Annihilation of positrons on organic molecules

The lifetime of positrons confined in a Penning trap is studied in the presence of a variety of chemical species. The large observed annihilation rates of the positrons on these molecules provide evidence that positron-molecule attachment is a general phenomenon, and distinct chemical trends are identified. For example, the annihilation rate on fluorocarbons is as much as 3 orders of magnitude smaller than that for the analogous hydrocarbons. An empirical scaling of the dependence of the annihilation rate on molecular ionization potential is identified.