Table of contents

Approximate expressions are derived for the dipole Coulomb matrix elements for atomic states with a large difference of principal quantum numbers.

The hyperfine structure of the 6sns ³S₁ (14<or=n<or=23) Rydberg states of ^135,137Ba has been measured. From these data the admixture of the 5d7d ³S₁ and ¹P₁ perturbers into the Rydberg states has been derived. A comparison with isotope shift data proves the hyperfine structure of Rydberg states to be the more sensitive tool to detect small admixtures of doubly excited configurations.

Radiative lifetimes have been measured for 15 odd parity levels of neutral platinum using the technique of laser-excited fluorescence from sputtered metal vapour. The results differ significantly for two of the four levels measured previously by the beam-sputtering method. The new lifetimes for four levels are combined with the recent branching ratio data of Lotrian and Guern (1982) to derive revised transition probabilities for 15 lines and new transition probabilities for six lines.

A generalisation of the Kramers-Heisenberg formula is used to demonstrate the evolution of inelastic X-ray scattering into fluorescence across the K absorption edge. As an illustration the results of a recent synchrotron radiation study of the inelastic scattering in the vicinity of the Mn K absorption edge in KMnO₄ by Briand et al. (1981) are analysed and found to be in qualitative agreement with the theory.

The photoemission of atomic Cu has been obtained in the photon energy range 65 to 90 eV. The 3d⁸4s²¹G, ³F, ¹D satellite lines are resonantly enhanced at the energy of the 3p⁶3d¹⁰4s to 3p⁵3d¹⁰4s² transitions. The intensity of the satellites as a function of photon energy can be approximated by two symmetric lines separated by the spin-orbit splitting of the 3p core. The satellites are driven by the 3p⁵3d¹⁰4s² to 3p⁶3d⁸4s² epsilon l super-Coster-Kronig decay, interference effects are negligible.

The semiclassical approximation model (SCA) with hyperbolic projectile trajectories, including target recoil and symmetrisation, is used to study Coulomb ionisation in the incident energy range from 50 to 300 keV for a Ni target. Combined with corrections for the binding effect and the relativistic behaviour of the target electrons, the first-order SCA theory yields total cross sections consistent with recent measurements. No variation of parameters is performed.

Measurements of the L₃-subshell alignment of gold by low velocity C⁺ and N⁺ impact are reported. The alignment parameter has been found to be positive and it is surprisingly large (A₂=0.47+or-0.09) for 2.4 MeV N⁺ ions.

Hyperspherical coordinates are used to describe the asymmetrical collinear reaction F+H₂ to FH+H. Compound-state resonances are characterised accurately in the adiabatic approximation.

Electron impact ionisation of atomic hydrogen is calculated in the energy range 13.6-27 eV by numerically following the time evolution of a wavepacket. All dynamical degrees of freedom are included and total angular momenta L<or=2 have been considered. Plots of differential probability against ejection energy and the angle theta between the outgoing electrons reveal a strong influence of the interelectronic repulsion. Although angles theta approximately 180 degrees are favoured, another more isotropic mechanism can be distinguished. The slope of the total cross section near threshold has been extracted and found to agree with experiment.

The perturbation approach, based on the space translation approximation (STA), to treating the bound states of atomic hydrogen in the presence of a laser field is analysed. It is shown that the first-order approximation, STA1, is unsuitable and does not modify the bound states of the hydrogen atom in the presence of the field. The authors derive the second-order approximation, STA2, which leads to changes in the bound-state spectrum.

It is suggested that in elementary atomic processes involving identical particles the presence of a strong external perturbation reduces the importance of the exchange effects if identical particles respond to the external field in different ways. The case of elastic electron-hydrogen collisions in a laser field is considered as an example. The laser field is taken to consist of: (i) a single mode, (ii) a large number of uncorrelated modes (chaotic field).

The attachment rates for thermal electrons to SF₆ and CFCl₃ have been determined with an accuracy of +or-3%. There is general agreement with previous authors for the results for SF₆ and with the latest corrected value for CFCl₃.

A general approach to N-electron systems in superstrong magnetic fields (B>or approximately=5*10⁵ T) is presented within Hartree-Fock theory. Starting from Slater determinants with single-particle wavefunctions that are products of Landau states and arbitrary longitudinal functions g_mi(z), the system of equations for self-consistently determining the g_mi(z) is derived, and the expansions of the matrix elements of the electron-nucleus and electron-electron interaction which occur between arbitrary Landau states are given in terms of numerically feasible basis functions. The well known Froese-Fischer code had to be modified considerably to solve the HF equations, and the numerical stability and accuracy are carefully checked. As a first application the ground-state energies of helium-like systems with nuclear charges up to Z=26 are calculated for magnetic field strengths in the range 5*10⁵ to 5*10⁹ T.

Using a 1322-term configuration interaction wavefunction, potential energy curves were computed for the ground (X¹ Sigma ⁺) and first excited (A¹ Sigma ⁺) states of the BeH⁺ molecule. Spectroscopic constants were obtained by means of a Dunham analysis.

The authors have studied phenomena related to the ionisation of indium Rydberg atoms by an external electric field. The indium atoms are excited in an atomic beam apparatus by pulsed dye laser light. The high-lying states are populated in one step from the ground state by an ultraviolet, frequency-doubled dye laser or by stepwise excitation using two dye lasers in the visible region. Shortly after the excitation, an electric field is applied in the interaction region which causes ionisation of the highly excited atoms. The authors have investigated (i) the lowest adiabatic ionisation threshold over a wide range of n values and compared the results with model predictions, (ii) the structure in the adiabatic part of the ionisation spectrum, and (iii) the transition from adiabatic to diabatic ionisation as a function of the slew rate of the ionising field pulse and the quantum defect and quantum numbers of the atomic state concerned. The ionisation signal is demonstrated to shift considerably towards higher field strengths with increasing slew rate. A qualitative explanation is presented and consequences for the detection efficiency are mentioned. Differences in the field ionisation behaviour between nearly degenerate excited states are demonstrated and exploited to achieve selective detection of fine-structure and even hyperfine-structure states.

The K_alpha and K_beta radiation from Fe, Ni and Ti plasmas in a low inductive spark has been investigated both experimentally and theoretically. A high-dispersion and high-resolution Johann crystal spectrograph has been used for the measurements. The theoretical analysis is based on relativistic perturbation theory with a zero-order model approximation. Some K_alpha and K_beta spectra features are qualitatively explained and some K_beta lines are identified for the first time. It has been shown that the observed time and space integrated spectrum is due mainly to the low M-shell ions. The calculation shows that this qualitatively explains the long-wavelength K_alpha plasma radiation shift relative to the solid state K_alpha line. The K_alpha radiation from high ions would be shifted to the opposite side.

Interaction configuration theory is used to study the photoionisation of neutral lithium. The energy levels and oscillator strengths for some bound states of neutral lithium and singly-ionised beryllium have been calculated in a multiconfiguration LS coupling scheme. The autoionisation widths and resonance energies have also been obtained.

The time dependence of the two-photon absorption process is experimentally investigated in the intermediate intensity rate equation regime, where incoherent relaxation plays an important role, as a function of laser linewidth and Doppler width. The saturation effects observed are in excellent agreement with the predictions of Allen and Stroud (1982).

Within the Briggs perturbed united-atom (UA) model a simple cross section formula has been derived for ionisation of the 1s sigma molecular orbital (MO) to s continuum states from straight-line trajectories, in a form which exhibits the coherent contribution of both collision partners to this process. The corrections for Coulomb and relativistic effects as used in the present work preserve the structure of this formula. Interesting behaviour of the ionisation due to the target motion as a function of the target atomic number is predicted and explained by the relativistic contraction of the 1s sigma MO. By means of an approximate scaling law, the calculations are compared with 520 published experimental cross sections (30<Z_UA<110, asymmetry parameter Q<0.3) spanning more than five orders of magnitude. The excellent agreement obtained suggests that the simulation, within the Briggs model, of the 1s sigma MO by a relativistic 1s atomic wavefunction with extended nuclear charge distribution and perturbed binding energy provides a good approximation in describing the direct ionisation in slow ion-atom collisions.

Ion-photon and ion-Auger-electron coincidence measurements have been performed to study the impact parameter dependence of Xe M-shell excitation in 1.05 MeV Xe³⁺-Xe collisions. The experimental results are found to be consistent with the prediction of the molecular orbital model of atomic collisions. The average fluorescence yield for the Xe M shell is found to be strongly dependent on the impact parameter. This is ascribed to the production of highly charged Xe ions in close collisions.

High-resolution energy-loss measurements of product ions resulting from single electron capture collisions between 540 eV Ar²⁺ ions and Ar, Kr and Xe atoms are reported for 0 and 0.4 degrees scattering angles. Significant contributions from long-lived excited states of Ar²⁺ are detected. Somewhat surprisingly, violation of the Wigner spin rule is not observed, even for the heaviest rare-gas target where there is significantly strong coupling between electron spin and orbital angular momentum.

Two-state coupled equations within the continuum distorted-wave representation (CDW) are derived using the time-dependent second-order Euler-Lagrange variational principle of Sil (1980). Matrix elements are simplified using generalised non-orthogonal coordinates. It is shown that the original CDW theory of Cheshire (1964) is a variation-perturbation approximation, analogous to the Oppenheimer-Brinkman-Kramers approximation (OBK) in the travelling atomic eigenstate representation. It is also shown that unlike OBK the CDW wavefunctions are in general not normalised and the CDW wavefunction normalisation integrals are therefore calculated numerically using a complex Laplace transform. The implications of a variational CDW theory of ionisation are briefly considered.

The distorted-wave Born approximation is used to calculate the 2p_3/2 vacancy alignment in ionisation of Mg and Ar by electron impact. The distortion of incoming and outgoing electron waves by a static atomic field is taken into account. The distorted-wave calculation allows one to explain the experimentally observed drastic increase of the alignment with decreasing incident electron energy. The threshold behaviour of the alignment in the Born approximation and in the distorted-wave approximation are discussed.

A simple procedure is given for readily obtaining the total ionisation cross section and rate for electron impact ionisation of any highly charged sodium-like ion. Excitation-autoionisation contributions are included. Illustrative numerical results are given for ten Na-like ions with nuclear charge number Z in the range 18<or=Z<or=42.

The dielectronic recombination cross section is estimated for the system e^-+Mg⁺ in the region of resonance arising from the target excitation 3s to 3p ( Delta n_t=0) with capture of the continuum electron into a high Rydberg state (nl). Dominant contributions come from n<or approximately=500; for an energy bin size of 0.01 Ryd the cross section is as large as 2*10^-17 cm².

A non-linear rate equation describing superfluorescence in many-level systems is presented. It is shown that when a large atomic population is excited into Rydberg levels the cascade back to lower levels by cooperative spontaneous emission leads to a number of new effects including 'trapping' of population in excited states, alteration of branching ratios, truncation of superfluorescent pulses and multiple pulsing of a single transaction.