Table of contents

The oscillator strengths and excitation energies for the ns²¹S₀-nsnp ³P₁, ¹P₁ transitions in neutral cadmium and mercury calculated in the configuration interaction (CI) approach based on numerical relativistic SCF wavefunctions are compared with the results of a multiconfiguration relativistic Hartree-Fock (MCRHF) scheme, which employs the same set of configurations. It is found that with careful choice of the method used to generate the valence orbitals, the relativistic configuration interaction treatment yields results comparable with those of the MCRHF approach. The influence on oscillator strengths and excitation energies of the additional nd² and npnd configurations, which could not be included in MCRHF calculations because of difficulties with the convergence of the SCF process, is also studied and is found to be very small.

Expansion of the recently suggested relativistic two-body equation (Pilkuhn) to the order 1/c² does not reproduce the Breit potential. Nevertheless, the equation gives the known result for the quadrupole moment in the ground state of the hydrogen atom contrary to Pilkuhn's assertion (Ibid., vol.17, p.4061, 1984).

A reply is made to a criticism by Yelkhovsky of an earlier paper by Pilkuhn.

The scattering of electrons by the N⁶⁺ ion has been studied in the vicinity of the intrashell autoionisation levels (3l3l') of the N⁵⁺ ion using a 14-state pseudostate close-coupling approximation. The resonance parameters (energy and width) are determined for the autoionisation states. The present results are in good agreement with those obtained by the complex coordinate rotation method. The energy values of these states are supported by recent experimental measurements of the ejected-electron energies following the autoionisation.

Distorted-wave perturbation theory with special reference to CDW1 (Crothers and McCann (1983)) is used to calculate the absolute cross section for electron capture to the continuum. Results obtained for the impact of fast fully stripped ions of oxygen, carbon, helium and hydrogen upon helium atoms are compared with the measurements of Andersen et al. (1986). Theory and experiment are found to be in excellent accord for the cross section magnitudes, but are in disagreement for the cusp asymmetry parameter.

The authors discuss the possibility of producing antihydrogen in collisions of antiprotons, p with positronium, Ps. Theoretical results of the cross section for this scattering channel are presented for the range of impact velocities from 0 to 4.6*10⁶ m s^-1 together with a discussion of the demands which must be met in order to make an experiment of this kind feasible.

Using the variational perturbation approach, mean static dipole polarisabilities for the first transition series, resulting from the contribution to the total polarisability of the polarisation of the 4s shell, are calculated. Two different sets of calculations are performed. With the exception of zinc, the polarisabilities calculated using a 4p polarising function, determined by minimising the second-order energy functional and representing the zero-order wavefunction by the Hartree-Fock configuration 3dⁿ(^2S+1L)4s² and each component of the perturbed function by the configurational function 3dⁿ(^2S+1L)4s(^2SL)4p, are within 2% of that obtained from the theoretically more correct calculation, consisting of a 4p correlating function, determined to describe the important near-degeneracy effect 4s² to 4p², and a 5p polarising function, determined by optimising the second-order functional. In the latter optimisation the unperturbed wavefunction is represented by the Hartree-Fock single configuration 3dⁿ4s², augmented by the near-degenerate configuration 3dⁿ4p², and each component of the perturbed function consists of the most important polarising and configurational functions. In addition, it is shown that the configurations 3d^n-14s²4p and 3d^n-14p³, from the 3d to 4p replacement, can be eliminated from the perturbed-states expansion, as their inclusion in the configuration interaction expansion of the perturbed states causes less than a 1% change in the polarisability.

Previous work on the relativistic Thomas-Fermi treatment of total energies of neutral atoms is generalised to heavy positive ions. To facilitate quantitative contact with the numerical predictions of Dirac-Fock theory, Darwin and Breit corrections are expressed in terms of electron density, and computed using input again from relativistic Thomas-Fermi theory. These corrections significantly improve the agreement between the two seemingly very different theories.

The authors derive exact expressions for the spectrum of spontaneous emission and the scattering probability of light from a regular lattice of two-level atoms, including collective decay and dipole-dipole interactions. The lattice may extend into one, two, or three dimensions. The decay rate and the lineshift are expressed as lattice sums, which are calculated numerically for a chain of atoms.

The resonance fluorescence of a two-level system irradiated by a smooth laser pulse is investigated. The author calculates higher-order correlation functions of the electromagnetic field explicitly in the limit of a very short pulse and demonstrate the presence of both antibunching and squeezing of the scattered light.

The authors describe measurements of spectral line intensities for pairs of transitions having common upper levels and thus derive the branching ratios of their spontaneous radiative transition probabilities. These are then combined with the results of measurements of the radiative lifetimes of the upper levels by other authors to obtain values of the individual probabilities. Care was taken to avoid the effects of the trapping of resonance radiation, which could have invalidated the results. A particular feature of their measurements was the use of two spectrometers, spanning between them the wavelength range from less than 100 AA to over 7000 AA. The results are for transitions in N IV, O V and Ne VII and are given with a claimed accuracy of between 7 and 38%. These are compared with theoretically calculated values. Good agreement is found for some of the simpler electric dipole transitions. On the other hand, for some of the other transitions, which in certain cases are only possible because o configuration interaction, disparities between their measurements and theory are as large as a factor of five.

Higher-order multiphoton ionisation enhanced resonantly by Raman-type transitions is studied theoretically. This higher-order process is found to be stronger than the non-resonant lowest-order one by approximately two orders of magnitude. The shape, position, width, height and area of the resonance peaks that appear due to this process are calculated. Qualitative agreement is found between the theory and the experiment of Feldmann et al. (1986).

Irreducible tensorial sets of the point-symmetry groups are used to determine the general formulae for calculating the matrix elements of the reduced dipole moment operator of a polyatomic molecule. These general expressions allow different types of intramolecular interactions and accidental resonances to be considered. Transitions from one type of molecule to another are accounted for by substitution of isoscalar factors and 9 Gamma symbols of the point-symmetry group in the formulae. As an illustration the general expressions obtained are used to analyse the dependences of matrix elements of the reduced dipole moment on the rotational quantum numbers for some vibrational absorption bands of molecules with D_3h, C_3v and T_d symmetries. For allowed transitions contributions to the reduced dipole moment operator which are linear and quadratic in J_alpha are taken into account. for forbidden transitions the main contributions are taken into account. It is shown that the general formulae obtained describe the generalised Herman-Wallis effect (1955) for polyatomic molecules.

Thin targets of Dy and Yb have been bombarded by 1.2 to 3.0 MeV deuterons. The resulting L-shell X-rays have been analysed to determine L_i-subshell ionisation cross sections, sigma _L(1)^exp. The measured cross sections and their ratios are compared with the predictions of the (ECPSSR) theory. Experimental sigma _L(1)^exp values display a sharper dependence on the deuteron energy than predicted theoretically. The ECPSSR consistently underpredicts sigma ₍₂₁₎^exp by as much as 30%. In general sigma _L(1)^exp is described reasonably although disagreements by 12% are noticed for Dy. Similar discrepancies are observed for the total cross section data in the case of Dy whereas for Yub agreement is closer.

L-subshell X-ray production cross sections were measured for Ta and W bombarded by 0.20 to 0.90 MeV u^-1 protons and deuterons and by 0.2 to 0.45 MeV u^-1 alpha particles. L_alpha L_{gamma 1} and L_{gamma 2,3} lines were selected to convert X-ray production cross sections into ionisation cross sections. The deuteron to proton-induced cross section ratios show that the Coulomb and energy loss effects are well accounted for in all subshells. The alpha to deuteron-induced cross section ratios show that the theory overcorrects the binding effect, the theoretical ratios being lower than the experimental ones by roughly 25% for the L₃ subshell and 75% for the L₂ subshell. It is argued that this difference between the ratios, and the poor agreement on the L₂-subshell absolute cross sections, may be accounted for by assuming a 10-20% deviation in the values used for the fluorescence and Coster-Kronig yields and for the emission rates; such a deviation may be induced by the projectile.

Single electron capture into selected states has been studied by means of translational energy spectroscopy for collisions of multiply charged Kr^q+ ions (q=2, 3 and 4) with rare-gas atoms, at collisions energies of 3q keV, with an energy resolution of 0.3q eV. The measurements show that as the atomic number Z of the target increases, i.e. as its ionisation potential decreases, electron capture into higher excited states of the projectile product ion become more important. State-selective single electron capture cross sections are measured for Kr⁴⁺ ion colliding with He and Ne in the collision energy range from 8 to 24 keV by means of translational energy gain spectroscopy. The single electron capture cross sections in collisions of 3q keV Kr^q+ (2<or=q<or=6) with He are found to be oscillating functions of q.

The authors present a brief discussion of first-principle calculations on the elastic scattering of low-energy electrons by Hg, Tl, Pb and Bi atoms. The calculations were carried out within a generalised Kohn-Sham one-particle theory which accounts for relativistic effects. Recently published data by Kaussen et al. (1987) on the scattering-induced spin polarisation of the projectile show good agreement with their results and suggest spin-orbit coupling, rather than the so called 'fine-structure effect', to be the dominant mechanism in the process of polarisation. This is further substantiated by comparing their calculations for Tl targets with those of Bartschat et al. (1984) on spin polarisation at very low impact energies. As distinct from the latter authors, they do not allow for the fine-structure effect but nevertheless arrive at qualitatively the same results.

The authors report accurate variational calculations of the ^1,3S and ^1,3P phaseshifts for the elastic scattering of electrons (or positrons) by positronium up to the n=2 threshold. Higher-order phaseshifts are evaluated both in a static-exchange model and in an adiabatic-exchange model to give the total elastic, momentum transfer, ortho-para conversion and elastic differential cross sections. The positions of the minima in the elastic differential cross section are given for several values of k². New ³P resonances are reported. The ¹P continuum wavefunction is used together with an accurate variational wavefunction for the ground state of Ps^- to obtain the photodetachment cross section of Ps^-, and both length and velocity results are checked by evaluating the sum rule S_-1.

Unpolarised electrons were scattered elastically from unpolarised Hg, Tl, Pb and Bi atoms and the polarisation of the scattered electrons was measured for scattering angles between 30 degrees and 130 degrees in the energy range from 6 to 180 eV. The influence of the outer-shell configuration of the target atom on the polarisation is discussed. The results are compared with a recent calculation by Haberland and Fritsche (1987). To a good approximation the elastic collision process can be described by potential scattering in the energy range studied.

The authors have studied the orientation and alignment of the 2¹P and 3¹P states of helium caused by electron impact at incident energies between 50 and 80 eV both experimentally and theoretically as a function of the electron scattering angle. The angular correlations between inelastically scattered electrons and VUV de-excitation photons were measured, particularly in the region where the orientation changes sign. The experimental results were compared with several distorted-wave models. The best of these models gave reasonably good agreement with the experimental data for excitation of the 2¹P and 3¹P states for scattering angles of less than 60 degrees and qualitative agreement for larger angles.

The ortho-positronium (o-Ps) quenching reactions promoted by K(CoEDTA) ((CoEDTA)^-=ethylenediaminetetraacetatocobaltate), K₂CrO₄ (Co(NH₃)₅H₂O)Cl₃ and (Co(DDTP)₃) ((Co(DDTP)₃)=diethyldithiophosphatocobaltate) were investigated in different solvents and at different temperatures. It was found that the reactions are diffusion controlled and that the rate constants k of the compounds undergoing ionic dissociation depend on the dielectric constant epsilon of the reaction medium. A relationship between k and epsilon is suggested. The observed effect, independent of its explanation, also has a practical bearing on the method applied up to now in order to ascertain whether an o-Ps quenching reaction is diffusion controlled or not.

The author predicts that bound atomic or molecular states can be dressed by an applied field in a fashion analogous to the pseudo-autoionising structure induced in a photoionising continuum. Just as the continuum structure can enhance the photoionisation rate, the induced virtual structure between real states can be used to enhance multiphoton processes by tuning one of the intermediate fields near this virtual resonance. The author concentrates on stimulated Raman scattering, but the connection to multiphoton absorption is pointed out.