Dissociative positronium attachment in halogen gases

TL;DR

This paper proposes that dissociative positronium attachment explains the high annihilation rates of ortho-positronium in halogen gases, supported by calculations of cross sections and resonances in Ps-F2 interactions.

Contribution

It introduces a theoretical model for dissociative Ps attachment in halogen gases, calculating cross sections and resonances, and links this process to observed annihilation rates.

Findings

Large annihilation rates of o-Ps in halogen gases explained by dissociative attachment.

Calculated resonance contributions align with observed high annihilation rates.

Rate constants are close to those in other halogen molecules like Br2 and I2.

Abstract

We suggest that the observed large annihilation rates of ortho-positronium ($o$-Ps) in halogen gases are due to the process of dissociative Ps attachment, ${\rm Ps} + X_2 \to {\rm Ps}X + X$, where $X$ stands for a halogen atom. This process is similar to dissociative electron attachment which leads to formation of negative ions. We calculate the cross section and rate of this process for the F$_2$ molecule, for which it is exothermic, and therefore, can occur at room temperature. We start with the Ps-F$_2$ scattering calculations which take into account electron exchange and correlations within the framework of the free-electron-gas model. The calculations reveal several resonances. Similar to the process of dissociative electron attachment, a $\Sigma_u$ resonance contributes to the dissociative Ps attachment at thermal energies. We determine the resonance position and width as functions of the internuclear separation, and use them as inputs for the local version of the quasiclassical theory of dissociative attachment. Our calculations yield an anomalously large rate constant for the $o$-Ps annihilation process which is only one order of magnitude lower than those observed for Br$_2$ and I$_2$.