Table of contents

Within a Hartree-Fock model it is shown that the collapse of excited atomic wavefunctions can be induced and, to a large extent controlled, by acting on the valence shell. The example of 4f wavefunctions in Cs I is considered, and controlled collapse is suggested as a means of producing a soft X-ray 'Q-spoiler'.

Ab initio calculations, within a perturbation treatment of the coupled Hartree-Fock equations are reported for the static dipole polarisability components of polar diatomics. Their dependence on internuclear-separation values around the molecule equilibrium positions is also computed and discussed. The relevance of the present results for vibrationally inelastic collisions of slow electrons pointed out in its relation with previous empirical estimates of the polarisability first derivatives.

A Z-expansion method, in which the spin-orbit and electron-electron Coulomb interactions are treated by first-order perturbation theory, is developed for ³P₁⁰-¹S₀ intercombination-line transitions in helium-like ions. Reliable results are obtained for ions with nuclear charge Z<or=14. It is shown that the range of validity of non-relativistic calculations is substantially larger for dipole-allowed than for intercombination-line transitions.

Highly excited levels of ytterbium have been investigated using two-photon absorption spectroscopy and space-charge detection. The authors tuned the 0.2 cm^-1 dye laser radiation over the range 3900-4100 AA and observed the even-parity states J=0 and J=2 belonging to the series 4f¹⁴6s ns ¹S₀, 4f¹⁴6s nd ¹D₂, 4f¹⁴6s nd ³D₂ from n=13-62, from n=11-64 and from n=11-21, respectively. Many strong perturbations appear due to configurations with a 4f¹³ core.

The Stueckelberg approximate two-state curve-crossing partial-wave scattering matrix is discussed and found to be a good approximation provided the parameters are correctly computed and the coupling region is well separated from the classical turning points. A novel parametrisation of the close-curve-crossing reduced scattering matrix for the BNO or DT model has been found. There are only two non-trivial parameters, but analytical results and preliminary numerical calculations show these to be slowly varying smooth functions of L.

Electron impact ionisation cross sections of the Li-like ions C IV and N V and the Be-like ions C III and N IV are calculated in a no-exchange Coulomb-Born approximation. Inner-shell ionisation is included but not autoionisation. Comparison with available theoretical and experimental data has been made. In C IV, the results agree with the recent calculations of Golden and Sampson (1977) and with exchange-classical impact-parameter calculations, which are about 30% higher than the crossed-beam experimental results of Crandall and co-workers (1978). In N V the results agree with those of Golden and Sampson but are now in close agreement with Crandall et. al.

The case of small inter-centre separations R in the two Coulomb centres problem is treated with the help of asymptotic methods. Particular attention is paid to the logarithmic terms in the expansions for the energy. It is shown that they correspond to the complicated behaviour of the wavefunctions near the origin. The agreement between the asymptotic formulae and the numerical results is discussed. An expression for the phaseshift in the continuum is obtained.

Calculations of the radiative properties of the Be I isoelectronic sequence, through Ne VII, have been carried out using a semi-empirical model-potential method. Bound-bound oscillator strengths among the lowest few states of the ¹S, ³S, ¹P⁰, ¹P, ³P⁰, ³P, ¹D and ³D symmetries are tabulated. Where accurate theoretical or experimental data are available, the agreement is excellent. Results are given for the ¹S₀-³P₁⁰ intercombination oscillator strengths for fine-structure splittings in the term 2s2p ³P⁰ and 2p²³P, and for the dipole polarisability of the ground-state ions.

The hyperfine structure of various isotopes of first-row atoms is investigated using configuration-interaction wavefunctions. The work extends earlier references by including systematically one- and two-electron excitations. Three-electron excitations were found to be unimportant. Generally good agreement is obtained with experimental A_J values.

Theoretical energy differences obtained from a multi-configuration Hartree-Fock calculation including spin-orbit interaction are reported for the levels of 2p⁵3p⁵ in Ar²⁺. The inclusion of configuration mixing leads to good agreement with the experimental energies of Nordgren et al. (1976) with respect to the absolute spread of the terms as well as many of the ratios of energy differences between the terms.

The relative transition probabilities of 27 lines of Ge I were determined from emission measurements. The branching ratios were measured for all the known transitions from five excited levels in the 4p5s configuration (4p5s-4p² transitions) and the relative intensities for some lines (4p4d-4p² transitions) in the 1900-4700 AA spectral range. To connect the relative transition probabilities of the different excited levels, the authors have studied the spectrum generated in a direct current arc plasma in an Ar-GeBr₄ mixture at high pressure. The emission spectrum has been analysed under the assumption that in this arc local thermodynamic equilibrium is established. The relative transition probabilities for six lines were calculated from radial emission coefficients obtained by side-on measurements.

Bands assigned to the gaseous FeF molecule have been photographed both in emission from hollow cathode discharges at low rotational temperature and in absorption in a King furnace. Observations of origin gaps and of lines of lowest J, combined with theoretical considerations, show that the ground state is X⁶ Delta _i.

For pt.I see ibid., vol.11, no.13, p.2281 (1978). Ab initio SCF-CI calculations of the electronic states of iron monofluoride have been performed with the aim of elucidating the ultraviolet spectrum of this radical. The bonding is ionic and the valence orbitals preserve their atomic character in the molecule. The ground state is found to be an inverted ⁶ Delta state associated with the (3d_Fe⁶( sigma pi ² sigma ³) 4s sigma _Fe2p_F⁶( sigma ² pi ⁴)) configuration. An energy-level diagram is built up which includes all the valence sextet states up to 60000 cm^-1. It demonstrates the close resemblance of the electron structure of FeF to that of the Fe⁺ ion. The observed systems of FeF are shown to originate from 4s-4p transitions corresponding to raies ultimes of Fe⁺: the observed ⁶ Pi and ⁶ Phi states both belong to a single configuration, (3d_Fe⁶( sigma pi ² delta ³)4p pi _Fe 2p_F⁶( sigma ² pi ⁴)).

The fluorescence response functions and quantum yields of four mixed crystal systems (naphthalene and p-terphenyl hosts; anthracene and tetracene guests) have been measured and analysed. The results are consistent with time-dependent exciton transfer, and the Powell-Kepler effect is not observed. It is concluded that the latter requires higher defect concentrations.

The present theory of low-energy scattering by a long-range potential has been examined by omitting the assumptions about the wavefunctions and taking the phase to be a function of the position of the scattered particle. The resulting integro-differential equation for the phase shift has been simplified for a particular case and it is found that the magnitude of the correction terms so obtained is quite large. It is shown that the two-potential formalism can be applied only to light pairs of scattering atoms.

The scattering of He²⁺ ions from molecular nitrogen has been studied in the angular range 0-20 degrees and in the energy range 200-600 eV. Elastic, simple inelastic, and single-charge-transfer scattering were observed using energy analysis of the scattered He ion to distinguish among the different processes at each observed scattering angle. The data are presented in the form of exit-channel spectra and cross section against reduced scattering angle. For scattering without electron capture, a binary collision model (in which one of the two nitrogen nuclei exchanges momentum with the projectile, the other acting as a 'spectator') is adequate up to a reduced scattering angle of approximately tau =E theta =6000 eV deg for describing the average elastic energy transfer. Evidence for spin-selection rules in low energy ion-molecule collisions was observed. For He²⁺+N₂ collisions the charge-transfer processes had the largest cross section throughout the range of energies and angles.

A beam-attenuation technique has been used to study the formation of fast metastable atoms in electron capture by 5-100 keV He⁺ ions in passage through both thick and thin targets of sodium, potassium and caesium vapour. Lower limits to the fraction of metastable atoms formed in the beam are obtained and used to determine cross section for metastable atom formation. Spectroscopic observations indicate that the 2¹S/2³S ratio in the collision products is very small. Cross sections for the collisional destruction of fast metastable helium atoms in H₂, He, Ar, and N₂ have also been obtained from the attenuation plots and compared with previous experimental data.

Differential cross sections for the elastic scattering of electrons from sodium have been measured with high angular resolution for incident energies of 54.4, 75, 100 and 150 eV and over an angular range of 12-140 degrees . The experimental data are compared with calculations based on the first Born approximation, the Glauber approximation and a close-coupling impact-parameter calculation. Calculations have been carried out for an optical model using the prescription of Vanderpoorten (1976) for localising the absorptive part of the potential. The optical model is found to reproduce best the general features of the cross section at all energies.

Free-free transitions have been observed in electron-argon scattering in the presence of the radiative field of a CO₂ laser. The ratio of the free-free and elastic differential cross sections has been determined at a scattering angle of 160 degrees and electron energies of 5, 11.15 and 16 eV. Good agreement was found with the soft-photon approximation at 11.15 and 16 eV. At 5 eV the correspondence was not as good, possibly indicating the expected failure of this approach towards lower electron energies.

Using an electron spectrometer and a CW CO₂ laser the author has measured free-free transitions (stimulated emission of a bremsstrahlung photon of 117 MeV) of electrons scattered elastically off argon atoms in the vicinity of the two resonances near 11 eV. An excitation function is obtained at 11-11.5 eV electron energy and 160 degrees scattering angle and it is compared to free-free cross sections calculated with the use of the soft-photon approximation. The agreement is excellent. For each resonance in the elastic cross section there are two resonance structures in the free-free cross section as predicted by theory.

Electron scattering by Ba atoms has been studied in a beam-beam configuration. Momentum-transfer and differential and integral cross sections for elastic scattering and for the excitation of the 5s5d¹D and 6s6p¹P states have been determined at impact energies of 20, 30, 40, 60, 80 and 100 eV in the 3 degrees to 130 degrees angular range.

Integral and differential cross sections for the parity-unfavoured excitations of the ³P_g and ^1,3D_u states of helium have been calculated using the Born-Oppenheimer approximation for the scattering amplitude and a variety of estimates for the ground-state wavefunction of the helium atom. These cross sections are found to be much more sensitive to the choice of target wavefunction than single-excitation cross sections which previously have been investigated. Thus, the integral cross section for excitation of the ³P_g state is only half as large when calculated with a 35-term CI helium ground-state wavefunction as with a Hartree-Fock function.

For pt.I see ibid., vol.11, no.13, p.2377 (1978). Regional wavefunctions of the type proposed by Rudge have been used to construct estimates of the integral and differential cross sections for the parity-unfavoured ¹S_g to ³P_g and ¹S_g to ^1,3D_u transitions of helium. Both single- and multi-configurational ground-state wavefunctions are tested. Results obtained by using the Born-Oppenheimer approximation are found to be quite insensitive to the choice of ground-state wavefunction. Cross sections computed in this way are similar to previous Born-Oppenheimer results based on conventional continuous wavefunctions for the target states. However, calculations based on the Rudge variation principle are found to depend very sensitively on the choice of target wavefunction and to exhibit several other unexpected characteristics.

The Fowler equation has been solved to obtain the mean energy expended per ion pair, omega , for electrons in molecular oxygen in the energy range from the ionisation threshold up to 500 eV. In the calculation the effects of Auger electrons are also considered. To test the accuracy of the continuous slowing-down approximation (CSDA) the values of omega , using the same set of inelastic cross sections, are also obtained from the ionisation threshold up to 10 keV. The study indicates that the Fowler equation values are always smaller than CSDA values. However, the difference between the two sets of values decreases with increasing incident energy. The results are in very good agreement with the available experimental data.