Excitation of He atoms by 10-30 keV He+ impact

The anticrossing spectra of the helium line lambda(1s4l-1s2p P-3) = 447.2 nm were measured for 10-30 keV He+-He collisions. The theoretical intensity functions were calculated taking into account cascade processes, the inhomogeneity of the axial electric field in the collision volume and the density distribution of the target He atoms. Comparing the theoretical intensities with the measured ones, the post-collisional states of He atoms were determined. The results indicate that for this projectile-energy range, the electronic charge distributions of the excited-atom states change from almost symmetric to strongly asymmetric ones with large electric dipole moments. This result suggests that for projectile energies of 20-30 keV, Paul-trap promotion becomes the main excitation mechanism.