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Journal of Physics B Atomic and Molecular Physics

Polarisation and angular correlation relations in electron-photon-photon coincidence events

E L Heck, Jerome P. Gauntlett

Published Nov 28, 1986• 13 citations
Publisher

Abstract

The polarisation and angular correlation relations for electron-photon-photon coincidence event of an atomic system, which is excited by electron impact and subsequently decays spontaneously via a two-photon cascade, is derived. The expressions for the two-photon and the electron-second-photon coincidence measurements are deduced from the general formulae. As a special case the cascade n=3 to n=1 of hydrogen excited by electron impact is considered.

DOI
10.1088/0022-3700/19/22/009

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•Jan 1, 1986•0 citations

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Journal of Physics B Atomic and Molecular Physics•Feb 28, 1984•5 citations
The polarisation and angular correlation relations for electron-photon-photon coincidence event of an atomic system, which is excited by electron impact and subsequently decays spontaneously via a two-photon cascade, is derived. The expressions for the two-photon and the electron-second-photon coincidence measurements are deduced from the general formulae. As a special case the cascade n=3 to n=1 of hydrogen excited by electron impact is considered.
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.
The delayed coincidence technique was applied to investigate angular and polarization correlations between electrons scattered inelastically from helium atoms and photons emitted in the decay of n1P states. \n \nElectron photon angular correlations were studied for the excitation of the 21P and 31P states of helium at an incident electron energy of 50 eV and for electron scattering angles in the range of 20° - 102° and 35° - 55o respectively. From the analysis of these measurements the excitation parameters λ = (|a¬o| 2)/(2| a1| 2+|ao| 2) = σo/σ and the magnitude |x| of the phase difference between the excitation amplitudes a_ML for the M_L = 1 and M_L = 0 states are obtained. \n \nUsing the 3¬¬1P - 21S decay light the electron-photon polarization correlation were measured for the 31P state of helium at electron impact energies 50, 80, 120 and 160 eV and for electron scattering angles between 27.5° and 108°. The Stokes parameters n1, n2 and n3 were derived and the sign change of the angular momentum transfer < Ly > to the atom was investigated using the circular polarization results (n2). A sign change was confirmed for all energies studied and the scattering angle of the sign change was found to increase with decreasing electron energy. The excitation parameters λ and x have been derived and the present results are compared with theoretical calculations and with previous experimental values obtained from angular and polarization correlation measurements.