Excitation of helium by He + and polarization of the resulting radiation

Abstract

Absolute excitation cross sections have been determined for He + incident on He by measuring the intensities of the singlet and triplet lines of the He I spectrum emitted by the target atom between 2900 Å and 6000 Å. The polarization of the photons has also been measured. The impact energy was between 5 and 100 keV. As a function of the energy the cross sections show a similar behaviour for different levels of a spectral series and the polarization of light tends to be the same for the corresponding emission lines. Generally more than one maximum is found in the cross section curves, and these maxima have a tendency to shift to higher impact energies for levels with increasing quantum number n, probably due to the corresponding increase of excitation energy. Moreover the energies of the cross section maxima decrease with increasing azimuthal quantum number l, which may be explained by means of electrical polarization of the target atom. Cross section results are compared with the theoretical calculations of Moiseiwitsch and Stewart, using Born approximation. A big discrepancy is present and it seems also probable that their neglection of the electron exchange process is in contradiction with our large experimental cross sections for excitation of triplet states. Comparing our results for He excitation by He +, H + and H impact, we find that levels with higher l values are relatively more excited if the effective charge of the impacting particle increases. This can be understood by taking into account a larger electrical polarization of the He target atom by projectiles with a larger effective charge.