Positronium in MOFs: the Atom out of the box

TL;DR

This study investigates positronium behavior in various metal-organic frameworks, providing evidence of its delocalization and emission characteristics, and introduces a model explaining its energy states within MOFs.

Contribution

The paper offers new experimental data on positronium emission and diffusion in MOFs, and demonstrates that a Kron--Penney potential model accurately describes Ps delocalization and energy.

Findings

Positronium exhibits delocalization in MOFs.

Measured Ps emission energies are lower than localized expectations.

A Kron--Penney potential model explains the Ps energy states.

Abstract

Recently, evidence for positronium (Ps) in a Bloch state in self-assembled metal--organic frameworks (MOFs) has been reported [Dutta et al., Phys. Rev. Lett. 110, 197403 (2013)]. In this paper, we study Ps emission into vacuum from four different MOFs crystals: MOF-5, IRMOF-8, FMA and IRMOF-20. Our measurements of Ps yield and emission energy into vacuum provide definitive evidence of Ps delocalization. We determine with a different technique Ps diffusion lengths in agreement with the recently published results. Furthermore, we measure that a fraction of the Ps is emitted into vacuum with a distinctly smaller energy than what one would expect for Ps localized in the MOFs' cells. We show that a calculation assuming Ps delocalized in a Kronig--Penney potential reproduces the measured Ps emission energy.