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Journal of Physics B Atomic and Molecular Physics

Electron-photon coincidence studies of the resolved argon resonance lines

I C Malcolm, J. William McConkey

Published Feb 14, 1979• 31 citations
Publisher

Abstract

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).

DOI
10.1088/0022-3700/12/3/026

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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).
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.
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 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 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 .
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.