Charge-Exchange Rates

        Relative measurements of the charge-exchange rates are reproducible over a wide range of experimental conditions. As shown, the scaling of the relative rates with alkali polarizability and reduced mass is consistent with the linear relationship predicted by the physics of low energy ion-neutral collisions. In the figure above, the rate is divided by n_v, the alkali oven vapor density.

        In the case of Li-C₆₀⁺ charge-exchange, Langevin capture collisions are clearly involved as evidenced by the growth of Li_nC₆₀⁺ clusters. Although cluster growth was not observed in charge-exchange collisions of Na, K, Rb and Cs with C₆₀⁺ ions, capture collisions are probably occurring followed by rapid dissociation. The occurrence of capture collisions has an important consequence. As the neutral executes a spiral capture trajectory to the ion surface from large (~10-13 Angstrom) impact parameters, the interaction samples all available electronic curve (v=0) crossings of C₆₀^* required for resonant charge-exchange. A heuristic model is as follows:

Dissociative Charge-Exchange

      This diagrams the RRKM unimolecular dissociation time of an alkali-C₆₀⁺ complex as a function of internal vibrational energy. This calculation suggests that only Li complexes provide sufficient time for energy transfer collisions with the dilute He background gas to stabilize the complex.

        Vibrational frequencies of the complexes were estimated using both C₆₀ vibrational frequencies and alkali stretch and bend modes estimates from an ion-induced dipole interaction potential. The internal energy associated with each alkali complex was taken as the binding energy plus the difference in ionization potentials. The binding energy was approximated by evaluating the ion-induced dipole potential at an ion-neutral separation determined by the ion radius.

        It is interesting to note that even though the ionization potentials of Na and Li differ by only 0.25 eV, N complexes are not observed experimentally. This result is consistent with the model of dissociative charge-exchange.