Anticrossing spectroscopy of fastly moving helium atoms after electron capture in 10 to 50 keV He+-He collisions

The symmetric (He+,He) collision system is investigated in the 10-50 keV range. In particular, we studied the formation of excited helium atoms by electron capture. By applying electric fields to the collision volume, the charge distribution of helium atoms immediately after the collision in a transverse electric field was measured. The results show that the projectile and target atoms are highly polarised after the collision. The centre of the charge distribution for the projectile atoms after the collision was found to have shifted in the direction of the receding target ion. The projectile atoms have electric dipole moments opposite to the excited target atoms. We measured the intensity of the fluorescence light of the triplet 4D-2P HeI line (447 nm) as a function of the strength of the external electric field. The intensity function contain sharp peaks at singlet triplet anticrossings in the region of 12 to 30 kV/cm. In this publication the anticrossing technique and all necessary equipment is discussed in detail. Comparing the anticrossing spectra of the projectiles with target spectra that were analysed in detail previously, suggests that also for the projectiles parabolic states with the largest dipole moment, jet opposite, are populated highly selectively for these projectile velocities. Some difference in projectile and target spectra were found. Interferences of even and odd states are discussed as a mechanism for these differences between direct and transfer excitation