Energetic states of positronium in liquids from the study of magnetic field effects and of spin conversion reactions

Abstract

The energetics of positronium (Ps) have been investigated in various liquids through measurements of both the positron lifetime spectroscopy and the Doppler broadening techniques, by studying the effects of magnetic field ( B) quenching and of a chemical spin conversion reaction promoted by a free radical. In the first case, two populations of Ps atoms annihilating in an intrinsic mode are present: the singlet atoms formed on a picosecond time-scale in the positron spur ( p-Ps) and a fraction of the m0 substrates of the triplet atoms ( o-Ps) as quenched by B over a nanosecond time-scale. In constrast with previous results, the momentum distributions of these two populations are found to be the same. This excludes the possibility of Ps thermalization over a nanosecond time scale and, as a consequence, leads to attribute the momentum distribution of the p-Ps atoms measured in the absence of field to the existence of a “bubble” state for Ps in the liquids. This is supported by correlations found between the lifetimes and momentum distributions of the o-Ps atoms with the surface tensions of liquids. In the presence of a chemical spin converter, it is confirmed that the p-Ps atoms formed in the spur have a much broader momemtum distribution than those emerging from the reaction cage in some cases (benzene between 294 and 353 K), but not in others (dioxane at 294 K).