Extra formation of positronium in amorphous polymers at low temperature

Abstract

Positron annihilation lifetime spectroscopy measurements of polystyrene and poly(methyl methacrylate) have been performed at 20 K during $400 h. An increase of the probability of positronium (Ps) formation with irradiation time is observed. This effect is due to the accumulation of shallow-trapped electrons which are stable at low temperatures. Detailed analyses of the lifetime spectra within the blob model framework show that these electrons induced by the previous positron tracks act as extra blob electrons: they either recombine with a cation or form Ps. This kinetic analysis introduced by the blob model indicates that no formation of Ps occurs after the blob recombination in these amorphous polymers. An increase of the free positron lifetime with the measuring time is also observed. This effect can not be explained in terms of artifact of discrete analysis caused by the existence of an ortho-positronium lifetime distribution.

Electronic structure of solid molecular complexes evaluated by positronium formation studies at low temperature

“…In the literature it is well known that the increase of Ps formation as a function of measuring time, at low temperatures, is a common effect in polymers [19][20][21][22][23] and in some solid longchain alkanes [24][25][26][27][28][29], which is correlated with the molecular structure of the compounds. In these situations the Ps formation phenomenon results from positron interaction with trapped electrons [19][20][21][22][23][24][25][26][27][28][29], i.e., during PALS experiments, the excess of free electrons produced during positrons' slowing down can be trapped in the intermolecular cavities.…”

“…In these situations the Ps formation phenomenon results from positron interaction with trapped electrons [19][20][21][22][23][24][25][26][27][28][29], i.e., during PALS experiments, the excess of free electrons produced during positrons' slowing down can be trapped in the intermolecular cavities. Trapped electrons in these shallow traps create, according the spur model, an additional probability of Ps formation by binding free positrons with one of them.…”

The free positron lifetime in amorphous polymers at low temperature

“…In this condition, the increase in t 2 could be explained by an analysis artifact. However, such a point had been studied and cannot explain the observed effect on t 2 [17].…”