Measurement of alignment and orientation parameters for low-energy electron impact excitation of the 21P state of helium

The authors have determined the anisotropy parameters for the electron impact excitation of the 2 1 P state of helium in the energy range from 26.6 to 40 eV and in the angular range from 30 degrees to 110 degrees . The results of the 29.6 eV measurements are in agreement with the results of McAdams et al. (1980) although the small-angle minimum in the value of lambda as a function of the electron scattering angle seems somewhat more pronounced in the authors' results. The authors have derived an alternative expression for the coincidence rate, based on a different choice of the quantisation axis (perpendicular to the scattering plane instead of in the direction of the incoming electron beam). The new expression displays a more transparent relation between the observed radiation pattern and the anisotropy parameters of the excited atom. In particular it allows a more straightforward determination of the errors in the derived parameters.