Electric dipole moments of He atoms excited to the 1s5l (l ≥ 2) states by He+-ion impact at intermediate energies

The post-collisional 1s5l (l >= 2) states of He atoms after He+-ion impact (10 keV-28 keV) have been investigated using anticrossing spectroscopy. In particular, the intensity of the spectral line lambda(1s5l D-3-1s2p P-3) = 402.6 nm emitted by the impact-excited He atoms was measured as a function of an axial electric field (which varied from -30 kV cm(-1) to +30 kV cm(-1)). By fitting the theoretical intensity functions to the measured ones, the post-collisional states of the atoms and their electric dipole moments were determined. The results indicate that for projectile energies below 20 keV, the electric dipole moments are small; however, for energies above 20 keV, mainly the parabolic Stark states with maximal electric dipole moments are excited. We conclude that in the upper section of the energy range investigated here, the Paul-trap promotion is the dominant excitation mechanism for He+-He collisions.