Alignment and orientation in He 21P and 31P excitation by electron impact

The authors have studied the orientation and alignment of the 2 1 P and 3 1 P states of helium caused by electron impact at incident energies between 50 and 80 eV both experimentally and theoretically as a function of the electron scattering angle. The angular correlations between inelastically scattered electrons and VUV de-excitation photons were measured, particularly in the region where the orientation changes sign. The experimental results were compared with several distorted-wave models. The best of these models gave reasonably good agreement with the experimental data for excitation of the 2 1 P and 3 1 P states for scattering angles of less than 60 degrees and qualitative agreement for larger angles.