User:Mroussenova/sandbox

Positronium is a metastable hydrogen-like bound state of an electron and a positron which can exist in two spin states. Para-positonium, p-Ps, ( the positon and electron spins are anti-parallel) is a singlet state with a characteristic self-annihilation lifetime of 125 ps in vacuum. Ortho-positronium, o-Ps, is a triplet state (the positron and electron spins are parallel) with a characteristic self-annihilation lifetime of 142 ns in vacuum. In molecular materials, the lifetime of o-Ps is environment dependent and it delivers information pertaining to the size of the void in which it resides. Ps can pick up a molecular electron with an opposite spin to that of the positron, leading to a reduction of the o-Ps lifetime from 142 ns to 1-4 ns (depending on the size of the free volume in which it resides). The the size of the molecular free volume can be derived from the o-Ps lifetime via the semi-empirical Tao-Eldrip model.