Table of contents

Several experiments on Rydberg series of atoms in electric and magnetic fields have shown a pattern of equally spaced resonances around the ionisation limit. A unified picture, giving characteristic spacings and their dependence on the external fields is presented. Such patterns of equally spaced levels near zero energy are to be expected quite generally in physics when there is mixing of fields. Further experiments in atoms and in the analogous system of image-potential-induced states at the surface of liquid helium are suggested.

Measurements have been made of the lifetimes of excited sodium 3²P atoms in some premixed O₂-H₂-Ar flames operated at atmospheric pressure. Absolute fluorescence yields and cross sections for the quenching of the sodium D line fluorescence by oxygen, hydrogen and nitrogen are derived.

Circularly polarised components of the resonance fluorescence of ⁸⁵Rb excited with the pi polarised D₂ line are investigated in a magnetic field in the range 0-170 G. A difference in the intensities of the sigma ^- and sigma ⁺ components is reported. As a result orientation can be obtained in the ground state of alkali atoms.

A description of ionic recombination in a low-density ambient gas in the form of a variational principle is formulated, and its equivalence to the quasi-equilibrium condition is proved. Application to the computational procedure of the quasi-equilibrium statistical theory is discussed.

Plasma densities and temperatures are deduced from soft X-ray emission spectra of neon-filled glass microballoons imploded by a 0.2 terawatt laser. Spatial resolution allows separate characterisation of the ablation plasma, the imploded glass and the compressed gas. Comparison with computer simulations of the implosion shows sensitivity to models for energy transport by hot electrons.

A version of quantum-defect theory appropriate to the analysis of the spectra of highly ionised atoms is obtained from the single-electron Dirac equation. Quantum defects mu _n are defined as the principal quantum number n increases along the Rydberg series using Sommerfeld's relativistic one-electron energy level formula. Considering the analytic properties of solutions to the Dirac equation, the authors establish that mu _n can be extended smoothly away from the bound-state energies just as in the non-relativistic theory. Moreover, the relativistic mu ( epsilon ) can be analytically continued beyond the threshold epsilon =mc² and related to the non-Coulomb scattering phaseshift delta ( epsilon ). At the threshold the authors establish the result delta (mc²)= pi mu (mc²) well known in non-relativistic quantum-defect theory. Several examples of the relativistic quantum-defect theory are given for highly stripped Na-like and Li-like ions.

Plasmas produced by the laser irradiation of solid targets and in a plasma focus device have been employed as sources of X-ray spectra of the elements Kr(Z=36)-Mo(Z=42). The Ne-like isoelectronic sequence has been investigated by comparing observed wavelengths with ab initio atomic structure calculations and isoelectronic interpolation. Previous identifications in Ne-like ions have been extended to krypton, rubidium and strontium. The Na-like satellite structure of these elements has also been studied and a detailed classification of these satellites is presented.

The fine-structure splittings of the 3s3d³D and 3s4d ³D terms in Mg I have been measured by fast-beam level-crossing spectroscopy. The fine structures were found to be inverted. With E(³D₃)=0, the measured values for the 3s3d³D term are E(³D₁)=379+or-4 MHz, E(³D₂)=-532+or-4 MHz, whereas E(³D₁)=2377+or-12 MHz, E(³D₂)=1146+or-9 MHz for the 3s4d³D term. These splittings deviate considerably from values estimated on the basis of the nd²D splittings in Mg II.

The model potential method developed by Valiron et al. (1977, 1979) is applied to the calculation of the potential energies of the ground and excited states of the Na-He molecular system. The excellent agreement with the ab initio calculations and the stability of the results to variations in the model potential parameters establish the reliability of the method. Interesting features in the more highly excited states are observed. The calculated interaction energies are used to compute the profile of the fluorescence spectrum from optically thin Na vapour in the presence of He at moderate pressures. The theoretical results for the Na resonance line profile are compared with the experimental data of York et al.

The Dirac equation in a 'tumbling frame' is shown to yield, in its non-relativistic form, a non-inertial rotational coupling, quadratic in angular velocity of the frame, besides the usual Coriolis potential. The new 'quadratic Coriolis coupling' contains spin-orbit interaction and is numerically important problems involving fast-rotating frames. The customary analysis of radiative transitions in rotating diatomic molecules and quasimolecular X-ray emission in ion-atom collisions is generalised to include the effects of the new coupling term.

The Monte Carlo classical trajectory method has been used to study the distribution of excitation in projectile ions resulting from electron capture in collisions between fully stripped carbon, oxygen and boron ions and atomic hydrogen at intermediate collision energies (v_rel=1-1.7 au). In this approach, the classical distributions in angular momentum and binding energy obtained for the post-collision hydrogenic projectile ion are converted to effective n and l distributions. The author finds that at v=1 au, the n levels tend to be selectively populated (n=4 for C⁵⁺, n=5 for O⁷⁺ and a combination n=3 and n=4 for B⁴⁺), but as the collision velocity increases, the n distribution broaden considerably. In general, the most important specific excitations correspond to the highest quantum mechanically allowed values of l(4f for C⁵⁺, 5g for O⁷⁺, 3d and 4f for B⁵⁺). Studies of the impact parameter dependence of the C⁶⁺+H excitation at v=1 show that the angular momentum is strongly correlated to the impact parameter with the dominant excitation shifting from 4d to 4f as the impact parameter increases above 2 au.

Differential cross sections for charge transfer and excitation in low-energy (2-5 keV)Be⁺-He, Ne collision systems have been measured. For both collision systems, excitation of the Be⁺ ion to the 2p state is found to be the dominant inelastic process in soft collisions (reduced angle below 10 keV deg), whereas charge transfer and target excitation are the main processes in violent collisions. In the Be⁺-Ne case, the Be⁺ 2p excitation probability shows an oscillatory structure which is interpreted within a simple two-state model of the collision. The charge-transfer and target-excitation processes are discussed in terms of the electron-promotion model.

The triple differential cross section for electron impact ionisation of the ground state is calculated for atomic hydrogen in the Born, Coulomb-projected Born (CPB) and generalised Coulomb-projected Born (GCPB) approximations including exchange. The present CPB calculation is in agreement with the earlier calculation of Geltman and Hidalgo (1974). The authors have compared the present results with the experimental data of Weigold et al (1973) on the angular correlation of the scattered and ejected electrons. The CPB and GCPB calculations are in fairly good qualitative agreement with the shape of the experimental five-fold differential cross section (often called the triple differential cross section) for atomic hydrogen.

S- and P-wave phaseshifts are calculated by the R-matrix method for e-He scattering from 0 to 16.5 eV. The calculation uses a multiconfiguration He ground state and pseudo P and D states constructed from optimised 1s through to 4f pseudo-orbitals. Derived cross sections confirm experimental momentum transfer and differential cross sections and also some total cross section measurements. A detailed but non-rigorous error analysis of the results is carried out and best theoretical estimates of the phaseshifts and cross sections are presented.

In a coincidence experiment with coplanar geometry, angular correlations of the two outgoing electrons have been studied for the ionisation of helium atoms by electrons with an impact energy 10.5 eV above the first ionisation threshold. The measured angular dependencies of the triple differential cross section are compared with calculated results of the Coulomb-projected Born approximation including exchange. Neither the measured data nor the calculated ones show the features that have been found at medium and high impact energies. The momentum transfer K_0a loses its importance as a means of classification. For these low-energy collisions the amplitudes of direct scattering, exchange scattering and electron capture are of similar size and only the direct scattering amplitude has the momentum transfer vector K_0a as a symmetry axis (in Born approximation). Therefore the angular dependence of the triple differential cross section are no longer symmetrical with respect to the momentum transfer. Rapid variations of the angular distributions for different scattering angles theta _a of one of the two outgoing ionisation electrons indicate interference of the different scattering amplitudes. Moreover the measurements demonstrated that the angular correlations for fixed scattering angle theta _a are independent of the ratio of the energies of the two emerging electrons.

A set of total cross sections for scattering of electrons by Ne, Ar, Kr and Xe has been evaluated over the energy range of about 20 to 3000 eV by means of the analysis of experiments and theories on total cross sections for elastic scattering, ionisation and excitation, and on differential cross sections for elastic and inelastic scattering. The total cross sections for scattering of electrons are evaluated by adding those for ionisation, excitation and elastic scattering and they are accurate to about 5-10%. They appear to be in very good agreement with the recent experimental results on total electron scattering of Wagenaar et al. (1978).

The polarisation of selected Lyman and Werner bands and also of Lyman- alpha radiation are measured as a function of impact electron energy in the range from threshold to 300 eV. Polarisations in excess of 0.2 are obtained in certain instances. The results for Lyman- alpha confirm and extend those of Ott et al. (1970). Extrapolations of the results to the thresholds are compared with estimates based on angular-momentum conservation and symmetry arguments. For the Werner emission a further comparison is made with a 'pseudo'-threshold polarisation measurement using the scattered-electron-delayed-photon coincidence technique. The apparent excitation functions for the two molecular emissions are derived from the measurements of polarisation. In both cases good agreement is obtained with the semi-empirical data of Gerhart (1975). Comparison is made with the results of other workers wherever possible.

The authors report close-coupling calculations of the total integrated and momentum transfer cross sections for the scattering of electrons from KOH and CsOH in the energy range 0.01 to 10 eV. A cut-off dipole potential is used to account for the long-range interaction. The authors nominally chose the cut-off radius R_c such that the potential has a minimum in the vicinity of the nucleus nearest to the centre-of-mass (COM) of the molecule. The results for the total integrated and momentum transfer cross section exhibit oscillations about a mean value. The choice of a different value for R_c in additional calculations for KOH led to results for the mean value and amplitude which were virtually identical for k²<<1/(2r_c)². A simple semi-quantitative understanding of this behaviour is presented in the context of Fabrikant's effective-range theory (ERT). Analytical expressions are given for the interaction parameters (conductivity integrals) which may be used for calculating electrical properties of gas mixtures according to the popular algorithm of Demetriades and Argyropoulos (1966).

The authors have extended their work on the polarised-orbital approximation to the calculation of scattering and annihilation processes for the positron-argon system in the elastic-scattering region. They present results for the phaseshifts; the elastic, diffusion and annihilation cross sections; and the angular correlation of photons produced during annihilation. As in previous calculations the results are in very good agreement with various experimental measurements. The authors have also investigated the angular distribution of the scattered positron and the possibility of forming a bound state of the system. The model predicts that there is no bound state of the positron-argon system.

A spectroscopic study of the relaxation of electron-beam-excited discharge plasmas in helium at pressures between 1 and 5 atm is reported. Initial charge densities produced exceeded 10¹⁴ cm^-3 and the studies were conducted at 300K. A kinetic model describing the afterglow evolution was formulated and found to agree well with the experimental results. The role of electronic recombination of He₃⁺ is discussed.