Table of contents

Using the well known relationships for the isoelectronic changes in the total Hartree-Fock energy, nucleus-electron attraction energy and electron-electron repulsion energy in atoms a simple polynomial expansion in Z is obtained for the sum of eigenvalues which can be used to calculate the total Hartree-Fock energy. Numerical results are presented for 2-10 electron series to show that the present relationship is a better approximation than the other available energy-eigenvalue relationships.

Effects of laser bandwidth on two-photon ionisation are studied in the case that two distinct near-resonant channels of ionisation exist, using the phase-diffusion model of laser bandwidth. An application of atomic caesium is presented.

The position and the width of the '2s2p⁶3s²' term are calculated by analysing the resonance obtained in the Na⁺+e^- scattering problem, which is solved by the close-coupling method.

Fluorescence of ¹³⁰Te₂ and ¹²⁸Te₂ excited by lines of an argon-ion laser have been recorded by Fourier transform spectroscopy. The resolution and precision afforded by this technique has enabled several new features of this complex spectrum to be observed for the first time, including the characterisation of the excited state, B 1_u. It is concluded that this is a method of some general utility.

Using the approach previously developed by the authors (1979) a simple expression for the charge capture cross section from H(2s) into an arbitrary principal shell of a highly stripped energetic projectile is derived. The formula is exact in the eikonal approximation. Total cross sections for various projectiles are graphically displayed.

Cross sections calculated in a six-state close-coupling approximation are given for electron impact excitation of the 1s inner shell of the Li-like ions C³⁺, N⁴⁺ and O⁵⁺ for energies from near threshold to three times the excitation threshold. The 1s²2s to 1s2s2l excitation followed by autoionisation gives an abrupt and significant contribution to the total ionisation cross section, in agreement with recent measurements of Crandall et al. (1979). When combined with theoretical direct ionisation cross sections, the calculations agree with the observation that the relative contribution of the excitation-autoionisation process increases with increasing ionic charge for the three cases calculated for the Li isoelectronic sequence.

The polarisability derivatives of the nitrogen molecule are calculated using both SCF and accurate CI wavefunctions. The CI values of these derivatives are in good agreement with experiment. However the SCF values are noticeably in error, and it seems probable that SCF wavefunctions are inadequate for the calculation of polarisability derivatives.

Pade approximants to the diagrammatic many-body perturbation expansion for electron correlation energies in molecules are discussed. The numerical convergence properties of the perturbation expansion and the Pade approximants which can be formed from it are compared. Calculations of the ground-state potential energy curve for the CH⁺ ion are reported. Systems, such as CH⁺, which contain low-lying excited states, can be handled by forming (2/1) Pade approximants from third-order calculations using 'shifted' denominators.

Configuration interaction wavefunctions for the 1s²2s²¹S^e, 1s²2s2p ^1,3P⁰; 1s²2p²³P^e, ¹D^e, ¹S^e; 1s²2s3s ^1,3S^e; 1s²2s3p ^1,3P⁰; 1s²2s3d ^1,3D^e states are calculated for C III, N IV, O V and Ne VII and are used to calculate oscillator strengths, both length and velocity forms, for transitions between these states. Some significant differences from previous calculations are obtained. Comparison with recent beam-foil experimental data is generally good.

The authors calculate energy levels and transition probabilities for Ca XVII, Fe XXIII, Kr XXXIII and Mo XXXIX. With reference to beam-foil experiments they briefly discuss decay curves for Fe XXIII. They also compare their calculated 2s²¹S₀-2s2p ³P₁⁰ inter-combination probabilities with those deduced from beam-foil experiments for Fe XXIII and Kr XXXIII.

An examination of Hartree-Fock results for the 5p⁵6s² configuration of caesium shows that the assignment of the ²P_1/2 level should be revised.

Experimental and theoretical results on far UV transitions between levels of configurations 2s2p⁴3s and 2s2p⁵ in Mg V, Al VI and Si VII spectra are reported. Theoretical analysis by means of ab initio and semi-empirical methods leads to a new identification of a number of lines, nine of which pertain to the Si VII spectrum. These are situated in the 70-90 AA range, and have not been observed previously. The energy levels of the upper configurations of Al⁵⁺ and Su⁶⁺ ions are calculated in the intermediate coupling scheme. The correlation effects expressed by configuration interactions do not greatly influence the relative energy position of the analysed lines but they do have significant consequences on their relative intensities.

Radiation emitted from silicon beams at 35, 38 and 45 MeV excited by passage through thin carbon foils has been studied in the range 3-9 AA using a curved-crystal X-ray spectrometer. More than 100 lines have been observed, of which only one quarter had been observed or predicted from solar or laboratory plasma spectra. Most of the previously unobserved lines may be attributed to satellites of H- and He-like transitions arising in three- and four-electron Si ions. The lifetime of the Si XIII 2³P₁ state has been measured, and a preliminary experimental value of 6.5+or-1.3 ps obtained.

Atomic photoionisation cross sections including both relativistic and correlation effects for the helium and beryllium isoelectronic sequences are calculated in two-channel relativistic random-phase approximation (RRPA). By studying the relativistic E1 transition along the sequence, the coupling between the low-energy photoelectron and the core electron (electrons) is found to deviate from relativistic jj coupling even for high Z elements. The deviation of the sum of dipole oscillator strength from the non-relativistic Thomas-Reiche-Kuhn rule is also investigated.

The authors report self-broadening and self-shifting measurements on the J=0 to 1 transitions of ¹⁵N₂O and the J=0 to 1 and J=1 to 2 transitions of CF₃H as a function of temperature. The experimental results are compared with calculations based on the Anderson-Tsao-Curnutte-Frost theory and the modified Murphy-Boggs theory, (1968), as formulated by Cattani, (1972). The modified Murphy-Boggs theory offers in all cases the best agreement with experiment as far as the width is concerned. With regard to the shift, however, both theories seem to be equally valid. The authors wish to emphasis particularly that the K dependence, as predicted by the modified Murphy-Boggs theory, of the linewidth and lineshift of the J=1 to 2 transition of the oblate symmetric top CF₃H is confirmed by experiment for the first time. The recent Mehrotra-Boggs theory, (1977), appears to be identical to the modified Murphy-Boggs theory for molecules in a pure rotational state.

A technique in which the H(2s) metastable population of a fast hydrogen atom beam is modulated by electric field quenching has been used to study the collisional de-excitation of 5-26 keV H(2s) atoms in passage through H₂, He, Ne, Ar and Kr. Cross sections sigma _2s.2p for 2s-1s de-excitation via the 2p state have been determined for the first time and shown to account for no more than about 15% of the total cross section for de-excitation. Checks have been made on the authors' previously measured total cross sections for one-electron loss by H(2s) atoms.

In the Coulomb-Born-Oppenheimer (CBO) approximation, terms of the first order in the interaction energy between the colliding particles are omitted from the exchange scattering amplitude and consequently, near the threshold energy, the cross sections become highly overestimated, especially for s-s transitions. If these terms are retained the resulting scattering cross sections at low energies are considerably improved. A first-order Coulomb exchange approximation, where some allowance has been made for the neglected terms in the CBO approximation, has been developed and applied to investigate the 1s-ns excitation of hydrogen-like ions by electron impact for any arbitrary value of n. The behaviour of the scaled excitation cross sections ( sigma *n³) in the asymptotic region (n to infinity ) has also been analysed.

A distorted-wave model originally applied to ionisation of helium is extended to the calculation of the ionisation of atomic hydrogen. Comparison is made with recent experimental data and with the results of other models.

Differential cross sections for the electron impact excitation of the 2¹S state of helium at impact energies 200 and 300 eV are calculated in the two-potential model and using the modified Born approximation. Exchange is considered within the framework of the Ochkur-Rudge approximation. The results thus obtained are in reasonable agreement with other theoretical models and with recent experimental measurements.

Differential cross sections for the electron impact excitation of the 3²P_1/2,3/2 resonance levels of sodium have been measured for incident energies of 54.4, 100, 150 and 217.7 eV over an angular range of 2 to 145 degrees using a modulated crossed-beam technique. The results are compared with the first Born approximation and a close-coupling impact-parameter calculation. None of these approximations are in agreement with the data over the entire angular range. Integrated total cross sections are presented at each energy which show excellent agreement with previous experimental and theoretical work.