Table of contents

Relativistic wavefunctions for K-shell electrons have been introduced in the binary encounter approximation for atomic ionisation together with an approximate relativistic correction of the collision dynamics. A comparison with experiment is given.

Ab initio calculations of spin-orbit coupling constants are reported for a number of first-row atoms and ions using a universal systematic sequence of even-tempered basis sets of exponential-type functions. The convergence of the calculations with respect to size of basis set is examined and extrapolation to the basis set limited made. The importance of the present atomic calculations to the determination of the fine structure in molecules is discussed.

The results of HXR calculations of the 3d subshell absorption spectra of Rb I and Sr I are presented. Both the transition energies and their intensities are compared with experiment and the resulting assignments of the experimental peaks are discussed in relation to those obtained by self-consistent field calculations and by the Z+1 approximation. The limitations in the applicability of the Z+1 model in cases where strong s-d mixing is induced by wavefunction collapse are indicated.

The 3d^-1 to 4p^-1 X-ray emission spectrum of Xe has been calculated from first principles by diagrammatic many-body theory within the framework of a free-atom calculation. The agreement of the theoretical spectrum with experiment is good. The spectrum is entirely dominated by the strong dynamical effects of a final 4p hole. The one-electron picture of a 4p hole breaks down by a strong dynamical giant Coster-Kronig 4p^-1 to or from 4d^-2m, epsilon f process.

Coupled Hartree-Fock perturbation theory is applied in accurate ab initio calculations of molecular dipole moment, polarisabilities, first hyperpolarisabilities, electric field gradients and nuclear electric shielding tensors in LiH. Their dependence on internuclear separation is also analysed. According to the theoretical investigations, the molecule is characterised by a very high hyperpolarisability, very strong hyperpolarisability gradients and enhanced electronic shielding about the hydrogen nucleus.

Oscillator strengths of 43 W I lines and 27 W II lines in the wavelength range 2400-5600 AA have been measured with a wall-stabilised arc. The arc was operated at 10-80 A in argon with an admixture of tungsten hexafluoride. A set of relative W I f values was determined by a combination of hook and emission measurements requiring no assumptions concerning the plasma state. The relative set has been converted to an absolute scale by means of radiative lifetimes. The overall uncertainties of the oscillator strengths are within 10-36%. The relative W II oscillator strengths have been measured by applying the standard emission technique, which requires the determination of the arc temperature in addition to measurements of the line intensities. The experimental uncertainties are about 10% but increase to 20% for weak lines.

The effect of binary collisions on the fluorescence spectrum of an atom in a nearly resonant monochromatic radiation fievd is evaluated in terms of formal relaxation and correlation operators that refer to a single collision. The driving field can be included in the atomic Hamiltonian, and its time dependence is transformed away, giving rise to a modified atom. The authors relate the strengths and the widths of line occurring in the fluorescence spectrum to the rate of optical collisions, and to other elements of the scattering matrix of the modified atom.

The effect of resonant two-photon ionisation processes on the AC Stark-split spontaneous emission spectrum is determined using master equation methods.

The population dynamics and spectra of a three-level system driven by two applied fields of arbitrary amplitudes and detunings are examined using dressed-state techniques. The authors report the existence of novel dispersive features in the variation with detuning of the intermediate-state population.

For a spin-1 atomic system subjected to an oscillating radiofrequency field applied at right angles to a DC magnetic field, the Block-Siegert shift has been calculated to sixth order in V, the amplitude of the oscillating field, and the three-quantum resonance has been calculated to eight order. The results are obtained for the three different polarisations- pi , sigma and t (unpolarised)-and expressions for the maxima are also obtained. For simplicity the atomic levels were assumed to have zero natural lifetime. It is shown that the level-crossing criterion may be used to calculate the positions of the p-quantum resonance minimum to the order of 2(p-1) in the oscillating field amplitude. Expressions are given for this quantity to order V⁶, and the separation of the maxima is calculated to order V^p. The author's analytic results are compared with the numerical and experimental work of Jacobson (1980) and reasonable agreement is found.

New measurements of coherent (Rayleigh) scattering of photons of energies in the vicinity of K-absorption edges of gold and lead atoms together with other recent measurements with a coverage in the momentum transfer region up to 1.0 mc are presented for a critical evaluation of (i) the latest relativistic calculation of coherent scattering factors, (ii) the anomalous dispersion correction to the scattering factors and (iii) the exact numerical Rayleigh scattering amplitudes of inner electron shells. This evaluation reveals the ranges of applicability of these calculations and indicates the trend of behaviour due to the proximity of K-absorption edges of scatter atoms.

Seven Rydberg electronic states of SiF have been observed in the energy range 42000 to 50000 cm^-1. Five of them, three ² Sigma ⁺ and two ² Pi , have never been reported before. They have been rotationally analysed and, for the two states already known, C'² Pi and D'² Pi , the rotational analysis has been appreciably extended.

For pt.I see ibid., vol.14, p.985 (1981). A general theory of the Rydberg atom inelastic scattering by perturber atoms has been developed previously, in which the leading term is the simple impulse amplitude, followed by the various correction terms. Theoretical estimates of these correction terms are presented in this paper, resulting in a general scaling formula. The non-impulse correction to the impulse approximation varies as n^-2 at large principal quantum number n and depends on the signature of the electron-perturber scattering length. The distortion correction due to the interaction between the perturber and the Rydberg atomic core is large for the intermediate values of n<or approximately=20 behaves as n^-4, and depends on the perturber-core total cross section. The scaling formula derived here has been tested for the scattering systems consisting of Na and Rb targets and inert gas perturbers.

The authors recently developed 'one and a half centered' expansion (OHCE) method for impact parameter coupled-states calculations is applied to ionization and charge transfer in H⁺+He⁺ collisions. Their cross sections are compared with other theoretical calculations, and with the experiments of Gilbody and co-workers (1978). Some disagreements are noted. By applying time reversal to the authors' H⁺+H3⁺ calculations, they obtain He²⁺+H total capture and He⁺(2s) capture cross sections in good agreement with experiment.

Calculations have been made of cascade effects on Zeeman quantum beats for the case of beam-foil excitation of certain levels in Ar II. It is noted that cascade contributions significantly affect the quantum beat or oscillatory part of the total light intensity. However, whilst determination of the lifetime of the intermediate level via quantum beat measurements is significantly affected by cascade contributions, the determination of the Lande g factor for the intermediate level is essentially unaffected.

The authors have measured the impact parameter (b) dependent probability for the production of electrons from collisions of 32 MeV S⁵⁺ and S¹¹⁺ beams with an argon target. Electrons having energies between 0.3 and 3.5 keV were detected at a mean laboratory angle of 130 degrees in coincidence with scattered projectiles and with K X-rays from both target and projectile. The argon K Auger electrons are clearly separable in the coincident electron spectrum and show an impact parameter dependent probability, P(b), identical to that measured earlier for K X-ray production. It is thus established that disagreement at small b between the experimental P(b) and the 2p pi -2p sigma rotational coupling P(b) may not be attributed to a b dependence of the K fluorescence yield. An attempt to isolate direct K electron ejection to the continuum was unsuccessful because of the strong yield of continuum electrons apparently originating from projectile and target L electrons. In this interpretation, the target electrons show the interesting feature of decreasing production probability at small b.

The energy dependence and the absolute value of the cross section for the P_1/2 to P_3/2 fine-structure transition (FST) in Rb(5P) induced by H₂ or N₂ molecules and He atoms has been measured for the first time in a crossed-beam apparatus. The results for He, in excellent agreement with previous experimental data, are used for comparing atomic and molecular target behaviour with regard to FST processes in heavy alkalis. One observes much larger cross sections for H₂ and N₂ at low energies with a much weaker energy dependence. Theoretical work suggests two seemingly competing mechanisms for including large FST cross sections for molecular targets. One, involving an intermediate ionic complex, accounts for the general trend (absolute value and energy dependence) of the experimental data for N₂ while the other, recently suggested by Hickman seems to play the major role when H₂ is the target.