The rates for free ion production through electron transfer in K(np)/SF6 collisions are measured as a function of Rydberg atom velocity for low-to-intermediate values of n, together with the lifetime of the product SF6- ions against autodetachment. The data show that postattachment interactions between the K+-SF6- product ion pair become increasingly important as n, and the Rydberg atom velocity, are decreased. These result in the conversion of internal energy from the SF6- ion into translational energy of the ion pair and in a dramatic increase in SF6- ion lifetimes. Analysis of the data using a Monte Carlo collision model suggests that the energy transfer is sizable, and is more than sufficient to stabilize the product SF6- ions against autodetachment. (C) 2003 American Institute of Physics.