Table of contents

It is shown that by pure scale transformations the non-relativistic Coulomb calculations can be made relativistic, and vice versa, and also, bv scale transformations, all orbits can be mapped on the same energy shell.

Preliminary studies have been made of a system which consists of a molecule placed in an intense electromagnetic field resonant with a molecular transition frequency, and a probe field containing frequencies in the neighbourhood of the resulting dynamic Stark splitting frequency 2 epsilon . In these studies, only a single decay process was considered. Both emission and absorption of radiation by the system are predicted at frequencies near 2 epsilon , provided that the molecule has different permanent dipole moments in its lower and its upper states. A correct treatment, which allows for a cascade of emitted photons, is expected to bring about some quantitative changes in the shape of the predicted spectrum, but to leave unchanged the basic prediction of transitions at frequency 2 epsilon .

The two-photon decay of metastable atomic hydrogen has been observed for the first time. A series of tests which confirm the observations are described, including preliminary measurements of the angular distributions and spectral distribution for the two-photon decay.

The authors have observed zero-field quantum beats between hyperfine levels in the 3p ³P term of ¹⁴N IV. They obtain an A value of 705+or-30 MHz for this term.

The partial-wave Coulomb T matrix has been determined in analytical form and its high-energy behaviour has been investigated.

It is shown that the approximation which gives the equivalence of the results obtained by Zont (1975) and by the present authors is the low-frequency approximation.

Evidence is presented, in the context of the positron-hydrogen atom zero-energy scattering problem treated with approximate (distorted) hydrogen wavefunctions, that it is important in a low energy scattering calculation involving an approximate target wavefunction that this target function should display the correct polarizability. The calculations have been carried out through 'variant IV' of Peterkop and Rabik (1971) and through the 'method of models' of Drachman (1972).

The absolute differential excitation cross section of the 2³S state of helium at 90 degrees scattering angle is obtained from threshold to 3.6 eV above. The energy transmission efficiency of the post-collision analyzer of the electron impact spectrometer is obtained from the known characteristics of the ionization process near threshold. The results are made absolute by normalization to accurate elastic cross section of helium. Recently calculated cross sections are in good agreement with these measurements.

Two He^- resonances have been observed near 60 eV in the total cross sections for the formation of metastable neutral helium. Comparison of measured resonance energies with values obtained by other methods shows that they result from the perturbation by the (2s²2p)²P and (2s2p²)²D negative ion states.

Kohn's variational method is used to calculate low energy positron-helium p-wave phaseshifts and p-wave contributions to the annihilation rate parameter Z_eff for the two helium models considered previously by Humberston (1973). The phaseshifts for both models are less positive than the lowest set of Drachman, but the cross sections and annihilation rates obtained from the present results and Humberston's s-wave results, for the more elaborate helium function, are in good agreement with the experimental values of Canter et al. (1973) and Coleman et al.

Autoionizing levels in potassium have been observed by measuring the energy spectrum of electrons at 90 degrees to the direction of an incident electron beam. Spectra were taken at 29 eV and 500 eV incident electron kinetic energies. Where possible comparisons have been made with the ultraviolet absorption spectra of Beutler and Guggenheimer and Mansfield (1975).

A group of metastable autoionizing levels in beryllium has been observed after Be⁺ ion bombardment of 7 mu g cm^-2 thin carbon foils at 300 keV incident ion energy. Two peaks appearing at 102.7 and 106.7 eV are interpreted to be related to the Auger decay of the Be I(1s2s2p²) ⁵P^e term. Lifetimes of the (1s2p ²) ⁴P^e state in Be II and the (1s2s2p²) ⁵P^e state in Be I have been determined. Different decay mechanisms are discussed.

In 1938 a band system was observed by Barrow and Jevons and assigned by them to the SiS radical, but the nature of the two states involved remained unknown. Using high resolution spectra the authors have been able show that these bands represent the longer wavelength end of a well known system of SiS: the E¹ Sigma ⁺-x¹ Sigma ⁺ transition.

Compares the available theoretical predictions for the energy of the ¹ Pi _u(1 sigma _u2 pi _g) autoionizing state of H₂. The present results cast serious doubt on the previous assignment of this state by Bottcher (1974) as the state responsible for the 4 eV protons which are observed in the dissociative ionization of H₂ by electron impact.

Discusses the theory of Doppler-free two-photon transitions. The theory is worked out for the case of one arbitrarily strong field and one weak field. The Doppler limit is assumed. These simplifications allow the authors to discuss the experimental manifestations of various parameters. They consider power shifts and broadening the number of resonant peaks and their positions. The general case of an off-resonant intermediate state and the single-frequency case with a resonant intermediate state are discussed. The former is found to lead to a power shift, whereas the latter is unshifted in a certain region. In addition the resonant case gives an enhancement without an appreciable increase in width.

The angular distribution of photoelectrons predicted for elliptically polarized light is shown to be the same as that predicted for partially polarized light having incoherent perpendicular electric field components equal to the electric field components along the major and minor axes of the ellipse.

A simple and tractable quantum expression of the line wings is given when the upper and lower levels of the spectral lines are both perturbed. This quantum profile converges towards the impact regime at small distances from the line centre and towards the quasistatic limit when the usual semiclassical approximations are made. The correspondence between quantum and semiclassical expressions is examined, and the passage from the former to the latter clarifies the implicit assumptions which are always made in any unified semiclassical approach.

Current models of quasars assume the presence of strong magnetic fields An ion moving in this magnetic field also experiences an electric field in its rest frame. Perturbation by this motional electric field mixes parity so that the forbidden magnetic-dipole transitions can possibly be turned into the much faster electric-dipole transitions. The magnetic field itself also mixes state changing the transition probabilities. It is found (i) that an electric field above about 10⁸ V cm^-1 is required to cause a significant enhancement of the forbidden line intensities in the quasar spectra and (ii) no such effect can be expected from the magnetic field.

The molybdenum spectrum was photographed with Edlen's 5 m grazing incidence spectrograph at Lund (Sweden) from about 25 AA to 440 AA. Transitions from 5d, 6d and 6s, 7s to the ground levels of Mo VIII are identified for the first time, establishing all eight 6s and 7s as well as seventeen 5d and twelve 6d levels. The eighth ionization potential of molybdenum 4p⁵²P₃2/⁰-4p⁴³P₂ is evaluated from ns and nd Rydberg series to be 1157.9+or-8.0 kK or 143.5+or-1.0 eV.

Differential cross section for electron impact excitation of the 2s and 2p states of atomic hydrogen have been calculated at all angles for energies of 50 to 200 eV in a distorted wave polarized orbital approximation (allowing for target distortion). The results are compared with other theoretical models, and the total n=2 excitation differential cross sections are compared with recent absolute experimental data of Williams and Willis. There is complete agreement at small and moderate angles, but the method underestimates the experimental results in the backward direction. It is argued that the inclusion of final-channel distortion effects will remedy this defect. The effect of target distortion on the 1s to 2p total cross section is discussed.

Total and differential cross sections are calculated for electron impact excitation of the 2¹S, 2³S and 3¹S states of helium in a distorted wave model at impact energies between 30 and 300 eV. The results are compared with other theoretical models and with recent absolute experimental measurements.

The autoionizing states of helium that exist beneath the n=2 threshold of He⁺ have been extensively investigated using a low energy (10-200 eV) electron impact spectrometer. The instrument was purpose-built for studying the electron ejection spectra of autoionizing states and features a high intensity ( approximately 1 mu A) incident beam and a high resolution ( approximately 40 meV) electron energy analyser. The energies of a number of the autoionizing states of helium have been measured, and our values are compared with previous results of theory and experiment. In a few cases the resolution of the instrument has allowed us to estimate the widths of the very short-lived states. An outstanding feature of the spectra obtained at energies just above the thresholds for excitation of the autoionizing states is the phenomenon of displaced thresholds.

The single- and double-electron-loss cross section for He^- ions passing through H₂, He and Ar gas targets have been studied in the impact-energy range from 100 keV to 3 MeV. He^- ions were formed from He⁺ by charge transfer in a gas cell and the cross sections were determined by the initial-growth method. The experimental results for single-electron loss are compared with theoretical estimates based upon the semi-empirical classical impulse approximation of Bates and Walker (1966). For H₂ and Ar targets, theory and experiment agree well within experimental error while the present results for He targets are 20% higher than the theoretical values.

Proton impact excitation of positive ions is investigated using the quantum approach of the close-coupling method. The theory is applied to fine structure transitions of atoms with np² configurations. Results are presented for the ³P₀ to ³P₁, ³P₀ to ³P₂ and ³P₁ to ³P₂ transitions in Fe⁺¹² and are compared with those obtained by other methods at low and high energies.

The generation of a set of one-electron molecular orbitals suitable for the description of inner-shell excitation in heavy-ion-aton collisions is described. The collision of neon with oxygen is taken as an example of a slightly asymmetric (heteronuclear) collision. The adiabatic orbitals obtained from approximate Hartree-Fock solutions at varying internuclear separation exhibit avoided crossings at which the orbital character changes abruptly. These changes result in a singular behaviour of the dynamic couplings between the adiabatic orbitals. A transformation is prescribed, leading to a diabatic set of orbitals which preserve their character at crossings and between which the dynamic couplings vary smoothly with internuclear separation.

For pt.I, see ibid., vol.8, no.11 (1975). The differential and total cross sections for ionization of the K shells of neon and oxygen atoms in close collisions are calculated. The time-dependent electronic wavefunction is expanded in terms of the four lowest diabatic one-electron MO(1s sigma , 2p sigma , 2s sigma and 2p pi ) of the neon-oxygen molecule. These MO are derived from adiabatic Hartree-Fock orbitals. Vacancies are created in the K shell of oxygen, predominantly by the rotational 2p pi -2p sigma coupling at small distances of approach. The ionization of the K shell of neon is due largely to the subsequent transfer of these vacancies into the 1s sigma orbital by the 2p sigma -1s sigma radial coupling. It is shown that coupling to the 2s sigma orbital has little influence of the K-shell cross sections. The calculated total cross sections are compared with existing experimental results.

The interelectronic distribution function is studied as a function of x₁₂, of y₁₂ and of z₁₂ for the lowest ^1,3 Pi _u states of H₂ using simple uncorrelated wavefunctions. The coordinates are defined so that the z₁₂ axis is parallel to the internuclear axis, and so that the pi -electron density reaches its maximum in the x₁₂ direction. It is found that the distribution functions and their differences in the x₁₂ direction are substantially greater than those in the y₁₂ or z₁₂ directions. The triplet electrons are more likely to be found at intermediate x₁₂ values, while the singlet electrons have a greater probability of being found at intermediate x₁₂ values, while the singlet electrons have a greater probability of being found at small or large separations. The Pauli principle is thus shown to be local in effect, at least in the x₁₂ direction for the lowest ^1,3 Pi _u states of H₂. This is consistent with known results for the lowest ^1,3P states of the helium atom, and accounts for the higher interelectronic repulsion in the ³ Pi _u state.

The rotational spectrum of the nu ₃ states of OCS and N₂O, whose levels lie near 2000 cm^-1, has been observed through a vibrational energy transfer from active nitrogen. The frequencies of many transitions between 48 GHz and 158 GHz have been measured; a determination of the rotational and centrifugal distortion constants of these states has been derived.

A method, based on evolution operators and aided by diagrams is used to obtain a semiclassical expression for the transition probabilities in the time-dependent formulation of scattering theory. As a consequence of the Feynman correspondence identity, by which the time evolution of systems with quadratic Lagrangians is given exactly by semiclassical mechanics, the theory is exact for a simple model of vibrational excitation. This is demonstrated explicitly. The present form of the theory is not directly suitable for numerical computations.

The equivalence between the adiabatic-nuclei and the frame-transformation theories of e^--molecule scattering in certain approximations has been established. It has also been shown that in the adiabatic-nuclei theory it is immaterial the order in which one calculates the K and T matrices from the fixed-nuclei parameters.

The diffusion cross section and the mean scattering cross section have been calculated for slow electron scattering by a polar molecule considering the effect of short range attractive and repulsive potentials. The effect of an attractive short range potential is to reduce Altshuler's cross section (1957), while the repulsive short range potential increases Altshuler's cross section. It is observed that a Coulomb one-centred short range potential leads to a very small change in the cross section calculated by the Born approximation.

The thermal conductivity of carbon dioxide is measured from continuum heat transfer measurements in a carefully designed column in the temperature range 350 to 2000K and with an estimated maximum uncertainty of about 2%. The data are correlated with the following cubic polynomial in temperature, T: k(T)=-10.280+0.0942T-0.1937*10^-4T²+0.2619 *10^-8T³. Here k is in mW m^-1K^-1 and T in K. The conductivity values are compared with the other available data and with the predictions of various theories developed for polyatomic gas systems. The conductivity data are also employed in conjunction with theory to compute the effective diffusion coefficient for the transport of vibrational energy as a function of temperature.

Parameters describing the deviations from local thermodynamic equilibrium of the populations of excited levels of C II, C III, C IV and F II ions in isolated plasmas have been calculated. The values apply to plasmas with electron densities from 10¹⁵-10²⁰ cm^-3 and electron temperatures from 0.5-20 eV. Means of extending the data to lower densities and non-isolated plasmas at higher temperatures as well as to other ions in the same isoelectronic sequences are also described.

The lifetime of the ¹ Sigma _g⁺ oxygen state produced by a corona discharge in oxygen has been measured by a spectroscopic method. The value obtained by this method is about 80 times shorter than that deduced from known reaction rates with pure oxygen. This large difference can be explained by the presence in the discharge of species other than O₂. The ozone concentration is large enough to reduce the lifetime by such a factor.