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 MOF crystals: MOF-5, IRMOF-8, ZnO4(FMA)3 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.

What is the most significant result of this study? We introduce an atomic model coupling aT ao step potential with the crystallographic structure of am aterial. This enables unprecedented structural insight into the behavior of the positronium within zeolitic materials of distinct framework topology. How did the collaborationo nt his project start?

The experimental setup and results of the first search for invisible decays of orthopositronium (o-Ps) confined in a vacuum cavity are reported. No evidence of invisible decays at a level Brðo-Ps → invisibleÞ < 5.9 × 10 −4 (90% C.L.) was found. This decay channel is predicted in hidden sector models such as the mirror matter (MM), which could be a candidate for dark matter. Analyzed within the MM context, this result provides an upper limit on the kinetic mixing strength between ordinary and mirror photons of ε < 3.1 × 10 −7 (90% C.L.). This limit was obtained for the first time in vacuum free of systematic effects due to collisions with matter.

Precision spectroscopy of the Muonium Lamb shift and fine structure requires a robust source of 2S Muonium. To date, the beam-foil technique is the only demonstrated method for creating such a beam in vacuum. Previous experiments using this technique were statistics limited, and new measurements would benefit tremendously from the efficient 2S production at a low energy muon (< 20 keV) facility. Such a source of abundant low energy μ + has only become available in recent years, e.g. at the Low-Energy Muon beamline at the Paul Scherrer Institute. Using this source, we report on the successful creation of an intense, directed beam of metastable Muonium. We find that even though the theoretical Muonium fraction is maximal in the low energy range of 2-5 keV, scattering by the foil and transport characteristics of the beamline favor slightly higher μ + energies of 7-10 keV. We estimate that an event detection rate of a few events per second for a future Lamb shift measurement is feasible, enabling an increase in precision by two orders of magnitude over previous determinations.

Interest in new tools for the analysis of catalytic materials is growing due to the potential to enhance their functionality through the optimal nanostructuring, for example, of pore networks and surface properties. This prompts the need for improved descriptors to discriminate increasingly complex architectures. As a nondestructive, dynamic, and potentially, temporally, and spatially resolved tool, positron annihilation spectroscopy (PAS) can provide valuable complementary insights to already established (e.g., adsorption, spectroscopy, diffraction, and microscopy) methods. This is possible due to the specific sensitivity of positrons to the electronic environment, which determines their annihilation characteristics. However, despite growing enthusiasm, PAS is not widely known in the catalysis community. This review aims to highlight the many unique features, principles, and potential pitfalls of the technique, expanding on the outdated reviews on the topic, which are now over a decade old. After briefly introducing the principles, progress in the application of PAS to investigate various features of relevant catalytic materials is summarized. This includes the crystalline structure, presence of defects, pore connectivity and evolution, chemical properties, and adsorption phenomena. An improved understanding of the response will contribute not only to guiding the design of nanostructured materials but also to positioning PAS as a mainstream method for catalyst characterization.