The g-u interference oscillations observed in the emission cross sections and the optical polarizations in He+-He collisions

We have observed emission radiation from helium atoms excited in He+-He collisions by the direct and electron capture processes over the energy range 0.5-20 keV. The relative emission cross sections for transitions 21,3P-31,3S, 21,3S-31,3P and 21,3P-21,3D have been determined. Degrees of optical polarization have also been determined for the P- and D-state excitations. The emission cross section of the direct excitation and that of the electron capture excitation show oscillations against impact energy, which are in antiphase with each other. The polarization degrees for both processes are nearly the same magnitude and show weak oscillations in antiphase with each other. The oscillations of the cross section and those of the polarization degree are in phase in some cases and in antiphase in other cases. These oscillations are interpreted as due to the interference between the gerade and ungerade states of the helium quasimolecular ion. From the amplitude ratio and the phase correlation between the oscillations of the cross section and those of the polarization degree we find that the predominant g-u interference pair is Πg-Πu. (author)