Table of contents

The authors question the claim made by Heck and Gauntlett (ibid., vol.19, p.3633, 1986) that the spin polarisation resulting from the spin-orbit interaction in the hydrogen atom after electron impact excitation to the n=3 states significantly influences the evolution of the atoms in the intermediate n=2 state of the n=3 to n=2 to n=1 cascade. They also show how some of the authors' formulae can be simplified and hence analysed without using numerical methods.

The authors calculated configuration interaction wavefunctions for the ground (²P⁰) and the 3s3p⁶; 3s²3p⁴s; 3s²3p⁴nd (n=3,4,5,6)²S states of Ar II. The major correlation effects are included, as are both correlation and spectroscopic d functions. The wavefunctions are used to calculate energies and oscillator strengths of ²P⁰-²S transitions. The spectroscopic factors of the ²S states, calculated from the wavefunctions, are compared with values derived from recent (e,2e) and photoelectron spectroscopy (PES) experiments, The calculated values agree very closely with the PES results, but not with the (e,2e) values.

The ratio R* between the cross sections for transfer ionisation and single capture is determined experimentally for 1 MeV H⁺ and 0.35-1.5 MeV amu^-1 He²⁺ ions colliding with He atoms. The experimental data are compared with recent theoretical results based on direct and rearrangement mechanisms. For even higher projectile velocities, some capture processes lead directly to transfer ionisation, and that this fact should be taken into account when the asymptotic value of R* is estimated.

The first Born approximation is used for the calculation of cross sections for the ionisation of helium by fast (8 keV) electrons. Both Coulomb and distorted wavefunction are used to describe the ejected-electron. For the target ground state either Hartree-Fock or correlated variational wavefunctions are employed. Comparison with experiment seems to indicate that at low ejected-electron energy the choice of ejected-electron wavefunction is critical, but at higher (100 eV) energies the degree of correlation in the target initial-state wavefunction is more important.

A broad shape resonance built on the electronic ground state ¹A₁ of CH₃CN with an extra electron in the 3e molecular orbital (antibonding pi *C identical to N) is observed in the vibrational inelastic electron scattering around 2.8 eV.

The level distribution of an electronic configuration cannot be described in a simple and exact way according to the value of the total angular momentum J. Starting from the calculation of variances of the quantum numbers M_J and M_L, and using a Gaussian model, approximate formulae are derived for the number of levels with a fixed J, for the total number of levels, for the number of Russell-Saunders terms of a given (S, L), for the total number of terms with a fixed S and for the number of levels of a jj term. From these results, closed formulae are derived for the total numbers of electric dipole lines in any transition array between two configurations, in intermediate coupling and in Russell-Saunders coupling, with an uncertainty of about 2%. Then, the statistics of the line strengths of an array is studied, and a simple distribution function is presented. When this simple model is used, for example, to calculate how many intense lines are expected to appear on top of an unresolved transition array, the agreement with exact calculations is good, except in very special cases where a sophisticated model is needed.

A method, correct to order alpha ², is suggested for the relativistic treatment of pair correlation. Projection operators for positive energy states are applied to the two-particle equation which describes the interaction between the electrons. After elimination of two of the four spatial components-each with four spin components-of a relativistic pair function, two coupled equations for the 'large-large' and 'small-small' components, rho ^LL and rho ^SS, remain. By neglecting certain terms in the coupled equations, the occurrence of homogeneous solutions with one electron in the positive and one electron in the negative energy continuum is avoided, and an equation for rho ^LL alone is obtained, which can be solved using numerical methods developed for the non-relativistic problem. For the Coulomb interaction, this procedure is found to be correct to O( alpha ²) even without the use of projection operators, whereas for the lowest-order Breit interaction (exchange of a single virtual transverse photon), projection operators give corrections to the pair function already in O( alpha ²), thereby removing the well known alpha ² errors that occur if the Breit interaction is used in a higher-order calculation neglecting projection operators. If projection operators are included, the Breit interaction can be treated to higher orders without causing spurious alpha ² terms. By means of an iterative procedure, pair correlation effects, due to the instantaneous Coulomb interaction and the exchange of single virtual transverse photons, can be evaluated to all orders.

The authors report the first high-resolution measurement of hyperfine structure (HFS) in the 4f¹² configuration of ¹⁶⁷Er III. Together with recent measurements of HFS in the 4f¹²6s² configuration of ¹⁶⁷Er I and in the 4f¹²6s configuration of ¹⁶⁷Er II, these data constitute the first detailed description of HFS along several ionisation stages in a rare-earth element. Based on ab initio, multiconfiguration Dirac-Fock calculations of the magnetic dipole and electric quadrupole coupling constants for experimentally observed levels, several conclusions are drawn in a discussion of the present status of understanding of HFS in typical rare-earth system.

The spectra of the neon-like ions Ni XIX and Cu XX have been studied by beam-foil techniques. About 30 lines have been used in each spectrum to derive the energy levels of the 2p⁵3l configurations. The identifications are supported by systematic theoretical studies using the Slater-Condon parametric method with isoelectronic constraints in the fitting process, and the ab initio relativistic parametric potential method. Some earlier identifications have been revised in the sequence Ar IX-Cu XX.

A relativistic screened hydrogenic model is used to calculate the transition energy shifts of Ll, L eta , L beta ₁₅ and L gamma ₁ X-ray lines for a spectator vacancy in the M_v subshell for Z=57 and Z=60-95 in steps of 5. It is pointed out that, although not very pronounced, the relativistic effect if quite interesting for the L X-ray spectra. Relativity is seen to play an opposite role in affecting the energy shifts for LM and LN transitions. A possible explanation of this is presented in terms of the concept of direct and indirect relativistic effects on atomic orbitals. The authors' values for ⁵⁷La agree quite well with the experimental results of Pepper et al. (1975). By comparing these results with the appropriate values of Parente et al. they find that quantum electrodynamic corrections are quite important in calculating L X-ray transition energy shifts.

Spectroscopic experiments have been performed by focusing a 0.263 mu m wavelength laser beam on LiF targets with various transverse dimensions (40<or=OE22 cm^-3 with a gradient length of about 10 mu m.

DIfferential incoherent scattering cross sections for 279, 662 and 842 keV photons in lead, tin and copper at three scattering angle, 20, 30 and 40 degrees , have been measured. A photon counter with a uniform spectral sensitivity was used for the detection of the scattered photons. The scattering geometry used in this experiment was also of a special type, namely the surface of revolution geometry, which has several advantages over the conventional ring geometry. From the measured differential cross sections the corresponding incoherent scattering functions were estimated and were in good agreement with theoretical values.

The effect of an interloper level on a Rydberg series is examined emphasising the effect on q values. The conditions in which q reversals may occur are given, are are related to other Fano parameters.

The elastic scattering of intermediate velocity protons by atomic hydrogen and helium is analysed within the framework of the second-order optical potential method, taking into account the charge-exchange channel. For H⁺-H scattering the results are in good agreement with the measurements by Rille et al. (1984), while for the H⁺-He case there is a significant discrepancy with the experimental data of Peacher of el. (1982). Results for incident antiprotons are also presented.

The alignment and orientation of the excited electron cloud of the Li(3²D) state created at impact parameters between 0.2 and 1.1 au in 2.5-15 keV Li⁺-He charge-exchange collisions have been studied by means of Stokes parameter analysis of the photons emitted in the subsequent 3²D to 2²P transition. The experimentally determined coherence parameters are qualitatively in good agreement with calculated parameters, even in scattering regions where two-electron processes could be expected to contribute to the creation of Li(3²D). The calculated parameters are obtained by means of the molecular-orbital expansion method, taking molecular translations factors into account. The quantitative discrepancies are attributed to limitations in the calculations since two-electron processes and interactions between the Li(3²D) state and higher-lying states are neglected.

Partial and total cross sections of single electron capture processes from He by O⁶⁺ are calculated for the velocity range from 0.075 to 0.53 au . A molecular orbital expansion method including electron translation factors has been employed along with classical trajectories. The adiabatic potential energies and wavefunction for the (O+He)⁶⁺ system were generated using a pseudopotential technique. The cross sections were calculated using seven- and eight-channel close-coupling treatments. Calculated total cross sections are relatively independent of velocity, with a value of about 10^-15 cm². However, partial cross sections are more sensitive to velocity in the lower velocity region. These findings agree well with experimental observations and results calculated with an atomic orbital expansion method.

High resolution translation energy change spectra of product ions of electron capture collisions of CS₂²⁺ and CS₂³⁺ with rare-gas atoms and diatomic molecules, H₂, N₂ and O₂, have been measured using a modified, reversed geometry, double focusing mass spectrometer. Electron capture occurs selectively into the ground electronic state except in the case of CS₂³⁺ collisions with Xe and O₂. In the former case significant target excitation occurs whereas in the latter case the predominant reaction result is dissociative electron capture.

Absolute cross sections for double ionisation of Kr^q+ ions (q=1,. . ., 4) and for triple ionisation of Kr^q+ ions (q=1, 2) in single collisions with incident electrons have been measured by using a dynamic crossed-beams technique. Electron energies ranged from below threshold up to 1000 eV for Kr²⁺ and up to 700 eV for the other charge states. With increasing q the cross sections for double ionisation are determined by an increasing relative contribution of single ionisation of the 3d subshell and subsequent autoionisation. The authors' data for double ionisation of Kt⁴⁺ are in good agreement with measurements of Pindzola et al. (1984). For electron energies about 300 eV triple ionisation appears to be dominated by ionisation-double-autoionisation contributions from the 3p and 3s subshells. Between threshold and about 200 eV, however, the observed cross sections sigma _q,q+3 seem to be due to direct multiple-electron processes.

The excitation of nitrogen 1s electrons to the first unfilled pi 2p orbital has been studied in N₂, NO and N₂O by electron energy loss spectroscopy at incident energies of 1400 and 3400 eV. The absolute generalised oscillator strength and its dependence on the momentum transfer have been derived from the angular distribution of the differential cross section over a large range of the momentum transfer. The extrapolated optical oscillator strength has also been obtained. The results are compared with previous experimental results and calculations.

Differential and total cross sections for the excitation of n=2 sub-levels of hydrogen atom by positrons are presented. A consistent version of the distorted-wave approximation method (in which distortion due to static and polarisation effects are incorporated in both the channels) has been used for the calculation of the cross sections and the results are compared with other available theoretical calculations. Coincidence parameters for the 2p state excited by positrons are also reported at various positron impact energies.

Positron lifetime experiments have been performed on H₂ gas at temperatures between 77 and 297 K and in the density range from 12-160 Amagat. The extracted parameters are discussed in terms of current models. In the case of the positronium fraction it has been found that the observed density dependence can, in part, be interpreted using a combined Ore and spur model.

The authors studied in an earlier paper (1985) the production of a fast beam of H(3s) atoms. They investigated the particular problem of a coherent laser excitation of the 3s_1/2 and 3p_1/2 Stark states from the 2s_1/2 metastable Stark state. They measured the 3s_1/2 fluorescence outside the electric field and quantal interference effects have been observed as a function of the electric field strength. A simple theoretical interpretation is proposed which takes into account the quasi-adiabatic evolution of the atom in the electric field under the authors' experimental conditions.

The photon statistics of spectrum components and the cross correlation between spectrum components in resonance fluorescence are discussed. For the collective case anticorrelation between the central component and the sidebands is observed.