Table of contents

Wilson's proposal (see ibid., vol.12, no.5, p.135, 1979) to improve correlations energy calculations by using a scaling parameter is examined in the light of accurate variational-perturbation theory results.

The use of scaling in second-order electron correlation calculations is examined in the light of recent comments by Byron and Joachain (1979). The relation between the scaling parameter, Pade approximants and higher order terms in the perturbation series is discussed. Truncation effects in calculations of electron correlation energies by perturbation theory are considered and some remarks on the computational tractability of second-order calculations are made.

Using a single-mode dye laser to excite and a thermoionic diode to detect highly excited caesium vapour atoms, the authors have extended the range of measured term values in caesium for the levels n²P₁2/,₃2/(20<or=n<or=60), n²D(54<or=n<or=58), n²F(23<or=n<or=109) and n²G(27<or=n<or=36). The atomic transition wavenumbers were measured against the absorption spectrum of I₂ vapour, affording an estimated uncertainty of +or-0.01 cm^-1.

The K Auger spectrum of atomic Mg-including the K-LM and K-MM transitions-excited by 3.8 keV electrons has been measured with high resolution. The absolute energies and relative intensities of all normal K Auger transitions and most of the satellite lines due to 3s shake-up, 3s shake-off and 2p shake-off in the initial 1s ionisation process have been determined. The relative intensities of the normal K Auger transitions agree well with recent theoretical calculations of Howat (1978) including CI between the final ionic states as well as between the final (²S) continuum states.

The authors establish a relation between the three most frequently used non-perturbative methods proposed to study the ionisation of atoms by strong electromagnetic fields and determine the range of validity of these methods.

A thermal beam of helium (2³S₁) metastable atoms was used to excite coherently the ²P_3/2 levels of Ca, Sr and Ba in a Penning ionising collision. The coherent excitation of the ions appears as a linear polarisation of the optical emission from the excited ions. The degree of linear polarisation is 5.5, 3.5 and 0.5% for Ca, Sr and Ba, respectively, with the polarisation parallel to the beam direction. Hanle effect signals from the ²P_3/2 level of Sr were observed and the radioactive decay rate measured.

Cross sections for Cs²⁺ production from the combined processes of charge transfer and ionisation in Cs⁺-Cs⁺ collisions have been determined at centre-of-mass energies between 40 and 280 keV using an intersecting-beam technique previously developed in this laboratory. Measured cross sections are at least 1.5*10^-16 cm² and the results are directly relevant to proposed schemes for heavy-ion fusion.

The authors have measured absolute (e,2e) cross sections for electron impact ionisation of helium with a much higher accuracy than obtained before. In a coplanar symmetric geometry ( theta =45 degrees ) both outgoing electrons-with equal velocities-were detected in coincidence. The resulting experimental triple differential cross sections for incoming electrons of 200-2800 eV contain an experimental error smaller than 20%. At the same time the authors have determined the absolute double differential cross sections at 45 degrees to an accuracy of 10%. The data are compared with various theoretical results.

The fraction of positrons forming positronium in helium gas in a conventional positron lifetime system is enhanced by the addition of small quantities of any other gas. The magnitude of this increase at very low concentrations is used to measure the mean positronium formation cross section for a number of gases.

The effects of saturation of a bound-bound atomic resonance on bound-continuum decays in investigated using the dressed state concept. Two special cases are discussed: the modification of spontaneous radiative decay by coupling to a third bound atomic state, and the changes in single-photon photoionisation when the initial discrete state is coupled by an external field to another bound state. For strong external fields Rabi oscillations and dynamic Stark splittings profoundly alter the 'decay'; but for weak fields the alterations at line centre are no less dramatic.

Radiation properties of three atoms continuously pumped by an incoherent excitation mechanism are investigated by using Lehmberg's master equation (1970) and the quantum fluctuation-regression theorem. The main results are: (i) the radiation rate of three atoms within a volume of diameter very much less than lambda essentially depends on their arrangement; (ii) the effect for two atoms, i.e. that an increasing radiation rate can be accompanied by a decreasing spectral width, becomes even more pronounced for three atoms.

Photoionisation cross sections are calculated for the ground states of Ne II, Ne III and Ne IV using the length formulation of the dipole matrix element. In the close-coupling expansion for the total wavefunction, the author includes channels coupled to all terms dominated by the ground configuration and the next excited configuration of the residual ion. Resonances in the near-threshold regions are analysed using accurate interpolation techniques. A brief description of the computational method is given.

Equilibrium charge-state distributions observed for chlorine, bromine and iodine ions after passing through gaseous targets (Z_t>or approximately=7) with an energy per nucleon in the range 20-500 keV amu^-1 are very well fitted by a simple statistical distribution: a three-parameter chi-squared model. The three parameters are elementary functions of the mean, the standard deviation and the skewness of the distribution. Equilibrium charge-state distributions for heavy ions (Z_p>or=17) in gases may generally be predicted with a good approximation by this chi-squared model (particularly the proportions of ions having a very high charge-state which have generally not been measured) even if only the mean charge state of the distribution can be precisely estimated.

A tungsten-tube furnace containing highly dissociated hydrogen has been used, in conjunction with a technique previously used in this laboratory, to determine cross sections for collisional destruction, electron loss and electron capture by fast hydrogen metastable atoms in atomic hydrogen. Cross sections are not more than about 65% of the corresponding cross sections in H₂. While electron loss cross sections for H(2s) atoms are found to be considerably greater than those for H(1s) atoms, corresponding electron capture cross sections are not greatly different. Theoretical estimates of cross sections for collisional destruction and electron loss by H(2s) atoms in H are in poor accord with the present measurements.

Various variational approaches are derived from the least norm variational principle. The application of these approaches to the elastic scattering of electrons and positrons by atomic hydrogen shows that the corresponding phaseshifts are related to each other and agree very well with the results of other authors. A test for Kanellopoulos' inequality (1972, 1975) provides us with effective upper and lower bounds for the S-wave phaseshifts of e^--H elastic scattering (singlet-state).

The theoretic treatment of elastic electron-helium scattering is discussed in the context of the many-body method introduced by Schneider et al. (1970). An iterative equation for the optical potential is derived and the model obtained by using the first iterative solution is developed, paying close attention to similarities and differences with other theoretical models. A comparison with the results of other methods is illustrated at incident energies of 30, 50, 100 and 200 eV.

For pt.I see ibid., vol.12, no.20, p.3367 (1979). A second-order method developed from the many-body scheme introduced by Schneider et al. (1970) is applied to elastic electron-helium scattering. Three models derived from the method are discussed and results for differential, integrated and total cross section are presented at energies ranging from 30 to 300 eV and compared with absolute experimental data.

Discusses methods for calculating distorted-wave amplitudes. It is difficult to compare the relative merits of these calculations since widely different atomic potentials and wavefunctions have been used in the various calculations. The authors have examined these different approximations by calculating different distorted-wave amplitudes using a common Hartree-Fock basis for atomic potentials and wavefunctions. This procedure allows one to examine the relative effects of different types of approximations. For purposes of comparison, they have calculated differential cross sections and angular correlation parameters for electron impact excitation of the 2¹P and 3¹P allowed transitions of helium for incident electron energies in the range 40-200 eV. All of the calculations include both the direct and exchange amplitudes, and calculations were performed using both a static atomic distorting potential and a complex optical potential which included polarisation, exchange and absorption distortion in addition to the distortion of the static potential.

Calculations have been made of collision strengths for electron impact excitation transitions between the ground configuration terms of Ne⁴⁺. The authors have used a configuration-interaction target consisting of the lowest nine terms in Ne⁴⁺ (the configurations 2s²2p² and 2s2p³) explicitly in the close-coupling formalism, and three correlation configurations. The inclusion of the 2s2p³ term leads to many resonances in the near-threshold region and therefore affects the collision strengths significantly. The techniques used to solve the (Ne⁴⁺+e^-) collision problem are also used to calculate energies for bound states of Ne³⁺. The calculated ionisation energies for Ne³⁺ ground configuration terms agree with experimental values to within about 0.3 per cent. For excited configurations calculated and experimental quantum defects generally agree to within 5 per cent. Calculated energies are given for a large number of levels for which experimental energies have not been determined.

A recently developed 'effective potential' formalism is applied to the e^-H₂ elastic scattering and rotational excitation processes. The partial-wave analysis of the 'optical' and 'transition' potentials of the theory, relative to elastic and inelastic processes, respectively, is developed. The first quantity enters into a (small) set of coupled integral equations, which are solved numerically by a non-iterative technique. The second quantity is used to evaluate a matrix element, directly related to the T-matrix of inelastic transitions. A comparison of the results shows good agreement with exact close-coupling calculations, particularly for inelastic processes, which are dominated by higher partial waves.

Cross sections have been measured for the production of H^- ions by collisions between electrons and H₃⁺ at energies between 2 and 13 eV. The cross section reaches a maximum value of 1.6*10^-18 cm² at 8 eV. This implies that the reaction is unlikely to contribute significantly to the formation of H^- in ion sources.

Calculations of the spectral line broadening for 1s²¹S-1snp ¹p transitions in helium-like ions have been performed for emitters with relatively high Z (silicon and argon). These calculations illustrate the plasma dependent shifts and asymmetries which are due to the high emitter charge. A discussion of the effects is presented, with particular reference to the deviations from the corresponding hydrogenic line profiles.