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Journal of Physics B Atomic and Molecular Physics

Electron-polarised-photon coincidence experiments on the 31P state of helium

Johannes P. M. Beijers, J P van den Brink, J. van Eck, H G M Heideman

Published Sep 14, 1986• 11 citations
Publisher

Abstract

The authors present circular-polarisation measurements of 3 1 P to 2 1 S photons (501.6 nm) detected in delayed coincidence with electrons inelastically scattered upon excitation of the 3 1 P state of helium. They performed these experiments as a function of the electron scattering angle at incident electron energies of 80, 60 and 50 eV in order to study the orbital angular momentum transfer during the collision. The experimental results are compared with previous angular correlation data and with theory. For each incident electron energy they found a sign reversal of the angular momentum transfer as a function of the electron scattering angle.

DOI
10.1088/0022-3700/19/17/007

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Journal of Physics B Atomic and Molecular Physics•Aug 14, 1981•20 citations
The authors present circular-polarisation measurements of 3 1 P to 2 1 S photons (501.6 nm) detected in delayed coincidence with electrons inelastically scattered upon excitation of the 3 1 P state of helium. They performed these experiments as a function of the electron scattering angle at incident electron energies of 80, 60 and 50 eV in order to study the orbital angular momentum transfer during the collision. The experimental results are compared with previous angular correlation data and with theory. For each incident electron energy they found a sign reversal of the angular momentum transfer as a function of the electron scattering angle.
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.
We present linear and circular polarization measurements of $3^{1}P\ensuremath{-}2^{1}S$ (501.6 nm) photons detected in delayed coincidence with electrons inelastically scattered from the 31P3^{1}P31P level of helium. Vector polarization and coherence parameters of the coincident radiation provide direct experimental evidence of the coherent nature of the excitation process. Atomic orientation is obtained directly from the circular polarization measurements. Analysis of the data also yields $\ensuremath{\lambda}$ and $\ensuremath{\chi}$ parameters for 31P3^{1}P31P excitations, which are compared with previous data and theory.
The linear and circular polarisation of the K(4 2 P to 4 2 S) photons detected in delayed coincidence with the inelastically scattered K atoms are presented for K-He collisions at 93 eV as a function of the scattering angle. Data are analysed in terms of the Stokes parameters, and the coherence properties of the excitation process are examined. It is concluded that spin dependent forces do not play an important role in the excitation process. The lambda and chi parameters are also derived. Simple models for the excitation process are discussed in order to explain the impact parameter dependence of chi .
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.
An electron-photon coincidence technique has been used to study the coherence of the excitation of the 3 2 P state in sodium. The total polarisation of the decay photons has been determined by measuring the linear and circular components of the Stokes's vector. Analysis of the reduced degree of polarisation mod P mod shows that the excitation is coherent for electrons which have been scattered through the angular range from 3 to 10 degrees . The concept of reduced polarisation assumes that the Percival-Seaton hypothesis applies to the collision. Thus the authors deduce that the nuclear spin plays no role in the excitation process at this energy and over these scattering angles. Scattering parameters have been deduced from the individual components of the Stokes's vector. These have been compared with those predicted by calculations based on the first Born and distorted-wave polarised orbital approximations. It is found that neither theory adequately describes the interaction.
The polarisation of 2154 nm photons emitted from Hg atoms which are excited by electrons scattered in the forward direction has been measured for collision energies between 5.5 and 15 eV. From the results the relative contribution of exchange excitation could be determined. Exchange is found to be the dominant excitation mechanism from threshold up to 8 eV collision energy. The results are also expressed by the alignment parameter lambda and good agreement is found with a previous measurement using incident polarised electrons. The results are explained in terms of intermediate coupling.