Monoenergetic Positronium Emission from Metal-Organic Framework Crystals

Recently it has been discovered that positronium (Ps), after forming in metal-organic framework (MOF) crystals, is emitted into vacuum with a high efficiency and low energy that can only be explained by its propagating as delocalized Bloch states. We show that the Ps atoms are emitted from MOFs in a series of narrow energy peaks consistent with Ps at Bloch-state energy minima being emitted adiabatically into the vacuum. This implies that the Ps emission energy spectra can be directly compared with calculations to obtain detailed information about the Ps band structure in the MOF crystal. The narrow energy width of the lowest energy Ps peak from one MOF sample (2-Methylimidazole zinc salt ZIF-8) suggests it originates from a polaronic Ps surface state. Other peaks can be assigned to Ps with an effective mass of about twice that of bare Ps. Given the immense catalog of available MOF crystals, it should be possible to tune the Ps properties to make vastly improved sources with high production efficiency and a narrow energy spread, for use in fundamental physics experiments.

Figure 1

TOF spectra for Ps emission from the MOF crystals (a) MOF-5 and (b) ZIF-8. Spectra shown are the sum of scans taken with positron implantation energies in the range 0.2–5.2 keV, with the targets at room temperature (open circles, 300 K), and for ZIF-8 at low temperature (bold line, 90 K). Also shown in (a) is a background spectrum (solid line), resulting from ions generated in the detector due to scattered UV light, scaled to the first peak in the spectrum of MOF-5. The observed background is negligible for flight times greater than $3\mu \mathrm{s}$ (indicated by the dashed vertical line), corresponding to Ps kinetic energies $\ge 2\mathrm{eV}$ and has not been subtracted from the data.

Figure 2

Kinetic energy spectra for Ps emitted from the MOF crystals (a) MOF-5 and (b) ZIF-8, taken with the target at room temperature (band) and cryogenic temperatures (filled circles). The statistical uncertainty is represented by error bars in the low temperature data and by the width of the filled band for the room temperature data. The most prominent peaks are labeled E1–E4. Tentative band assignments are shown at the bottom of each plot. In the ZIF-8 spectrum, two additional features are indicated, labeled $\alpha$ and $\beta$. (c) A representation of the model of the observed Ps emission spectra from MOF targets. Ps thermalizing in the target cascades between Ps Bloch states, collecting in band minima. The lowest energy state of ZIF-8, labeled $S$, is not observed for MOF-5, but may exist and be too tightly bound to emit Ps.

Figure 3

(a)–(c) Peaks E1–E3 from the composite spectrum of Ps emission energy from 80 K ZIF-8. In (a) the lowest energy peak, E1, is shown along with a Gaussian fit, indicating a subthermal energy width of $9.7\pm 0.3\mathrm{meV}$. (b),(c) Peaks E2 and E3, and a fitted beam Maxwellian, which yield temperatures consistent with the target temperature.