It was recalled that the ortho-para conversion of positronium at paramagnetic reaction centers in condensed matter was often described in terms of simple chemical rate models which allowed for diffusion-limitation of the process due to diffusivity-dependent reaction rates. It was pointed out that, whereas this procedure could be justified for reactions between stable reactants, it disregarded essential features in the case of the reaction of positronium; where reactants were lost by positron-electron annihilation. A simple model was developed here which permitted an exact treatment of the ortho-para conversion of positronium within the framework of a diffusion-reaction theory. The diffusion-limitation gave rise to an infinite sequence of decay rates, of which only a few occurred with significant intensity in the positron lifetime spectrum. Close similarities were found between the present model and the diffusion-limited trapping of positrons at extended defects in crystals. The model included the rate-theory result as a limiting case of high positronium diffusivity.

R.Würschum, A.Seeger: Zeitschrift für Physikalische Chemie, 1995, 192[1], 47-61