with previous measurements of mutual neutralisation of H⁺, He⁺ and H₂⁺ with H^-.

The theory and applications of the hyperspherical harmonics (HH) method are considered for a Coulomb many-body system. An iteration scheme for obtaining the exact solution of the Schrodinger equation has been applied to the helium atom. This method enables one to take into account many thousands of basis functions in the HH expansion of the wavefunction. It is shown using an approximate 'supersymmetry' property of the HH expansion that, for K>or approximately=K_max, only a limited number of the HH may be retained in the expansion. Although the calculated energies have a stepwise dependence on K and they converge rather slowly as K to infinity , it is possible to develop simple asymptotic formulae which allow one to obtain accurate estimates of the eigenenergies. The error in the calculated energy of the 1¹S of ^infinity He is less than 10^-5, and that of the 2¹S state is less than 0.5*10^-4 (in relative units) when this method is applied.

The transition probabilities and radiation widths of the 4f¹³6sn_Rl_R (n_R=10-50, l_R=0; 1) Rydberg states of the Tm atom have been calculated in the electric dipole approximation. Special attention has been paid to the manifestation of non-Coulombic properties of the Rydberg states considered. The calculation has been carried out using relativistic perturbation theory with a model zero-order approximation. The formalism of the energy approach to the atomic processes theory has been used. The contributions of the one- and two-quasiparticle effects to the complex energy matrix have evidently been taken into account. The three-quasiparticle effects have been effectively accounted for.

The hyperpolarisability tensor gamma _nJM for an atomic level with non-zero angular momentum is presented in terms of irreducible parts. General formulae for the irreducible parts are derived. Their properties in constant and time-dependent monochromatic fields with linear and circular polarisation are examined. Some peculiarities of gamma _nJM for atoms in Rydberg states are discussed.

Using HFS-calculated data on doubly ionised atoms in inner shells, the satellite gamma ' observed in M X-ray emission spectra has been assigned to the superposition of the transitions M₃N₂³S₁ to N₂N₅³P₂, M₃N₄³D₂ to N₄N₅³F₃, M₃N₅³D₃ to N₅N₅³F₄ and M₃N₅³F₄ to N₅N₅³F₄. It has been suggested that a comparatively weak feature corresponding to the transition M₃N₃³D₃D₃ to N₃N₅³F₄, expected on the high-energy side of the satellite gamma ', should be looked for.

An ultrasoft X-ray spectrum of Xe in the photon energy region 52-94 eV was studied at various energies of bombardment electrons from 0.14 to 2 keV. Spectrum emitters, the processes of their formation and electronic transitions, at which ultrasoft X-ray radiation appears, were determined. The spectrum peculiarities which are caused by correlation effects were detected: in a number of cases the nature of the correlations was disclosed.

For pt.I see ibid., vol.18, p.3811 (1985). This paper is the second of a theoretical study of subradiance, i.e. of the cooperative inhibition of spontaneous emission by a destructive interatomic interference, a phenomenon which has recently been observed. It is devoted to the particular case of atomic systems with degenerate transitions, i.e. with several transitions of the same frequency and the same polarisation. The interatomic interference which then occurs between the different transitions is responsible, when destructive, for a new sort of subradiance. The phenomenon is studied for a few typical level configurations, using both quantum mechanical and semiclassical models. The first example concerns a four-level configuration with two transitions of the same frequency and polarisation. The other examples concern three-level configurations with a degenerate transition; in the case of the 'degenerate' cascade, new superradiant beats and subradiance effects are predicted.

Complex-energy resonance poles of the S matrix for scattering from a purely repulsive exponential potential are analysed by semiclassical phase-integral techniques. The method leads-by analytic summation of reflections from an infinite number of classical turning points-to a simple closed-form expression for the resonance poles. Results are compared with recent exact computations.

The authors discuss orbiting resonances in two-state curve crossing systems. A semiclassical quantisation condition determining complex-energy resonance poles is derived. Applications to the electron capture collision process N³⁺+H to N²⁺+H⁺ show good agreement with recent exact quantum computations.

Single-charge transfer cross sections sigma _q,q-1, for Ar^q+ (q=2-7) on He and H₂, and for Kr^q+ (q=2-9) and Xe^q+ (q=2-11) on He, Ne, Ar, Kr, Xe, H₂, N₂, CO₂, CH₄, C₂H₆ and C₃H₈, have been measured at energies of about 0.286 keV amu^-1. The scaling properties of sigma _q,q-1 according to the initial charge state q and the first ionisation potential I₁ of the target are discussed, and a law of sigma _q,q-1 varies as q^1.3I₁^-2.0 is presented. This coincides with the prediction of the 'classical absorbing-sphere model' (CASM). The oscillatory behaviour of the cross sections with q as well as I₁ is consistent with the concept of the 'classical one-electron model' (COEM).

Using an electron spectrometer and a CW CO₂ laser the author has measured free-free transitions influenced by scattering resonances. The author has measured this effect in the case of the p-wave neon and argon resonances around 16.1 and 11.1 eV respectively. Resonance structures have been obtained at scattering angles of 20 and 160 degrees for neon and 20, 30 and 50 degrees for argon. The measurements were compared with calculations based on the soft-photon approximation for free-free transitions and on the Fano formalism for the description of the resonances. In the case of neon the agreement is very good whereas in the case of argon a high non-resonant background in the free-free scattering at small angles occurs which is not predicted by the soft-photon approximation.

Differential electron scattering cross sections have been measured for dipole-forbidden and resonance transitions in Mg II, Zn II and Cd II in the angular range 4 degrees <or= theta <or=17 degrees at 50 eV. These provide the first recorded angular distributions for an optically forbidden transition. It is found that while the cross section for excitation of the 4s ²S to 3d⁹4s²²D transition in Zn II is small, those for the 3s ²S to 3d ²D, 4s ²S (unresolved lines) in Mg II, and the 5s ²S to 4d⁹5s²²D in Cd II are comparable in magnitude with the cross sections for resonance excitation. In addition, for Cd II one finds that the allowed and forbidden transitions have very similar angular distributions, and it is proposed that excitation to the ²D state may be dominated by a virtual 'double-dipole' transition via the ²P state. Also, the total excitation cross section of the resonance ²P state in Cd II is a factor of four higher than that predicted by the Gaunt factor approximation, suggesting that the accepted value for the oscillator strength may be too low.

The excitation of the O (3s ³S⁰ to 2p ³P; lambda 1304 AA) resonance transition by electron impact on atomic and molecular oxygen has been studied. The ratio of the cross sections for the specific atomic and dissociative processes, sigma _A/ sigma _D, has been measured accurately from 60 to 300 eV by direct normalisation to the ratio of the specific O⁺ and O₂⁺ ionisation cross sections. At 100 eV, sigma _A/ sigma _D=3.00+or-9%. The experiment was carried out using an apparatus that combined a high-density diffuse gas source (n(O) approximately 10¹² cm^-3), an electrostatically focused electron gun, a vacuum-ultraviolet monochromator and a quadrupole mass spectrometer for simultaneous optical and composition measurements. The present sigma _A/ sigma _D values are smaller by a factor of 2.1 than the original results of Stone and Zipf (1974) and Mumma and Zipf (1971). When revised sigma _A(1304 AA) values are calculated from these new data using contemporary calibration standards, it is found that the shape of the cross section for the excitation of the O (3s ³S⁰) state by electron impact on atomic oxygen agrees well with the earlier work but the absolute magnitude of sigma _A(1304 AA) must be reduced by a factor of 2.8. The revised cross section values are in good agreement with recent quantum calculations when cascade excitation of the 3s ³S⁰ state is taken into account.