Absolute cross sections for excitation of helium by electrons (20–2000 eV) and the polarization of the emitted radiation

Abstract

Absolute cross sections for excitation by electrons from the ground state of helium to 4 1S, 5 1S, 6 1S, 3 1P, 4 1D, 5 1D, 6 1D, 4 3S and 3 3P states of helium have been determined by observing the resulting light emission. The energy region covered extends from 20 to 2000 eV. Two devices were used, one for electron energies from threshold up to 100 eV and another for higher impact energies. All line-intensity measurements were corrected for the polarization of the radiation, the polarizing effect of the monochromator and space-charge effects. In the evaluation of the excitation cross sections cascading effects were taken into account. The energy dependence of the 1 1S-3 1P cross section is in very good agreement with theoretical calculations above about 200 eV. The experimental values for the n 1S and 3 1P levels are in the asymptotic high-energy region about 10% larger than the theoretical ones according to the Born approximation. The energy dependences of the n 1S and 3 1P cross sections are in good agreement with the results of Moustafa Moussa et al.. Our n 1D cross sections are, although 14 to 35% larger than the theoretical values, closer to the Born values than previous experimental results. In a plot of cross section times electron energy against the logarithm of the electron energy the n 1D curves exhibit a maximum. This effect becomes less pronounced for higher principal quantum numbers. The energy dependences of the 4 3S and 3 3P cross sections do not agree with the E −3 el law as predicted by Ochkur. For energies larger than 100 eV we find a decrease proportional to E −2.0 el for the 4 3S level and to E −2.4 el for the 3 3P level. The polarization results for the 3 1P-2 1S light emission show very good agreement with the Born theory. The energy dependence of the polarization degree of the radiation of n 1D-2 1P transitions is over a large energy range (150–1000 eV) in agreement with theoretical calculations; however, the magnitude of the polarization degree differs with theory which is also the case in previous experiments of other investigators.