Measurements of complex scattering amplitudes for electron impact excitation of the 31D and 31P states of helium

Abstract

In a scattered-electron cascaded-photon coincidence experiment the authors have studied the electron impact excitation of the in=3 singlet states of helium for incident electron energies between 28 and 46 eV and for a scattering angle of 35 degrees . This was done by measuring the angular distribution of the 3¹P to 1¹S photons (the later resulting from the 3¹D(3¹S) to 2¹P to 1¹S cascade) in coincidence with the n=3 scattered electrons, both in the scattering plane and in the plane perpendicular to the direction of the incoming electrons. At an incident electron energy of 45.6 eV they found a small 3¹D contribution to the coincidence peak. The 3¹P results at this energy are in perfect agreement with previously reported experimental data. At an energy of 34.6 eV this is not the case. At this and lower energies they observe, apart from the dominant 3¹P contribution, a significant 3¹D contribution to the coincidence signal. The authors present new experimental data for the orientation and alignment of both excitations at 34.6, 31.5 and 28.5 eV and for the ratios of the cross sections at these energies.