Reactions of Protons and Hydrogen at Electron-volt Energies.*

We combined a simple time-of-flight (TOF) mass spectrometer in tandem with a cylindrical rf ion trap to monitor the temporal evolution of an ion population as it reacts with a neutral gas buffer following an electron impact interval which is used to create the ions. At precisely determined times following electron impact, the stored ions are extracted from the trap into an 82-cm flight path terminated by an active-film detector. As charge-exchange (CX) reactions typically dominate ion losses from the trap for neutral gas pressures $\ge $ 10$^{-9}$ Torr, an analysis of the TOF spectra as a function of the extraction time and neutral gas pressure will yield the relevant CX collision-rate coefficient. Initial measurements are being carried out using H$^{+}$ and H$_{2}$. A discussion of the apparatus, data associated with the reaction, and systematic issues associated with these measurements will be presented.