Table of contents

The paper is concerned with techniques for the computation of functions which are solutions of the coupled equations for electron collisions with positive ions, and which have known asymptotic forms. Previous work is reviewed, a new iterative technique is described and some illustrative numerical results are obtained.

The resonant differential scattering of electrons by helium has been investigated for energies close to 19·3 ev. A data-logging technique was used to obtain exceptionally high signal-to-noise ratios. Analysis of the results gave δ₀ similar,equals 111°, δ₁ similar,equals 17° and δ₂ similar,equals 3° for the first three phase shifts for potential scattering at the resonance energy, and 8 mev for the natural width at half-height. Structure was observed at 19·45 ± 0·05 ev which had the properties of a p-wave resonance with a natural width of order 0·25 mev.

A method is suggested for measuring the polarization of low-energy electron beams by observing the polarization of the light emitted by mercury atoms following their excitation by electron impact. It is shown that the asymmetry of the left and right circularly polarized components of the 2537 Angstrom radiation of the even isotopes is approximately equal to the longitudinal polarization of the incident electron beam. The efficiency of such a detector is estimated for incident electron energies near the excitation threshold and its expected performance compares favourably with that of a typical Mott detector. Some distinctive advantages of the proposed scheme are pointed out.

The process of emission of radiation in the optical continuum, due to radiative capture of an electron by a positive ion, is discussed using a theory similar to the radiation damping theory of Weisskopf and Wigner for transitions between bound states. The results obtained are required for the development of a rigorous quantum-mechanical theory of di-electronic recombination.

A variational method for resonance widths suggested by Bransden and Dalgarno is examined in detail. Its formal equivalence to other theories is demonstrated. The equations which result from the use of a linear combination of trial functions are developed. Calculations are presented for several low-lying resonances in H^- by use of simple representations of the quasi-bound wave functions. The widths are in reasonable agreement with those found by the close-coupling method.

Charge equilibrium fractions F_{1 infinity}, F_{0 infinity} and F_{1 infinity} and relative charge-changing cross sections σ₁₀ and σ₀₁ were measured for the four possible combinations among hydrogen and deuterium beams and targets, namely H⁺-D₂, H⁺-H₂, D⁺-D₂ and D⁺-H₂. Within experimental errors of less than 0·3% for F_{0 infinity}, 1·2% for F_{1 infinity} and 2% for both F_{1 infinity} and the relative cross sections, no systematic differences for these isotope combinations were found in the velocity range 1·38 × 10⁸ to 2·24 × 10⁸ cm s^-1. Neglecting the small negative charge fraction, the ratio σ₁₀/σ₀₁, given by F_{0 infinity}/F_{1 infinity}, agrees to within 1·5% for the different combinations studied.

The drift velocities of positive and negative ions of sulphur hexafluoride in their parent gas have been measured in a time-of-flight drift tube over a wide range of electric fields E and gas pressures p (E/p <or=80 v cm^-1 torr^-1) at room temperature. Three species of negative ions and one species of positive ions were observed. The zero-field mobilities of the ions were found to be 0·56(4), 0·47(8), 0·54(5) and 0·59(0) cm² v^-1 s^-1.

In a number of investigations of the mobilities of hydrogen ions in hydrogen zero-field mobilities approximately equal to 12·6 cm² v^-1 s^-1 have been recorded, and it seems to be currently accepted that these are inaccurate determinations for H₃⁺ ions, the zero-field mobility of which is approximately equal to 11·1 cm² v^-1 s^-1. The present paper suggests that the ions of mobility approximately equal to 12·6 cm² v^-1 s^-1 are H₃O⁺ ions and discusses the available evidence. Approximate values for the lateral diffusion of the H₃O⁺ ions in hydrogen are reported, together with lower limits for the lateral diffusion of H⁺ ions. The implications of the identification of the ions of mobility approximately equal to 12·6 cm² v^-1 s^-1 as H₃O⁺ ions in studies of recombination in hydrogen are briefly considered.

Data for the drift velocity and lateral diffusion of potassium ions in hydrogen are also reported for 2·0 × 10^-17 < E/N < 300 × 10^-17 v cm² at room temperature.

An x-band microwave cavity method has been employed to study the decay of electron density in low gas pressures of very pure xenon. The main electron loss process in the late afterglow is via ambipolar diffusion affected by diffusion cooling of electrons. Extrapolation to zero pressure yields a value of (0·54 ± 0·03) cm² v^-1 s^-1 for the mobility of Xe⁺ diffusing in xenon at 295 °K, in good agreement with previous work. A value of 2·46 × 10^-14 cm² is deduced for the diffusion cross section, in reasonable agreement with a theoretically calculated value.

Valency-electron transitions with up to 4 kev energy in highly stripped ions are produced in the transient pinch which occurs in a coaxial plasma gun `Plasma Focus'. New or improved wavelengths for transitions in H-like and He-like neon and argon ions are measured and are identified by comparison with Hartree-Fock (HX) calculations of atomic structure made by Cowan in 1966 and 1968. Observed wavelengths of the He-like resonance lines lead to extrapolated values for the ionization potentials of Ar XVII and Ne IX. Inner-shell transitions in less highly stripped ions are also identified.

The values of the screening constant σ₂ for L_IIL_III, M_IIM_III and M_IVM_V have been calculated over the entire range of z values, including the transuranium elements, using the more recent data on x-ray atomic energy levels reported by Bearden and Burr in 1967. It is found that the anomalies reported by Karnatak and Varshni in 1956 for z = 66 and z = 51 do not exist in the present calculations.

A technique which employs a carefully controlled electron-impact ion source is used to show that the two ground doublet states of both Kr⁺ and Xe⁺ undergo symmetric charge transfer with near equal efficiency in the energy range 300-2500 eV.

A Doppler shift analysis of the emissions arising from excitation in fast Ar⁺-Ar collisions shows that collisions involving charge transfer simultaneous with excitation and direct excitation of fast Ar⁺ ions are of comparable importance.