A UHF cavity probe was used to determine the line densities and collision frequencies of electrons deposited in the wakes of spherical aluminum pellets. Density data were obtained over a wide dynamic range of up to six orders of magnitude. These density curves are characterized by a sudden drop of about three or more orders of magnitude which occurs when the wake has cooled to a critical temperature that is a function of pressure. The sudden drop is attributed to the oxygen-molecule attachment-detachment equilibria, while the final leveling portion of the wake is regulated by the positive-to-negative-ion recombination rate. The collision frequencies through out the entire density range are in agreement with those obtained through mobility studies.