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Journal of Physics B Atomic Molecular and Optical Physics

Coherent excitation of Ar 4s(3/2)o1by low energy electrons

J J Corr, S Wang, J. William McConkey

Published Nov 28, 1992• 8 citations
Publisher

Abstract

Electron-photon coincidence measurements are reported of the excitation of the 4s(3/2) o 1 state of Ar at incident energies of 20 and 30 eV and electron scattering angles up to 80'. Isolation of the 4s(3/2) degrees 1 state was accomplished in the photon channel using a LiF filter. Significant non-zero values of the spin-flip parameter, rho 00 , are reported at both energies as predicted by theory but the level of agreement with available perturbative theories becomes poorer (as might be expected) as the incident electron energy is reduced.

DOI
10.1088/0953-4075/25/22/025

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Electron-photon coincidence measurements are reported of the excitation of the 4s(3/2) o 1 state of Ar at incident energies of 20 and 30 eV and electron scattering angles up to 80'. Isolation of the 4s(3/2) degrees 1 state was accomplished in the photon channel using a LiF filter. Significant non-zero values of the spin-flip parameter, rho 00 , are reported at both energies as predicted by theory but the level of agreement with available perturbative theories becomes poorer (as might be expected) as the incident electron energy is reduced.
The excitation by electron impact at 50 eV, of the 104.8 and 106.7 nm lines of Ar, have been studied separately using the electron-polarised-photon coincidence technique. Polarisation correlations have been measured for electron scattering angles of 0 and 5 degrees . From the measurements at 5 degrees the authors derived the parameters lambda (0.72+or-0.06, 0.60+or-0.05) and mod chi mod (0.99 rad+or-0.19, 0.0) for the 104.8 and 106.7 nm lines, respectively. Thus, contrary to the assumptions of Arriola et al. (see ibid., vol.8, p.1275 (1975)) significant differences may occur between the two lines. The value of chi obtained for the 106.7 nm line implies total coherence at the energy and angle studied. The measurements at 0 degrees yielded 'threshold' polarisations of 0.41+or-0.13 (104.8 nm) and 0.94+or-0.14 (106.7 nm).
We present experimental values of the electron impact coherence parameters (EICP) and reduced Stokes parameters for excitation of 51P1 state of cadmium atoms. The results have been obtained using electron–photon coincidence technique for incident electron energies 80 eV and 60 eV and electron scattering angles in the range of 5° to 50°. We also present an additional set of data for electron energy 100 eV and scattering angle 50° which complements our previous results. All the experimental values are compared with theoretical relativistic distorted-wave approximation (RDWA) calculations. The first Born approximation (FBA) predictions of the alignment angle are also presented. The theoretical results are in good qualitative agreement with the experimental data.
In an electron-polarized photon coincidence experiment the Stokes' parameters P 1 , P 2 , P 3 and P 4 have been determined for four Ne * (3p) to Ne * (3s) transitions at a fixed electron energy of 50 eV and in the electron angular scattering range 30 degrees -70 degrees . The results indicate a tendency towards positive values for P 3 at the lowest scattering angles studied and a strong tendency towards zero and even negative values for P 4 . In all cases a drop in the total polarization degree between 30 degrees and 50 degrees electron scattering angle is observed, indicating that there is a decrease of coherence at these scattering angles.
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.