Table of contents

It is shown that some recent results due to Coulson and Nash (see abstr. A39588 of 1974) are in fact independent of the particular basis set used by them.

Quantum electrodynamic calculations involving the resonant Stark effect are reported. By including the effects of multiple spontaneous emissions for a two-level atom in a resonant applied field solutions are obtained which are valid over many radiative lifetimes. The resonance fluorescence spectrum is also calculated as a function of the detuning parameter. The theoretical results are in agreement with the experimentally observed long term fluorescence spectrum.

The nucleus-electron, as well as the electron-electron potentials, in all atomic systems are shown to have a simple expansion into hyperspherical harmonics. This expansion is useful for the calculation of matrix elements.

Collision strengths for electron impact excitation of Nv at an incident energy of 6.0 Ryd are calculated in a number of close-coupling approximations. Coupling effects are found to be more important than exchange effects for the 2s-3p transition which result in a close-coupling collision strength 40% larger than that calculated in a distorted wave approximation by Flower (see abstr. A76970 of 1972). For 2s-3s and 2s-3d transitions exchange effects are more important than coupling effects, and the close-coupling and distorted wave results agree to within 13%.

Results are presented for calculations of the photoionization cross section of the 4d subshell of Ba I in the Hartree-Fock and random phase approximation with exchange. Both results are in qualitative agreement with the experimental data of Connerade et al. (1974).

Initial measurements of ArKLL and KLM Auger electron lines produced by 4 MeV proton bombardment are reported. Energies and relative intensities of all observed peaks are tabulated. For the identifiable KLL transitions, the measured relative intensities are compared to theory.

The flatness of the energy differential cross section in impact ionization is derived analytically in the Wannier theory. However it is shown that the Wannier zone is confined to a region of the order E/5<or= epsilon <or=4E/5, E is the available energy and epsilon is the energy of one of the electrons. By contrasting the known results of photoionization and photodetachment it is argued that in the complementary region where the electrons share their energy very unequally the cross section rises to a value independent of E (aside from a modulation factor) and that this region determines the form of the threshold law.

Relative cross sections for the production of Lyman-alpha radiation in collisions of atomic metastable hydrogen with molecular hydrogen have been measured at twenty-five velocities between 6*10⁵ and 3*10⁶cms^-1. The measurements were carried out using time of flight techniques in combination with a recently developed target pressure modulation. Results for the Lyman-alpha emission cross section exhibit the same velocity dependence as the total destruction cross section (see abstr. A78529 of 1973) and are in complete disagreement with earlier measurements of Comes and Wenning (see abstr. A7068 of 1970).

The energy and angular distribution of H^-(D^-) produced by a dissociative attachment reaction of 6.5 eV electrons with H₂O(D₂O) has been determined. The angular distribution of H^- peaks at 100 degrees and the OH fragments is rotationally and, in a large part, vibrationally excited.

The variation of two-electron properties such as (1/r₁₂), (r₁₂) upon variation of the internuclear distance has been investigated for some excited states of the hydrogen molecule. A non-monotonic behaviour of these quantities in the ³ Pi _u state is contrasted with the monotonic behaviour in the ¹ Pi _u as well as in the ³ Sigma _g. The larger correlation energy in the ³ Pi _u than in the ¹ Pi _u is related to the differences in the Pauli precorrelation between them.

Zirconium and niobium spectra were recorded on the 5m grazing incidence spectrograph from 125AA to 440AA. All the eight 6s levels are found in both the spectra. Other levels determined were four 7s, twenty 5d and eleven 6d in Zr VI, and six 7s, twelve 5d and thirteen 6d in Nb VII. The analyses are based on 52 classified lines of Zr and 45 of Nb. It is suggested from isoelectronic comparisons that the designation of the 6s²P_3/2 and ⁴P_3/2 levels of Rb III should be interchanged. The authors evaluate the ionization potential 4p⁵²P_3/2-4p⁴³P₂.

In atoms a distance z from a perfectly conducting plane, electric dipole frequencies omega suffer shifts of order (d²)/z³ for small z, where d is the dipole operator. A totally different expression has been proposed recently, behaving like (d²) omega ³ log( omega z); it can emerge from some calculations on a two-level atomic model. One reason is an unrepresentative cancellation, peculiar to the model, between leading terms in the shifts of the two levels. A deeper failure of the model is its inconsistency with basic quantum mechanics, in that the upper level cannot obey the Thomas-Reiche-Kuhn sum rule, allowing results to depend on just how the calculation is performed.

The Doppler effect reduces the radiative interaction between identical atoms so that only those for which k. Delta v <or approximately= pi Gamma can couple coherently (where Delta v is the relative velocity, k=2 pi / lambda and Gamma is the lifetime of the isolated atom). The authors conclude that this coherent interaction cannot explain the 'extrapolation anomaly' of resonance broadening.

The lifetime for a pair of atoms with one quantum of excitation is calculated by conventional perturbation theory without the assumption of fixed separation between the atoms. Several cases for the initial preparation of the state of relative motion are considered. For plane wave states there is no coherence, for the state with locked-in relative displacement the usual coherence is deduced, while for a nearly classical state with minimum dispersion in both relative displacement and relative momentum the result of Cooper and Stacey (see abstr. A19687 of 1974) is confirmed. The resonance energy is shown to be unaffected by the relative motion.

A unified theory of collisional line profiles has been applied to the case of the D₁ and D₂ lines of caesium perturbed by xenon for which experiments are available. A Lennard-Jones potential has been used and interaction anisotropy has been included. It is shown that Hund's c case is valid, which allows us to simplify the formalism. For intermediate pressures the computed profiles show multiple satellites whose reality is corroborated by an experimental filtered profile. The asymmetry parameter is shown to be a good choice to use for testing interatomic potentials.

An expression is derived for the intensity of electrons inelastically scattered from an atomic state prepared by optical pumping. The expression is a sum of state multipoles, the coefficient of each multipole being a spherical harmonic in the polar coordinates of the light direction for circularly polarized light, or the direction of the electric vector for linearly polarized light times a weight function depending on the population of the initial state. These weight functions are given in closed form for both linearly and circularly polarized light.

Complex coordinates are used in the development of a variational principle for Siegert resonant states. The principle is first tested by application to a potential scattering problem. The position and width of the lowest ¹S resonance in e-H scattering are calculated to be 9.557 eV and 0.048 eV respectively. The corresponding values for the lowest ²S resonance of He^- are 19.398 eV and 0.012 eV.

For pt.IV, see abstr. A5820 of 1972. A phase shift analysis of electron-helium scattering in the energy range from 10^-2 to 10² eV and of electron-neon scattering from 3*10^-2 to 60 eV is reported, using new measurements of the elastic differential cross sections by McConkey and Preston (see abstr. A71429 of 1973). Error estimates are assigned to the derived s, p and d wave phase shifts for electron-helium scattering, and the conjectured dispersion relation further tested. The results of the analysis of electron-neon scattering remain in good agreement with adiabatic-exchange theory, though the s-wave phase shift shows anomalous behaviour at intermediate energies. Estimates of the imaginary part of the optical potential are obtained for both systems.

For pt.V, see ibid., vol.7, no.16, 2203 (1974). A phase shift analysis has been made of elastic electron-mercury scattering in the energy range below the inelastic threshold, 0.5 to 4.5 eV. Varying the 0, 1^+or-, 2^+or-, 3^+or- phase shifts a good simultaneous fit to the polarization, differential cross section and total cross section could not be found, apparently because of inconsistencies between the various sets of experimental data. In particular, it appears that the available measurements of the total cross sections are in error. The computed phase shifts agree reasonably with the relativistic, polarized orbital calculation of Walker (see abstr. A75726 of 1971), at 3.5 eV, but not with calculations made without allowance for atomic distortion.

The mechanism of fine structure transitions in atom-atom collisions is discussed, within the framework of the molecular representation, in terms of the evolution of the colliding system from one Hund's coupling case to another. Simple models of the process can be constructed. The results are compared in the case of Na(3p)-He collisions with the results of exact calculations. Choosing the molecular potentials given by Nikitin or by Bottcher and taking into account trajectory effects, the calculated cross sections are in close agreement with experiment. The discrepancy with the quantal results of Reid, which is about 20% for the fine structure transition, is primarily due to the use of different potential and to trajectory effects.

Relative integrated emission intensity measurements have been made photoelectrically on the bands in the wavelength region 2580 AA to 4170 AA. These measurements have been interpreted with the aid of computed arrays of Franck-Condon factors and r-centroids to determine the variation of electronic transition moment with effective internuclear separation. A smoothed array of relative band strengths was thereby obtained. The array was placed on an absolute scale using radiative lifetime measurements of Smith (see abstr. A20137 of 1970) on the B³ Sigma state of SO. Arrays of Einstein A coefficients and band oscillator strengths are also presented for these bands.

Absorption bands of the gaseous SeS molecule have been observed using separated selenium isotopes ⁷⁸SeS and ⁸⁰SeS. Rotational analysis of bands in the region 3700 to 4300 AA shows that they arise from three sub-systems, B 0⁺, A 0⁺ from X 0⁺ and B 1 from X 1, in which the Omega =0⁺ and 1 sub-states of the states X and B are formed by the F₁ and F₂, F₃ components respectively of ³ Sigma ^- states. The upper levels are all perturbed and there is a homogeneous interaction between A 0⁺ and B 0⁺. It has been possible to determine the vibrational numbering in the B-X system from the isotope effect between ⁷⁸SeS and ⁸⁰SeS.

An approximate semiclassical treatment of rotational transitions in the close-coupling domain has been applied to the system Ar-CO. The distortion of the curved planar trajectory is accounted for by the use of the weighted mean energy and angular momentum in the equations of motion. The close-coupling between several rotational levels is considered in the higher order matrix terms and the total transition probability from a given state never exceeds unity. Cross sections were computed at several energies and different values of the repulsive anisotropy parameter. The results bring out some interesting features of rotational transitions in atom-diatom collisions and in particular the important role of the repulsive anisotropy in the interaction potential.

An attempt has been made to test the validity of the theory of inelastic collision effects on thermal diffusion of eccentrically loaded polyatomic gaseous molecules. Moments of inertia of ten molecules with varying amounts of eccentricity have been calculated, and compared with experimental values of the moments of inertia. The comparison shows that the theory is not widely applicable.

Breakdown in the rare gases caused by picosecond laser pulses from a Nd:glass laser is investigated. Threshold intensity for breakdown is measured as a function of pressure and measured values compared with those expected from theory. It is shown that an implicit assumption has been made in past theoretical treatments which may not apply on picosecond time-scales. Under certain conditions, this can lead to a multi-photon breakdown process which is density dependent. The experimental results are explained on this basis.