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.