Resonant and non-resonant charge transfer in C⁺₆₀ + C₆₀ and C⁺₇₀ + C₆₀ collisions

Charge transfer cross sections σ_CT were measured for the single-electron charge transfer reactions C⁺_60/70 + C₆₀ → C_60/70 + C⁺₆₀ + ΔE as a function of the collision energy E in the range 200-2100 eV. For both processes the absolute values of the cross sections are similar to the geometrical cross sections and decrease with increasing collision energy below E = 400 eV. In the case of C⁺₆₀ projectiles, the charge transfer cross section remains almost constant for larger collision energies while σ_CT increases in the case of capture by C⁺₇₀ beyond 400 eV and saturates around 1350 eV. From an interpretation of the measured cross sections using very simple models it can be concluded that the electron transfer is resonant in collisions between C⁺₆₀ and C₆₀, as would be expected, and has both resonant and non-resonant contributions for C⁺₇₀ projectiles. The qualitative agreement of the experimental results with very simple one-electron models developed to describe the general trends of the charge transfer cross sections determined in ion-atom collisions in the intermediate energy range (where the semiclassical impact parameter method can be applied) is quite surprising.