Excitation of helium by 0.05–6 keV electrons and polarization of the resulting radiation

Abstract

Absolute cross sections have been determined for excitation of helium to n 1S ( n = 3, 4, 5 and 6), n 1P ( n = 2, 3, 4 and 5), n 1D ( n = 3, 4, 5 and 6), n 3S ( n = 3, 4 and 5), n 3P ( n = 3, 4 and 5) and n 3D ( n = 3 and 4) levels by 0.05–6 keV electrons determining the light intensities of the resulting spectral lines. The measurements have been carried out at low gas pressures to avoid all the effects due to collisional excitation transfer and the absorption of resonance radiation. The polarization of the resulting radiation has also been measured. The same apparatus has been used as before in ionization experiments, where detailed attention has again been paid to the elimination of disturbing effects caused by secondary electrons. Only small modifications were necessary for the observation of light. As far as the relative energy dependence is concerned, the experimental singlet excitation cross sections obey the asymptotic energy dependence predicted by the Bethe or Born theory. For 1S and 1D levels deviations from the Born energy dependence start at impact energies below about 1000 eV and for 1P levels below about 400 eV or possibly lower energy. As far as the absolute values of the singlet excitation cross sections are concerned, in the energy region where the Born approximation is valid, the agreement between experiment and theory varies from a few percent to about 50%, dependent on the level. The experimental polarization of n 1P-2 1S is in reasonable agreement with the Born calculations; for n 1D-2 1P there is no agreement. Cross sections for singlet excitation of helium by electrons are compared with those for protons, also measured by our group. At large impact velocities they become almost equal. In the case of triplet excitation, no agreement has been obtained with the Ochkur approximation; our cross sections decrease slower with increase of impact energy than E -3 el, similar as in other optical experiments.