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Physical Review A

Resonant shifts of positronium energy levels in MgO powder

L. Gurung, B. S. Cooper, S. D. Hogan, D. B. Cassidy

Published Jan 2, 2020• 3 citations
Publisher

Abstract

We report measurements of shifts in the frequencies of $1\phantom{\rule{0.16em}{0ex}}^{3}S_{1}\ensuremath{\rightarrow}2\phantom{\rule{0.16em}{0ex}}^{3}P_{J}$ and $2\phantom{\rule{0.16em}{0ex}}^{3}P_{J}\ensuremath{\rightarrow}n\phantom{\rule{0.16em}{0ex}}^{3}D/n\phantom{\rule{0.16em}{0ex}}^{3}S$ transitions optically driven in positronium atoms while they are inside the open volumes of MgO smoke powder. The observed intervals are larger than the corresponding vacuum excitations, but, surprisingly, the transitions to Rydberg states are less strongly affected, and the energy shifts exhibit no dependence on the principal quantum number $n$ of the final state. We attribute these shifts to resonant interactions between Ps atoms and MgO surfaces, mediated via spectrally overlapping MgO ultra violet (UV) photo-luminescent absorption bands. Since many insulating materials suitable for Ps confinement exhibit similar broadband UV absorption characteristics, the observed phenomena have implications for optical diagnostics and laser cooling schemes of relevance to studies of high-density Ps ensembles in insulating cavities, including the production of a Ps Bose-Einstein condensate.

DOI
10.1103/physreva.101.012701

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We report experiments in which the line shape of the Lyman-alpha ($1S\mathrm{\text{\ensuremath{-}}}2P$) transition was measured for positronium (Ps) atoms both inside and outside a porous silica target. The energy interval $\ensuremath{\Delta}E$ for confined atoms was observed to be larger than that of free Ps by $1.26\ifmmode\pm\else\textpm\fi{}0.06\text{ }\text{ }\mathrm{meV}$. A configuration interaction calculation yields results that are consistent with our $\ensuremath{\sim}5\text{ }\text{ }\mathrm{nm}$ sample, and suggests that $\ensuremath{\Delta}E$ decreases dramatically for larger cavity diameters. The linewidth of the transition, $(0.066\ifmmode\pm\else\textpm\fi{}0.004)\text{ }\text{ }\mathrm{nm}$ (FWHM), is about half of what one would expect for free Ps at room temperature due to the Dicke line narrowing effect of confinement. Such measurements can be used to determine void sizes in porous films and Ps dynamics therein, and elimination of the Doppler spread of atoms in a porous film could be useful for the efficient excitation of a Ps gas.