Table of contents

The classical Kramers-Gaunt factor g for electric dipole Coulomb excitation is given as an asymptotic expansion to fifth order in the relevant expansion parameter. This formula, together with the known asymptotic behaviour for very small argument, obviates the need to use tabulated values for the classical g in Stark broadening calculations of isolated line widths from multiply charged ions.

The velocity-diagram method is used for determination of critical angles corresponding to three possible mechanisms of Thomas exchange scattering. Formulae for angles and relative velocities before and after collision, as well as kinematically allowed regions, are considered. Generalisation to relativistic collisions is considered using the Lobachevsky geometry. The singularities of the scattering amplitude for the Hulthen interactions (which give as limiting cases both the Coulomb and short-range forces) are studied. These singularities are simple poles in the Coulomb case but have logarithmic terms in the general case. Possible experiments are discussed to search for these peculiarities in atomic and nuclear collisions.

For pt.I, see ibid., vol.19, no.10, p.L331 (1986). The authors present the energy levels of the doubly excited ¹P⁰ states of H^- and He which belong to the manifolds of the N=3-7 threshold of H and He⁺, respectively. They calculate these energies using the hyperspherical coordinate method with a set of basis functions of 100 terms. The energy levels obtained are specified in terms of the supermultiplet classification based on the collective representation of the rovibrator model for the doubly excited states. Their calculated energies are found to be in agreement with other ab initio calculations available at present within an accuracy of a few per cent.

The theory of the electric dipole moment (EDM) of an atom caused by an EDM on the electron is discussed in detail. The absence of an atomic EDM is the non-relativistic limit is examined both formally and numerically. The two strategies which are used in practice to calculate the leading relativistic terms of order alpha ² are extended by the inclusion of all terms of this order. One-transverse-photon exchange involving the EDM and effects due to the Breit interaction are included; the terms so generated are new, occur only in many-electron atoms, and can be identified semiclassically with the nu *B and delta A/ delta t contributions to the electric field seen by the atomic electrons. The need for projection operators to avoid negative energy states is examined. Numerical calculations are made for caesium and thallium; these show that the contribution from the central potential is dominant.

The ionometrically recorded electron excited M X-ray emission spectrum of tungsten has been reported. The gamma region of the spectrum and the line arising due to the transition 3d^-1_3/2 to 5p^-1_1/2 have been studied for the first time by an ionometric method. A satellite, referred to as gamma ' in the literature, has been recorded and assigned to the superposition of some of the transitions of 3p^-1₃2/4l^-1 to 4l^-14d^-1_5/2 group. The line arising due to the 3d-5p transition has been observed to have a large asymmetry towards the lower energy end. The origin of this low-energy component has been discussed on the basis of an interaction between a 5p hole and an unfilled 5d subshell. The possible hypersatellites which fall in this region of the spectrum and which could account for the structure of this line have also been indicated.

In the wavelength range 280-700 nm the authors have observed more than 60 resonances in the production of Ba²⁺ from neutral Ba. All the strong resonances can be associated with Ba⁺ transitions originating in the 6s₁2/, 5dj or 6p_j states. The production of Ba²⁺ is highest for the wavelength range 450-650 nm, where numerous two- and three-photon resonances occur in the multiphoton ionisation of Ba⁺.

In the present paper the author investigates the scattering of soft coherent X-rays by a beam of relativistic free electrons in the simultaneous presence of a strong low-frequency laser field. The author evaluates the laser-induced nonlinearities of the scattering process and discusses the intensity-dependent sum rule emerging in the limit of very low laser frequencies. This paper amends some earlier work on the same problem in the nonrelativistic regime. Some numerical examples of the differential cross sections for the laser-induced nonlinear scattering processes are presented.

Electron detachment in Li^--He collisions in the 1-20 keV amu^-1 energy range has recently been studied theoretically by Vu Ngoc Tuan and Gauyacq (1987), using an 'independent-scattering model'. A detailed test of the independent-scattering model has been carried out by studying the orientation and alignment of the Li(2²P) states created in 2-25 keV Li-He and 3-12 keV Li^--He collisions, using the polarised photon-scattered particle coincidence technique. In the region of maximum excitation probability, a strong preference for population of Li(2p_-1) is observed for both types of collisions, supporting the independent-scattering model for Li^--He collisions. The degrees of polarisation are, however, different for the two collision processes. Correlation between the two outer electrons in Li^- may account for the different polarisations. The scaling law for H^-, Li^-, Na^-, K^- destruction cross sections, which initiated this study, is discussed.

For the collision systems 1.4 MeV u^-1 U³²⁺ and 5.9 MeV u^-1 U⁶⁵⁺ on Ne, transverse (with respect to the beam axis) momentum distributions of recoiling target atoms have been measured applying a time-of-flight technique. In the case of isotropic electron emission, the transverse momenta of the recoil ion p_{R perpendicular to} and the projectile p_{p perpendicular to} after the collision are identical. This allows the transformation of measured p_{R perpendicular to} distributions into projectile related differential scattering cross sections d sigma /d theta . Using such an analysis, the authors have measured differential cross sections in the scattering angle regime of 1*10^-6 rad<or= theta >4*10^-5 rad. The shape, as well as the absolute magnitude of the derived experimental d sigma /d theta is in reasonable agreement with Rutherford differential cross sections at large theta using a projectile nuclear charge of Zp=q_P and a target nuclear charge of Z_T=5. For theta <or approximately=1*10^-5 rad, n-body classical trajectory Monte Carlo (nCTMC) calculations predict that the balance between p_{R perpendicular to} and p_{P perpendicular to} is strongly influenced by the momenta of anisotropically emitted ionised electrons. From the comparison between the derived experimental cross sections d sigma /d theta with the theoretical values, as well as from the agreement between experimental and theoretical transverse recoil-ion momentum distributions, they conclude that the momenta of the ejected electrons have a considerable influence on the scattering dynamics of the heavy nuclei. Furthermore, the calculations indicate that the projectile is scattered to negative angles at impact parameters of b>or approximately=3 alpha ₀ due to anisotropic electron emission and strong polarisation of the target electron cloud by the Coulomb potential of the projectile. The possibility of rainbow scattering at theta approximately=1*10^-6 rad is predicted.

Differential cross sections and Stokes parameters are reported for electron-magnesium scattering computed in the coupled-channels approximation. A configuration-interaction representation is used for the five lowest singlet states which are included in the close-coupling expansion. The calculated differential cross sections for elastic scattering and the resonance (3s²)¹S^e-(3s3p)¹P⁰ transition are in fair accord with experimental data, as are total cross sections for the resonance transition. There are only minor discrepancies in the comparison of the computed Stokes parameters with experiment.

Components of the Stokes vector have been measured using an electron-photon coincidence technique for radiation emitted normal to the scattering plane in the electron excitation of the 3²P state in sodium at several energies from 5.1 to 30 eV. The coherence of the excitation process is probed through the concept of the reduced polarisation. The data are compared with those derived from optical pumping experiments, which are the time-reversed equivalent of the present experiments. In general, very good agreement is found with the data from the two techniques. Scattering amplitudes derived from the components of the Stokes vector are compared with theoretical predictions. It is observed that there is only moderate agreement between theory and experiment.

The R-matrix method is used to obtain effective collision strengths for the beryllium-like ion C III for all transitions between the six n=2 states with configurations 2s², 2s2p and 2p² and the six n=3 states with configurations 2s3s, 2s3p and 2s3d. In addition effective collision strengths between all pairs of these n=3 states are calculated. The calculations are carried out including the first twelve target eigenstates in the R-matrix expansion of the total wavefunction. In order to obtain converged cross sections involving the n=3 states a new no-exchange R-matrix package was developed and is described in this paper. This package, combined with the Burgess sum rule, is used to determine the high partial wave contributions, to the cross section. Results for the effective collision strengths are presented in tabular form for temperatures in the range 10⁴-10^5.8 K.

The authors calculate the light-induced drift velocity of multi-level atoms in the strong collision approximation. The solution is presented in a general form which can be easily applied to various systems. Specific calculations of drift velocities are presented for the system Rb:Ar for the cases of single- and multi-frequency laser excitations. A calculation is also presented for the case of a white light excitation spectrum with a spectral dip, which could be caused by absorption of light by another substance in the vapour, for instance in stellar atmospheres.

Amplitude-squared squeezing of the electromagnetic field is obtained in the Jaynes-Cummings model of an atomic transition coupled to a single-mode quantised electromagnetic field.