On the high-energy limits for the double-to-single ionization ratio of helium and helium-like ions in collisions with charged and neutral particles

We compare ionization of helium and helium-like ions in collisions with highly charged ions and neutral atoms in the asymptotic high-energy limit γ --> ∞, where is the collisional Lorentz factor. In the case of ionization of helium and helium-like ions by charged particles, the asymptotic high-energy double-to-single ionization ratio is independent of the properties of the ionizing agent and depends on the nuclear charge of the helium-like system. In contrast to that, in the case of ionization of helium-like ions by neutral atoms, this ratio is nearly independent of the nuclear charge of the helium-like ion but strongly depends on the atomic number of the ionizing agent (neutral atom), provided the condition Z1Z2>0.4c is fulfilled where Z1 is the nuclear charge of the helium-like ion, Z2 is the atomic number of the neutral atom, and c=137 is the speed of light