Dissociative Charge-Exchange

Dissociative charge-exchange was measured by observing the decay of C₆₀⁺ ions following collisions with Na, K, Rb and Cs atoms.

        In the cases of Rb-C₆₀⁺ and Cs-C₆₀⁺ collisions, resulting alkali ions could be trapped simultaneously with C₆₀⁺ ions. The simultaneous trapping of masses with such a large mass ratio (8.42) as C₆₀ and Rb is possible because of the low ion kinetic temperatures. This permits the growth rate of trapped Cs⁺ and Rb⁺ ions for comparison with the corresponding C₆₀⁺ decay rate.

        The second panel displays the C₆₀⁺ ion decay and corresponding growth of Rb⁺ ions as the Rb flux exposure time is increased. The exponential time dependence is modeled by assuming that the decay of N₀ initial C₆₀⁺ ions (N) and growth of alkali ions (A) are expressed by N = N₀ exp(-k_expt) and A = N₀[1-exp(-k'_expt)] respectively, where k_exp = <F_A sigma_exp> is the experimental rate for the decay of C₆₀⁺ and k'_exp is the alkali ion formation rate as a result of charge-exchange. The decay rate k_exp is expressed here as an implied velocity average of the alkali flux F_A and the charge-exchange cross-section sigma_exp. Identical rates for C₆₀⁺ decay and Rb⁺ growth indicates that charge-exchange is the single process responsible for the measured time dependence. This result was also observed for Cs collisions.

        Trapped C₆₀⁺ lifetimes in the absence of alkali flux are greater than 10 min. Thus trap loss does not significantly contribute to the measured loss of C₆₀⁺ ions.