Table of contents

Single-channel quantum defect theory is extended to handle rapid variations of the quantum defect in a case where the functional form can be inferred, namely when a shape resonance occurs among the bound states, and an experimental example is analysed.

The X-ray intensity K_beta /K_alpha of 3d elements has been found to vary with excitation mode (photoionisation (PI) and electron capture (EC)) by a few per cent. The experimental data are complete by X-ray measurements of Mn. The effect of the excitation modes is discussed on the basis of the 3d excess electron in EC and the stronger shake processes accompanying PI excitation.

A study of the intensity distribution of almost 6000 spectrum lines indicates that the number of lines with a given intensity is inversely proportional to the square root of that intensity. The law appears to apply to smaller samples of only a few hundred lines as well as to the complete set. The analysis uses data from an iron hollow cathode lamp and is principally based on lines of Fe I, Fe II, Ne I , Ne II, Ar I, Ar II and He I lines are also included, and the wavelength range is from 2.6 to 0.28 mu m. The results is of application both to the calculation of plasma emissivity and to line blanketing in stellar atmospheres.

Radiative lifetimes of none levels of neutral rhenium are reported with an accuracy of +or-5%. Improved oscillator strengths derived from the lifetimes bring the spectroscopically determined upper limit for the solar abundance of Re closer to the abundance of Re in chondritic meteorites.

An exactly soluble model is presented for calculating the mean density matrix of a charged harmonic oscillator in the field of a multimode, high-power laser. Possible applications include multiphoton excitation of the vibrational degrees of freedom of a molecule, dissociation, and the excitation of optical modes in a crystal by chaotic laser light.

A time-of-flight technique was used to measure energy-dependent cross sections for the inelastic two-body collision: O(⁵S)+Xe+E_kin to O(³P)+Xe(³P₁) to O(³P)+Xe(¹S)+hv(146.9 nm). The kinetic energy range of the projectiles was between 0.5 and 10 eV. The cross section for this exoergic reaction decreases monotonically from 3*10^-16 cm² at the lowest available energy to roughly 1*10^-16 cm² for higher energies. A comparison is made with results for endoergic reaction channels in O(⁵S)+Xe collisions which show steep thresholds. At higher energies all cross sections are of the same order, but Xe(³P₁) excitation in the predominant process.

Reports a number of calculations for the ²P⁰ resonances in He^- below the 2³P⁰ state of He. These calculations indicate that there is no ²P⁰ shape resonance associated with the 2³S state of helium that the Feshbach resonance associated with the 2¹S state of He (as calculated by Chung (1981), and Bhatia and Temkin (1981)) is shifted down to the vicinity of the resonance computed by Burke et al. (1969) and Oberoi and Nesbet (1973), and that there appears to be no second ²P⁰ resonance between the 2¹S and 2³P⁰ states of He.

Ultraviolet transitions from helium-like neon recoil ions have been observed. The recoil ions were produced in a gas target by collisions with incident beams of krypton, xenon and bromine. The wavelengths of the 1s2s ³S₁-1s2p ³P_2,0 transitions in Ne⁸⁺ were measured to be 124.816+or-0.003 and 127.779+or-0.008 nm respectively. The results agree with previous measurements in a theta pinch, and with recent calculations.

Chemical influences on the relative K_beta /K_alpha X-ray intensity ratio up to five percent were found for different chemical constitutions of Cr, Mn, Fe and Cu by very precise PIXE measurements. In the discussion of these effects, screening of 3p electrons by a varying 3d valence charge, as well as polarisational effects are of importance.

The helium I photoelectron spectra of iron, cobalt and nickel have been recorded in the gas phase. Bands associated with ionisation from the outermost 3d and 4s orbitals have been observed and estimates have been made of the sigma _3d: sigma _4s photoionisation cross section ratios recorded at right angles to the photon beam. For all three metals evidence for ionisation from excited electronic states has been detected in the observed spectra and for iron and nickel it was possible to estimate the effective excitation temperatures as (1500+or-200)K and (1600+or-200)K respectively.

The angular distribution and the polarisation of radiation emitted following atomic inner-shell photoionisation by elliptically polarised radiation is investigated.

The spectra of HDO from 2500 to 3000 cm^-1 has been analysed at a resolution of better than 0.02 cm^-1. It was recorded at the Fourier transform spectrometer of the Kitt Peak National Observatory, Tucson. Ground-state constants of the reduced Hamiltonian, ground-state energies and transitions for nu ₁ and 2 nu ₂ bands are reported. The effects of a strong Fermi resonance on the spectra are discussed.

Ten Rydberg states of SiF have been observed below 2000 AA. Five of them have not been reported before. They all have been rotationally analysed.

Some aspects of the VPS theory for direct excitation in heavy particle collisions are critically examined. It is shown that the integral representation of the VPS amplitudes cited in the literature is in error, and its correct form is given. The theoretical basis for the use of the peaking approximation to simplify the mathematics of the VPS amplitudes is examined and it is shown that the terms thus neglected are related to the second-order and higher-order terms of a perturbative expansion. It is also shown that neglect to the term containing the projectile-core interaction from the VPS theory can not be justified, and this term plays an important role of cancelling a class of singularities which make the post form of the VPS amplitudes computed in the peaking approximation discontinuous. It is also pointed out that the VPS amplitudes for s to s transitions have logarithmic divergence which results from the well known cause of the mean excitation energy being equal to zero in the second-order perturbation theory term. The relationship of the VPS theory with eikonal approximations is also clarified.

Sharp interference structures in the angular distributions of autoionisation electrons from He⁺+He collisions are measured and interpreted quantitatively. A comprehensive derivation of a semiclassical theory is given which describes energy spectra and angular distributions of autoionisation electrons from collisionally excited atoms. The description includes post-collision interaction. Special emphasis is given to those cases where electron emission from target and projectile cannot be distinguished from each other, giving rise to interference effects. The experimentally observed interferences are reproduced without an adjustment of the free fit parameters, demonstrating the validity of the theoretical description. The author analysis shows that spin effects have to be taken into account for an understanding of the shapes of the observed interference structures.

Reports measurements of the distribution of final n states that are populated in charge transfer collisions of Na⁺ ions with Na in highly-excited Rydberg states. Relative cross sections for final principal quantum numbers n'=18-42 are shown for laser-excited Na initial states 24d, 25s, 28d, 29s, 33d and 34s at kinetic energies 550-2000 eV. These results span the velocity-matching region in which projectile and orbital speeds are comparable. At the matching velocity the final-state distribution exhibits a maximum close to the n' that corresponds to the smallest change of internal energy relative to the initial state. The distribution shifts toward lower n' and becomes broader as collision velocity is increased. Initial s and d states yield similar distributions above but not at the matching velocity. The results are consistent with excitation of only mod m₁ mod <or=1 sublevels referred to the incident beam direction, but several l' orbital angular momentum values appear to be present. Results for Ne⁺ and Ar⁺ incident on Na(24d) are similar to Na⁺ at equal velocity. The range of orbital speeds in a final-state distribution is approximately proportional to the projectile speed irrespective of initial state. The electronic orbital speed that corresponds to the centre of the distribution is a linear function of the projectile speed and initial orbital speed. The distributions disagree with predictions of the Brinkman-Kramers approximation.

A second-order model employing a pseudostate expansion in intermediate states is applied in a 1s-2s-2p coupled-channel formalism to electron scattering by atomic hydrogen in the energy range 54 to 200 eV. Although the model predicts cross sections for elastic scattering, and excitation of the n=2 levels, which are in reasonable accord with the experimental data, the predicted results for the angular correlation parameters show little improvement over the 1s-2s-2p close-coupling model.

The theory of electron-photon coincidence measurements is presented in the most general case where polarised electron beams are used to excite the collision target, a spin-orbit coupling effect during the excitation is present and plane symmetry is not assumed for the atomic collision process. The formalism is developed in such a way that natural and man made reflection asymmetry effects are represented separately. In particular, the correlation between the electron polarisations and the natural plane asymmetry effect is expounded. It is then shown that this correlation can be exploited to enhance the plane symmetry violation effect for detection. The formulae obtained are equivalently applicable to the superelastic scattering case.

A new, differentially pumped, multidetector electron spectrometer has been constructed and has been used to study the energy-loss spectrum of molecular oxygen. Spectra are presented with an energy resolution of approximately 30 meV (FWHM), in the energy-loss range 14.8-20.3 eV. Observation of the relative intensities of spectral features as a function of scattering conditions leads to the assignment of four new Rydberg series. Transitions from the ground state of the molecule to states in these series are symmetry forbidden. Two of the series converge on the fourth ionisation potential at 18.17 eV and two on the level at 20.296 eV, each pair comprising an ns sigma _g and an nd sigma _g series. Several other isolated, spin-forbidden transitions are also observed.

The authors study four-wave mixing when both the two-photon transition between the ground state and an excited state mod 0> and the one-photon transition between that excited state and an autoionising state mod 1> are nearly resonant. When mod 0> and mod 1> are strongly coupled, one of the eigenstates resulting from their mixing is almost stable against ionisation under certain conditions of frequency and intensity. The authors show that in this case an efficient frequency conversion should be obtained, since losses due to ionisation are expected to be strongly reduced.