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Integrated Stokes Parameter Measurements of Spin-Polarized Electron-Impact Excitation of the (3d104s5s)53s 1 State of Zinc
We measured the integrated Stokes parameters P1, P 2, and P3 of Zn (43PJ – 53S1) fluorescence resulting from transversely-spin-polarized electron impact excitation of the Zn (4s5s)5 3S1 state. This work was motivated by a similar study from several years ago by Pravica et al., in which they measured non-zero values of the integrated Stokes parameter P2 between the threshold for the (4s5s)53S1 excitation and the first cascading (4s5p)5 3PJ threshold. In our experiment, the electrons scattered in the excitation process are not measured (hence the designation “integrated”), instead the fluorescence is observed in the direction of the initial electron spin polarization. The atomic Zn target was produced by a Zn oven that directed an atomic beam at right angles to both the fluorescence observation direction and the electron beam axis. For this geometry, Bartschat and Blum showed that P2 must be identically zero, based on the symmetry properties of the 9j symbol used in the algebra required to describe the dipole fluorescence, and the assumption that the L and S angular momenta of the total wavefunction (atom plus incident electron) are factorizable throughout the scattering process. This assumption is invalid if (a) the excited state producing the fluorescence is not well LS -coupled or (b) the spin of the continuum electrons undergo precession during the collision under the influence of a motional magnetic field. State-of-the-art theory based on the Dirac, Maxwell, and Schrodinger equations rules out both possibilities, predicting P2 values of order 10-4. For this reason, the results of Pravica et al. are remarkable as they measured values three orders of magnitude larger. Using a standard GaAs polarized electron source we observe optical excitation functions in agreement with those of previous experimental and theoretical results. In the cascade-free range of excitation between 6.65 eV and 7.6 eV, we measured P2 and find its values in this range to be consistent with zero and inconsistent with those measured by Pravica et al.
Clayburn, Nathan Bennett, "Integrated Stokes Parameter Measurements of Spin-Polarized Electron-Impact Excitation of the (3d104s5s)53s 1 State of Zinc" (2017). ETD collection for University of Nebraska - Lincoln. AAI10607548.