Table of contents

The values of the A and B hyperfine structure constants for the 2s₂ and 2p₄ levels of ²¹Ne have been measured using Fourier transform spectroscopy. In addition it is found that the centre of gravity for the ²¹Ne line lies between the ²⁰Ne and ²²Ne components in agreement with the isotope shift theory.

Calculations of cross sections for elastic scattering of slow electrons by atomic oxygen in the energy range from threshold to 5 eV are in good agreement with experiments by Sunshine et al (1965). The cross sections are obtained in a close coupling calculation in which real and pseudo-states of the atom are included.

The first Born approximation is used to calculate the cross section for destruction of fast hydrogen metastable atoms, H(2s), passing through ground state atomic hydrogen. These theoretical results are in good agreement with the measurements of Gilbody and Corr (1974) for impact energies greater than 50 keV.

An experimental method is described which permits the determination of absolute differential cross sections for the autoionization of helium by electron impact. The analysis of this experiment in terms of the theory of Balashov is discussed, and illustrated for an incident electron energy of 80 eV.

Measurements of the relative excitation function and polarization of the 6¹S₀-6¹P₁ line of mercury are presented for electron impact energies between threshold (6.7 eV) and 15 eV. There is broad agreement with theoretical calculations above 9 eV, but a deviation from them in the region just above threshold.

The configuration mixing of the 3s3p⁶²S_1/2, 3s²3p⁴3d'²S_1/2 and 3s²3p⁴4d' ²S_1/2 states of Ar⁺ has been investigated using controlled electron impact excitation and detection of the optical radiation resulting from decay of the states. The coupling of the states is clearly demonstrated but the relative mixing coefficients differ from those obtained theoretically by Luyken (1972).

A beam attenuation technique developed previously in this laboratory has been used to study the formation of fast helium metastable atoms formed by two-electron capture neutralization of 400 keV He²⁺ ions in passage through both thin and thick targets of hydrogen. It is shown that the process: He²⁺+H₂ to He(2¹S)+H⁺+H⁺ has a cross section of 4*10^-18 cm² and accounts for about 76% of all collisions involving two-electron capture. This cross section is about the same as that for metastable atom formation in one-electron capture by He⁺ ions of the same energy.

A mass spectrometer was used to study atomic oxygen recombination following pulsed electrical discharges through O₂, CO₂ and a CO₂-N₂-He mixture, at pressures of a few torr. The surface recombination coefficient gamma for atomic oxygen in a pyrex discharge tube was found to be in the range 2.6*10^-4-5.2*10^-4, which is about an order of magnitude larger than previously determined values. For sealed CO₂-N₂-He lasers at pressures below 50 Torr the homogeneous recombination rate should be small compared with the surface recombination rate.

The harmonic oscillator has been used to test the efficiency of some off-diagonal hypervirial relations in providing approximate solutions of the wave equation. Inaccuracies have been deliberately introduced in both initial and final states to reveal the degree of error propagation.

A variational method for constructing polarized pseudo-states for the general LS coupled atom is discussed. These states, when used in a close-coupling expansion for low energy electron atom scattering, give the full static dipole polarizability of the atom, and hence the correct long range potential. Polarized pseudo-states for helium, neon, argon and nitrogen are given. The method may also be used for finding polarizabilities. The extension to higher multipole polarizabilities and to frequency dependent polarizabilities is straightforward.

Dirac-Slater calculations are used to predict the ratio of photoionization cross sections for the j=l+or-1/2 components of an atomic orbital. It is shown that the ratio is larger or smaller than the statistical value of l+1:l depending on whether the partial cross section is rising or falling. Two effects are shown to determine the deviation from the statistical ratio, one arising from small differences in the bound state orbitals, and the other from differences in the kinetic energy of the photoelectrons. Differences between the asymmetry parameter, beta , for the two ionizations are found to be small except at the Cooper minimum. In general the primary cause of the two values of beta not being the same at low photoelectron energies is the kinetic energy effect.

Some aspects of the theoretical description of the intensity and shape of resonances observed in the spectra of electrons ejected from atoms by fast electrons and protons are discussed. Numerical calculations have been performed for the (2s²)¹S resonance of helium atoms. Dependences of the resonance intensity and shape on the collision kinematics, i.e. incident-particle energy and electron ejection angle, have been obtained. The considerable variations revealed in the character of the resonance asymmetry with varying ejection angle are associated with a strong interference between separate components of the transition amplitude.

This series of articles will describe a novel method of utilizing optical detection of RF-induced fine structure transitions to investigate the population of excited states of fast hydrogen atoms produced by charge exchange collisions between an initial proton beam and various gas or foil targets. In Part I a model of the charge exchange collision and the interaction of the hydrogen beam with an external oscillatory field are considered within the framework of the density matrix formalism and expressions are obtained for the RF lineshapes in terms of the initial atomic density matrix elements.

The lineshape theory of optically detected RF-induced electric resonance transitions in hydrogen is concluded with the treatment of the detection process within the framework of the density matrix formalism. An optical detection function is derived from which can be determined the intensity, polarisation, and angular distribution of the detected decay radiation, subject to constraints of apparatus efficiency and geometry, in terms of the initial atomic density matrix elements and variables of the radiofrequency electric field.

For pt.II, see ibid., vol.7, no.6, 730 (1974). The apparatus and experimental procedure for optical electric resonance investigation of a fast hydrogen beam are described. Particular attention is paid to the design and performance of the radiofrequency system. The generalized rotating field model of the atom-RF field interaction developed in Part I and the optical detection function derived in Part II are employed in conjunction with computer simulation and least squares fitting techniques to extract information on the initial states from experimental optical electric resonance spectra of the H(n=4) manifold.