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On soft collisions of fast highly-charged ions with hydrogen and helium atoms

Ionization of hydrogen and helium atoms in small momentum transfer collisions with fast highly-charged ions is considered for the range of parameters , (Z and v are the ion charge and velocity, respectively, and is the characteristic velocity of the electron in the atom ground state). Analytical expressions have been derived for single and double differential cross sections of hydrogen ionization, accompanied by emission of slow electrons with the velocities . A generalization of the results obtained is proposed for helium single ionization. Soft collisions were shown to contribute mainly to hydrogen ionization and were found to determine practically the magnitude of the helium single ionization cross section. An asymmetry in the angular distribution of slow electrons is analysed. A formula for the helium single ionization cross section is proposed. A peculiarity of momenta transfer for soft collisions is briefly discussed.