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Journal of Physics B Atomic and Molecular Physics

Comment on 'polarisation and angular correlation relations in electron-photon-photon coincidence events

Stanisław Chwirot, J. A. Slevin

Published Apr 28, 1987• 2 citations
Publisher

Abstract

The authors question the claim made by Heck and Gauntlett (ibid., vol.19, p.3633, 1986) that the spin polarisation resulting from the spin-orbit interaction in the hydrogen atom after electron impact excitation to the n=3 states significantly influences the evolution of the atoms in the intermediate n=2 state of the n=3 to n=2 to n=1 cascade. They also show how some of the authors' formulae can be simplified and hence analysed without using numerical methods.

DOI
10.1088/0022-3700/20/8/001

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The authors question the claim made by Heck and Gauntlett (ibid., vol.19, p.3633, 1986) that the spin polarisation resulting from the spin-orbit interaction in the hydrogen atom after electron impact excitation to the n=3 states significantly influences the evolution of the atoms in the intermediate n=2 state of the n=3 to n=2 to n=1 cascade. They also show how some of the authors' formulae can be simplified and hence analysed without using numerical methods.
The authors present a theory of electron-photon coincidence experiments with polarised electrons, taking spin-orbit coupling effects during the excitation into account. The influence of the initial polarisation vector on the angular distribution and polarisation of the emitted photons is discussed in detail. As an explicit example the excitation process 1 S 0 to 3 P 1 in mercury is considered. Finally, the results are specialised to experiments where the scattered electrons are not observed. Using symmetry arguments and deriving additional selection rules they relate the degree of light polarisation to the initial polarisation. Two experimental results are presented.
The density matrix theory is applied to establish a connection between the configuration of the excited atomic states and the intensity and polarisation of the cascade photons emitted during the decay process. Explicit expressions are obtained for the P, D and F states of hydrogen and helium atoms. These formulae can be used to assist experimental design for the most efficient and revealing measurements. In addition, formulae for the integrated polarisation of the emitted photons from the P, D, F and G states of hydrogen and helium are presented in terms of partial excitation cross sections.
A brief review is given on development of electron-photon coincidence studies from electron impact of atoms. The theory of electron-photon correlations is outlined in detail in terms of cross sections of excited atoms and excited state charge clouds. The state of a target atom is determined by the various orientation and alignment parameters and the multipole moments of the excited atom. General considerations, such as coherence effects in impact excitation, the influence of fine and hyperfine structures in correlations are taken into account. The relationship between scattering amplitudes (including their phases) and target parameters is worked out for singlet and doublet transitions.