Positronium work function

In contrast to the case for metals, positronium can be formed in the bulk of many insulators and molecular crystals, and any positronium which subsequently diffuses to the surface can be emitted into the vacuum with a kinetic energy < — Ps, where (f)Ps is the positronium work function. Its value can be expressed in terms of the binding energy of the positronium when in the solid, EB, and the positronium chemical potential, /xPs, as (Schultz and Lynn, 1988)... 

Note that the use of a work function here, rather than the formation potential used for metals, is appropriate since positronium can exist in the bulk of the material. 

To date, it has been found that the positronium formation potential, see equation (1.14), is positive for nPs > 1, so that excited state positronium emission by a work function process is forbidden. Only positrons which reach a surface epithermally (which occurs predominantly at low impact energies where they do not penetrate far into the solid) may... 

Positronium in condensed matter can exist only in the regions of a low electron density, in various kinds of free volume in defects of vacancy type, voids sometimes natural free spaces in a perfect crystal structure are sufficient to accommodate a Ps atom. The pick-off probability depends on overlapping the positronium wavefunction with wavefunctions of the surrounding electrons, thus the size of free volume in which o-Ps is trapped strongly influences its lifetime. The relation between the free volume size and o-Ps lifetime is widely used for determination of the sub-nanovoid distribution in polymers [3]. It is assumed that the Ps atom is trapped in a spherical void of a radius R the void represents a rectangular potential well. The depth of the well is related to the Ps work function, however, in the commonly used model [4] a simplified approach is applied the potential barrier is assumed infinite, but its radius is increased by AR. The value of AR is chosen to reproduce the overlap of the Ps wavefunction with the electron cloud outside R. Thus,... 

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