Orientation and alignment of helium atoms by electron impact

Experimental studies have been carried out on the electron impact excitation of helium atoms. Coincidence methods are used to measure the angular correlations between the inelastically scattered electrons which have lost the excitation energy of the 21P and 31P states of helium and scattered in a given direction and the ultra-violet photons emitted in the decay of those states. The correlations are analysed to give the ratio of the differential cross-sections for excitation of the individual magnetic sub-levels and the phase difference between the excitation amplitudes for those sub-levels - equivalently the orientation and alignment of the excited state. The results for 31P excitation at 81.2 eV incident energy are the first to be made at large scattering angles for this state. Comparison between the data and a number of theoretical calculations is made, but no theory gives an adequate description of the scattering process at all angles. (author)