The feasibility of determining the Stokes' parameters for the electron impact excited 3 3 P, 3 1 D and 3 3 D states of helium is shown for several scattering energies and angles. An extensive analysis is made of the optimisation of the experimental variables. The methods used for extraction of the source parameters are given in detail, with emphasis on the use of polarisers and the analysis of the electron-photon coincidence spectrum. The Stokes' parameters for the 3 3 P state are determined at 39.7 and 23.5 eV for several electron scattering angles. There are no theoretical or experimental values for comparison. The lifetimes of the 3 3 P, 3 1 D and 3 3 D states are measured to be 102.8+or-2.4, 16.3+or-0.8 and 13.9+or-0.5 ns respectively. The method is free of cascade effects and the values are more accurate than previous determinations.
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Humphrey et al. (1987) studied this question.
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