Table of contents

The systematic variation of both exact and Hartree-Fock expectation values as asymptotically linear functions of the nuclear charge Z is derived, and the unusually high accuracy of certain Hartree-Fock values explained, by means of Z-dependent perturbation theory, the Hellmann-Feynman theorem and the virial theorem.

Partial plots of the transition from LS to J₁J₂ coupling are exploited as an aid to unravelling the complexity of d⁹s²p^k configurations in Ga, In, Tl, Ge, Sn and Pb. Although the full transition from LS to J₁J₂ coupling cannot be plotted in two dimensions for d⁹s²p² or d⁹s²p³, the partial plots enable consistent assignments to be made by comparison with individual intermediate-coupling calculations for each element.

Using laser excitation of the 6²P levels in the natural isotopic mixture of Rb atoms, modulation of superradiance intensity from some transitions to the lower levels was observed. On the transition 6²P_1/2-6²S_1/2 this effect is interpreted as a quantum beat between the hyperfine-structure transitions of the Rb⁸⁵ and Rb⁸⁷ isotopes. A nearly 'single-pulse' superradiant signal was obtained on the 6²P-4²D transitions. When a magnetic field was applied transverse to the direction of the laser beam, the pulses obtained showed the properties forecast by two-level superradiance theory. Preliminary results are reported.

Energy spectra of electrons produced in collisions of 100 keV Neⁿ⁺(n=1, 2, 3, 4) on He, Ne, Ar and Kr have been measured. The experimental data show that capture of two electrons into excited autoionising states of the projectile is important for the Ar and Kr targets. For the Ne and He targets capture of two electrons into excited states is less probable.

Variational calculations of e^--He s- and p-wave phaseshifts, together with the Born formula for higher partial waves are used to give the scattering amplitude to within one percent estimated accuracy for energies less than 19 eV. Coefficients are given of cubic spline fits to auxiliary functions that provide smooth interpolation of the estimated accurate phaseshifts. Data given here make it possible to obtain the differential scattering cross section over the energy range considered from simple formulae.

Measured cross sections for electron impact ionisation of Li-like ions of C³⁺, N⁴⁺ and O⁵⁺ are reported for energies from threshold to 1500 eV. For electron energies from near threshold through the first peak in the cross sections, the measured values are in reasonable agreement with Coulomb-Born calculations. However, in each case an abrupt increase in the measured ionisation cross section is observed at the electron energy associated with excitation of an inner-shell electron, a process which has not been included in the theories. The 1s²2s to 1s2s2l excitation followed by autoionisation is shown to give an abrupt and significant contribution to the total ionisation cross section. Further, the relative contribution of the excitation-autoionisation process increases with increasing ionic (or nuclear) charge for the three cases tested along the Li isoelectronic sequence.

The authors report measured post-collision interaction phenomena following K-shell ionisation of neon by electron impact. At near-threshold impact energy E_i electron spectra taken in the region of the K-L_2,3L_2,3(¹D₂) Auger line reveal a high-energy tail and an energy shift Delta E of this line, the latter obeying the Barker-Berry power law Delta E approximately (E_i-E_K)^-0.5 where E_K is the K-ionisation energy. The intensity ratio of diagram Auger-lines changes noticeably with near-threshold impact energy.

A conserved quantity is derived for ultrashort optical pulses propagating through a medium with which they are two-photon resonant. Some applications and consequences of this conserved quantity are noted.

A very simple method is proposed for calculating the radial matrix elements and the oscillator strengths for non-hydrogenic atoms in the Coulomb approximation. The method is valid for all cases of interest and in particular for transitions involving high Rydberg states for which previous methods cannot be applied.

The multi-centre integrals of spherical Gaussian atomic orbitals are expressed in an algebraically correct tensor form, which consists of invariants, 3jm symbols, and spherical harmonics. The invariants are calculated using the Talmi transformation and the Moshinsky-Smirnov coefficients of nuclear shell theory. Effective calculation procedures of the Moshinsky-Smirnov coefficients are discussed. The integral types investigated are: overlap, electric dipole transition, kinetic energy, three-centre nuclear attraction, four-centre electron repulsion, and three-centre spin-orbit interaction.

Pilot calculations of atomic spectroscopy are reported, being based upon an m-scheme configuration interaction method in which Hartree-Fock wavefunctions are used to generate interaction integrals. The many-electron energy matrix is diagonalised using the Lanczos algorithm. The results obtained agree with comparable configuration interaction calculations encouraging the extension of the use of this method to molecular and other atomic systems.

A semiclassical, self-consistent and unified formulation of core charge polarisation effects in atomic systems is presented. The method is immediately applicable to alkali-like atomic systems but may be more generally useful. The formulation encompasses the simple shell model of ionic crystals, the atomic dipole polarisabilities in electric fields, the effective dipolar scattering interaction, and the total effective one-electron potential energy for the valence electron. The expressions are presented in forms that can be readily assimilated into standard calculations. Some results are compared with previous calculations for atomic caesium. It appears that the authors have a valid and useful formulation and that, at least for atomic caesium, the major portion of the correlation effects, to be added to a Hartree-Fock calculation, are given simply by the dipolar contribution.

The semiclassical, consistent formulation of core charge polarisation effects presented in an earlier paper has been applied to atomic lithium. When the core polarisation potential is added to a Hartree-Fock calculation one obtains accurate atomic energy levels for the valence and excited states, reasonable values for the hyperfine splitting constant of the valence S state and oscillator strengths in excellent agreement with the tabulated values. This suggests that this procedure accurately describes the non-relativistic aspects of atomic lithium. The spin-orbit splitting and atomic g shift are not given well by the standard one-particle expressions. This suggests that the one-particle expressions are invalid for relativistic corrections in many-body systems.

The influence of core polarisation on relativistic oscillator strengths has been studied for the 4s²S₁2/-4p²P_1/2,3/2 and 4p²P_1/2,3/2-4d²D_3/2,5/2 transitions along the copper isoelectronic sequence. Core polarisation effects are included in a relativistic Hartree-Fock method as well as in a relativistic semi-empirical model-potential approach by introducing a core polarisation potential in the calculation of wavefunctions and by employing the corresponding correction for the dipole moment operator in the transition matrix element. The results obtained are compared with available theoretical and experimental data and the influence of core polarisation effects is discussed.

Theoretical line strengths for the transitions 3p⁶3d⁶⁵D₄ to 3p⁵(²P)3d⁷(⁴P,⁴F)⁵L_J are reported and compared with experimental values. The agreement is good in some cases, poorer in others.

The formulation of a model for the plasma line broadening of hydrogenic ions is presented. Calculations are performed and comparisons are made with experiments and theory.

A study of the spectral line-broadening of Lyman and Balmer series transitions in moderately dense plasmas is reported. Specific attention is given to effects of ion dynamic perturbations on the central regions of the profiles.

A simple model of a non-thermal RPA plasma is used to study possible collective resonances spectroscopically observable as satellites on spectral lines.

A new experimental method allowing the heavy-particle temperature in an arc plasma to change considerably in otherwise fixed plasma conditions was applied to the study of the Stark broadening of the line He I 4026 AA and its associated forbidden components at an electron density of n_e=10¹⁵ cm^-3. Consistent with an earlier experiment, strong deviations from static ion calculations were found. The whole line shape was found to depend sensitively on variations in the heavy-particle temperature and, by including the earlier experiment as well as using the static ion theory, systematic trends could be observed. Though improving some features of the static profile, a recent calculation including ion dynamics still gives poor agreement with experiment.

In the presence of coherent saturated near-resonant two-photon excitation, the ordinary Raman scattering line is replaced by a dynamic Stark shifted or Stark split doublet, and Rabi nutations appear in the time-resolved ordinary Raman spectrum. The relative intensity of the peaks of the doublet depends on the shape and on the average frequency of the exciting pulse. Striking new features arise from the interfering contributions of the upper and lower states of the two-photon transition to the Raman scattering process. The theory is applied to Raman scattering in atomic Na in the presence of the 3²S₁2/(F=1, M_F=1) to 4²D₃2,5/2/(F=3,M_F=3) two-photon resonances.

Coherent and incoherent photon scattering by bound electrons in low, medium and high Z atoms, over energies of 0.145-1.33 MeV and scattering angles below 15 degrees (momentum-transfer range 0-0.4 mc) and from 15 degrees to 150 degrees (0.1-4.0 mc), have been investigated experimentally. The results of the authors' relatively high precision measurements have also been compared with the predictions of new theoretical calculations for bound-electron incoherent and coherent scattering in the incoherent scattering-factor and form-factor approximations and for the Rayleigh and Delbruck scattering amplitudes over a high momentum-transfer range, to give a critical verification of the theory. It was found that for 0.145 MeV photons scattered with low momentum transfer, the F(q,Z) formalism is valid for low and medium Z atoms but not for the high Z (Z=82) atom. At all energies and in the low momentum-transfer range studied in the present work, the S(q,Z) formalism was found to be adequate for bound-electron incoherent scattering for all the atoms.

The Schwinger variational principle has been applied to solve certain scattering problems in atomic physics, and a separable approximation has been used for the Hamiltonian of the bound subsystems. The (e⁺H) scattering length and the cross section for p(d mu ^-) elastic scattering has been calculated using the second Born approximation.

The authors have measured the differential cross section for the elastic scattering, target and projectile excitation (corresponding to energy losses of about 21 eV and 40 eV) and for the double-excitation process (Q approximately=60 eV). Over the whole energy range, the elastic cross section shows oscillations of which extrema are located at particular values of tau ( tau =E₀ theta ). These oscillations are due to the interference between the waves scattered through the Sigma _u and Sigma _g states and they have particularly studied the phaseshift between these waves, starting from Everhart's calculations. Moreover, the minima of the elastic process appear for some energies and do not exist for others as is expected from Everhart's results. For inelastic processes, the general behaviour is roughly the same.

The quenching of O(¹D) by collision with N₂ at room temperature (T=300K) is studied. A statistical model, based on the Landau-Zener formula for obtaining transition probabilities between the singlet and triplet states, is used. In this, the number of times that the potential-energy curve crossing points are traversed by the system is taken into account. The cross section values encountered agree fairly well with the experimental results, it being possible to observe a high vibrational excitation of N₂ after collision.

The cross section for double charge transfer in H⁺-H^- collisions has been measured, using merged beams, over the energy range 30-200 eV. The cross section shows an oscillatory behaviour. A semiclassical calculation has been completed, the results of which show the same behaviour but overestimate the magnitude of the cross section.

The process of H formation as a result of electron capture by protons from hydrogenic ions such as He⁺ and Li²⁺ has been investigated using the Coulomb-Born approximation with the total interaction potential. The calculated cross sections of capture in the ground state show a significant difference from the corresponding Born cross sections in the intermediate energy region. Capture into the excited 2s state of hydrogen is also considered in each case.

A distorted-wave model, which takes into account polarisation and absorption in the initial state, is applied to the ionisation of helium by electrons in the energy range 100-400 eV. Double, single and total cross sections are computed and compared with the experimental data.