Photon angular correlations and pressure-dependent effects of helium 2'P and 3'P excitation by electrons

The orientation and alignment parameters of the 2'P and 3'P excited states of helium at an incident electron energy of 81,6 eV have been remeasured for electron scattering angles from 25° to 5° and extended down to 2°. The studies investigate a wide range of experimental conditions and measurement techniques, including beam alignment, electron and photon angular distribution asymmetries in both the singles and coincidence detection modes, in order to establish better precision in the measurements. Nevertheless the present coherence λ, x and γa parameters for He(2'P) and He(3'P) state excitation for scattering angles in the range θ_e=2° to 25° are collectively not in good agreement with any single distorted wave, first Born or close coupling model predictions, although there are ranges of scattering angles where the individual measured parameters are in good agreement with a particular theory. Radiation trapping effects on the 3'P state lifetime enabled the relative relaxation rates for the population, orientation and alignment, i.e. rank 0, I and 2 multipoles, of the excited state to be measured. The effects of these relaxation rates on the measured intensities and the coherence parameters are quantified. The lifetime, which can be measured an order of magnitude more quickly than the state parameters, reflects these relaxation rates and provides a reliable guide to radiation trapping effects. Radiation trapping influences the state parameters down to the lowest background operating pressure of 10^–7 Torr at which the measurements have been made.