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Physical Review A

Electron-impact excitation of He: Dependence of electron-photon coherence parameters on the principal quantum number

P. R. Hammond, M. A. Khakoo, J. William McConkey

Published Dec 1, 1987• 6 citations
Publisher

Abstract

Measurements are presented representing a complete set of electron-photon polarization correlation parameters for the excitation of n $^{1}P$ states of He at an incident energy of 80 eV and an electron scattering angle of 20\ifmmode^\circ\else\textdegree\fi{}. The data support the predictions of a recent theoretical paper that these parameters should exhibit little variation with n. However, disagreement in absolute values between experiment and theory indicates the need for additional theoretical input into the problem.

DOI
10.1103/physreva.36.5127

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Journal of Physics Conference Series•Nov 5, 2012•0 citations
Measurements are presented representing a complete set of electron-photon polarization correlation parameters for the excitation of n 1P^{1}P1P states of He at an incident energy of 80 eV and an electron scattering angle of 20\ifmmode^\circ\else\textdegree\fi{}. The data support the predictions of a recent theoretical paper that these parameters should exhibit little variation with n. However, disagreement in absolute values between experiment and theory indicates the need for additional theoretical input into the problem.
Electron-photon coherence and correlation parameters for electron-impact excitation of the n 1P^{1}P1P (n=2--8) states of helium at 80 eV are predicted to be essentially independent of the principal quantum number (n), and the available experimental data for n=2 and 3 show (semiquantitatively) the same behavior. The n independence of these parameters predicted by first-order many-body theory can be explained using basic concepts associated with quantum-defect theory and, if verified by further experimental results, has important consequences for understanding the physics of the electron-atom scattering process.
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.
Electron-photon angular correlations have been measured for excitation of the 2P2P2P states of hydrogen at an incident energy of 35 eV. The data presented relate to electron-scattering angles from ${\ensuremath{\theta}}_{e}=8\ifmmode^\circ\else\textdegree\fi{}$ to ${\ensuremath{\theta}}_{e}=120\ifmmode^\circ\else\textdegree\fi{}$ and yield values for the parameters $\ensuremath{\lambda}$ and RRR. The experimental results are compared with several theories. In general, none of the theories is found to be adequate at this energy.