Excitation of helium by protons and hydrogen atoms and polarization of the resulting radiation

Abstract

Absolute excitation cross sections have been determined for H + and H incident on He by measuring the intensities of the singlet and triplet lines of the He I spectrum. The energy range was mostly 5 to 35 keV, but for one 1D and one 1S level up to 100 keV. The polarization of the photons has also been determined. As a function of the energy the cross section shows a similar behaviour for different levels of a spectral series and decreases with increasing quantum number n. The polarization is almost the same for the emission lines of one series. These properties can be understood qualitatively by theory. Many maxima in the excitation cross sections are found at energies of about 15 keV for singlet and triplet excitation of helium. A tentative explanation is given in terms of formation of intermediate molecular states. Excitation of the 4 1D level by protons results into two maxima, one at 15 keV and the other at 55 keV. The excitation of the 3 1P level and the degree of polarization of the 3 1P-2 1S transition show an energy dependence which differs from the existing theoretical results. In agreement with the spin conservation rule of Wigner, we have measured that the 3 3P level is excited much more effectively by H atoms than by protons (at least a factor of 100).