The dynamics of free-ion production through electron transfer in K(np)/CH3CN collisions are examined through measurements using velocity-selected Rydberg atoms. The data show that Rydberg electron transfer leads to the creation of two groups of dipole-bound CH3CN-ions, one long lived (tau>85 mus), the other short lived (tau<1 mus). The velocity dependences associated with the production of both groups of ions are similar, the ion formation rate decreasing markedly with decreasing Rydberg atom velocity, principally as a consequence of postattachment electrostatic interactions between the product ions. The results are in reasonable accord with the predictions of a Monte Carlo collision model that considers the effect of crossings between the diabatic potential curves for the covalent K(np)/CH3CN system and the K+/CH3CN- ion pair. This model also accounts for the relatively small reaction rate constants, similar to0.5-1.0x10(-8) cm(3) s(-1), associated with the formation of long-lived CH3CN- ions. No velocity dependence in the lifetime of the CH3CN- ions is observed. (C) 2004 American Institute of Physics.