Ortho-Parapositronium Quenching by Paramagnetic Molecules and Ions

The quenching of the long lifetime of orthopositronium by a paramagnetic molecule such as NO is generally interpreted as due to a spin flip resulting from exchange of the positronium electron with the single odd unpaired electron of the molecule. This concept is re-examined and it is found that spin flip is actually irrelevant, the only essential condition for quenching being that exchange be possible with one or more unpaired electrons. A further example is the well-known strong quenching caused by the ${\mathrm{O}}_2$ molecule, whose two unpaired electrons can undergo exchange both with and without spin flip. In addition the quenching by paramagnetic ions in aqueous solution is briefly discussed. It is further noted that the quenching of orthopositronium by diphenylpicrylhydrazyl (DPH) in benzene, generally regarded as a prime example of exchange quenching, is probably of quite different origin. Evidence is presented that this quenching is more likely due to a simple capture of the positronium atom by the DPH molecule.