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Physical Review Letters

Coherent Excitation of the 4F1 State of Helium by Electron Impact

D Cvejanović, Albert Crowe

Published Apr 6, 1998• 12 citations
Publisher

Abstract

The Stokes parameters ${P}_{1}$, ${P}_{2}$, ${P}_{3}$, and ${P}_{4}$ have been measured in a scattered electron-polarized cascade photon correlation study of the $4^{1}F$ state of helium excited by 29.6 eV electron impact. This is the first detailed experimental study of electron impact excitation leading to the transfer of three units of angular momentum. The data are expected to be influenced by the breakdown of $\mathrm{LS}$ coupling for $L\ensuremath{\ge}3$ states, even in a simple system like helium, providing a further challenge for theory.

DOI
10.1103/physrevlett.80.3033

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The Stokes parameters P1{P}_{1}P1​, P2{P}_{2}P2​, P3{P}_{3}P3​, and P4{P}_{4}P4​ have been measured in a scattered electron-polarized cascade photon correlation study of the 41F4^{1}F41F state of helium excited by 29.6 eV electron impact. This is the first detailed experimental study of electron impact excitation leading to the transfer of three units of angular momentum. The data are expected to be influenced by the breakdown of LS\mathrm{LS}LS coupling for L\ensuremath≥3L\ensuremath{\ge}3L\ensuremath≥3 states, even in a simple system like helium, providing a further challenge for theory.
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 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.
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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.
Experimental studies of electron impact excitation, of atoms with closed ns shells are discussed in terms of electron impact coherence parameters, EICPs. Experimental and theoretical data leading to full determination of complex scattering amplitudes for the S-D excitation in helium and S-P in alkaline earth atoms are presented. Similarities and differences in the angular behaviour of EICPs within the alkaline earth group are discussed and compared with helium. These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.