Anticrossing spectroscopy of He atoms excited by 30-300 keV He+ impact

By measuring the anticrossing spectra of the 1s4l and 1s5l He I levels, we investigate single-electron excitation in 30-300 keV He+-He collisions. The post-collisional He I states are highly coherent superpositions of spherical states with large electric dipole moments. These superposition states vary gradually with the impact energy from purely parabolic states at 30 keV to superpositions of 1 less than or equal to 2 states at 300 keV. The results indicate that for the whole energy range single-electron excitation should be considered as a process mainly localized in the saddle region of the two-centre Coulomb potential of the (He+)(2) molecular core. A theoretical model based on the symmetry of the collision system is discussed.