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EPJ Techniques and Instrumentation

Fast decay of 23S1 positronium atoms in an MgO lined cavity

L. Gurung, Tamara J. Babij, D. B. Cassidy

Published Jan 15, 2021• 3 citations
Publisher

Abstract

Abstract We report experiments in which radiatively metastable $2\,^{3}\mathrm {S}_{1}$ 2 3 S 1 positronium (Ps) atoms entered a waveguide whose internal surfaces were lined with smoked magnesium oxide (MgO) powder. The waveguide was built such that time-delayed microwave radiation pulses, tuned to drive $2\,^{3}\mathrm {S}_{1}\rightarrow 2\,^{3}\mathrm {P}_{2}$ 2 3 S 1 → 2 3 P 2 transitions, could be applied to the Ps ensemble. The lifetime of $2\,^{3}\mathrm {S}_{1}$ 2 3 S 1 atoms was measured using time-delayed microwave induced decay, yielding ≈200 ns. This is considerably shorter than the Zeeman shifted lifetime of 1070 ns, indicating that Ps atoms in the $2\,^{3}\mathrm {S}_{1}$ 2 3 S 1 state do not survive multiple collisions with MgO nanocrystals.

DOI
10.1140/epjti/s40485-020-00058-z

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Physical Review A•Jan 1, 1974•20 citations
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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 nnn 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.
Positronium in the 23S2^{\phantom{\rule{0.16em}{0ex}}3}\mathrm{S}23S metastable state exhibits a low electrical polarizability and a long lifetime (1140 ns), making it a promising candidate for interferometry experiments with a neutral matter-antimatter system. In the present work, 23S2^{\phantom{\rule{0.16em}{0ex}}3}\mathrm{S}23S positronium is produced, in the absence of an electric field, via spontaneous radiative decay from the 33P3^{\phantom{\rule{0.16em}{0ex}}3}\mathrm{P}33P level populated with a 205-nm UV laser pulse. Thanks to the short temporal length of the pulse, 1.5 ns full width at half maximum, different velocity populations of a positronium cloud emitted from a nanochanneled positron-positronium converter were selected by delaying the excitation pulse with respect to the production instant. 23S2^{\phantom{\rule{0.16em}{0ex}}3}\mathrm{S}23S positronium atoms with velocity tuned between $7\ifmmode\times\else\texttimes\fi{}{10}^{4}\phantom{\rule{0.16em}{0ex}}{\mathrm{ms}}^{\ensuremath{-}1}$ and $10\ifmmode\times\else\texttimes\fi{}{10}^{4}\phantom{\rule{0.16em}{0ex}}{\mathrm{ms}}^{\ensuremath{-}1}$ were thus produced. Depending on the selected velocity, a 23S2^{\phantom{\rule{0.16em}{0ex}}3}\mathrm{S}23S production efficiency ranging from $\ensuremath{\sim}0.8%$ to $\ensuremath{\sim}1.7%$, with respect to the total amount of emitted positronium, was obtained. The observed results give a branching ratio for the $3^{\phantom{\rule{0.16em}{0ex}}3}\mathrm{P}\ensuremath{-}2^{\phantom{\rule{0.16em}{0ex}}3}\mathrm{S}$ spontaneous decay of $(9.7\ifmmode\pm\else\textpm\fi{}2.7)%$. The present velocity selection technique could allow one to produce an almost monochromatic beam of $\ensuremath{\sim}1\ifmmode\times\else\texttimes\fi{}{10}^{3}\phantom{\rule{4pt}{0ex}}2^{\phantom{\rule{0.16em}{0ex}}3}\mathrm{S}$ atoms with a velocity spread of $<{10}^{4}\phantom{\rule{0.16em}{0ex}}{\mathrm{ms}}^{\ensuremath{-}1}$ and an angular divergence of $\ensuremath{\sim}50$ mrad.
We report a measurement of the annihilation decay rate of positronium (Ps) atoms, . Ground state atoms optically excited to radiatively metastable states were quenched via Stark mixing by the application of a time-delayed electric field. Rapid radiative decay of the Stark mixed states to the ground state, followed by self-annihilation, was observed via the annihilation radiation time spectrum, and used to determine the number of excited state atoms remaining at different times, and hence the decay rate. We obtain , in broad agreement with the Zeeman-shifted theoretical value of .
We numerically study laser deceleration, cooling and trapping of host fast ortho-positronium (Ps) bunches as produced in silica microchannel targets using positron bunches from a Surko trap. While Doppler laser cooling routinely allows to cool neutral atom species below milli-Kelvin kinetic temperatures, the fast ground state annihilation and the high recoil shift to line-width ratio in Ps pose new challenging limitations on timing and the required laser power. Earlier theoretical work and first experiments already exhibit viable prospects for radiation pressure cooling to create unprecedented cold and dense Ps ensembles. Our extended numerical studies confirm very good prospects to simultaneously decelerate, cool and optically trap Ps atoms in a standard Doppler cooling geometry on the 13S1^3\mathrm{S}13S--23P2^3\mathrm{P}23P transition. Including the full Zeeman state manifolds and adding additional transverse lasers on the 23P2^3\mathrm{P}23P--33D3^3\mathrm{D}33D transition improves timing and final temperature to allow for optical trapping at the end. The required laser powers and geometry parameters to implement effective slowing and trapping within a time frame, where the majority of atoms is not annihilated, is in reach of current technology of mJ\mathrm{mJ}mJ energy pulses of approximately 100 ns100\ \mathrm{ns}100 ns duration. Future extensions to collectively enhance cooling and trapping in optical cavities or hollow-core fibres should finally allow fast preparation of ultra-cold Ps systems as future basis of superradiant lasing and Bose--Einstein condensates.
We have measured the lifetime of the 3P1^{3}P_{1}3P1​ metastable state of Mg and obtained a value of 2.2 \ifmmode\pm\else\textpm\fi{} 0.2 msec. The level was excited with a dye laser tuned to the intercombination line ($3s3p^{3}P_{1}\ensuremath{\leftrightarrow}3{s}^{2}^{1}S_{0}$). In addition to the 3P1^{3}P_{1}3P1​ state, the 3P2^{3}P_{2}3P2​ and 3P0^{3}P_{0}3P0​ levels were populated by mixing collisions owing to the presence of an inert buffer gas. The populations of all three metastable levels were monitored as a function of time by observing the absorption of Mg resonance radiation ($^{3}P_{0,1,2}\ensuremath{\rightarrow}^{3}S_{1}$) out of each level. The mixing time was short compared to the 3P1^{3}P_{1}3P1​ lifetime, so that the populations of the three metastable levels were in thermal equilibrium with each other. This produced an apparent lengthening of the 3P1^{3}P_{1}3P1​ lifetime, which would introduce a large systematic error if the degree of mixing were not known. Cenventionally, one observes the fluorescence from the excited levels to obtain their lifetimes. Because the lifetimes of the 3P2^{3}P_{2}3P2​ and 3P0^{3}P_{0}3P0​ states are too long, fluorescence from these states cannot be observed, and the observation of fluorescence from the 3P1^{3}P_{1}3P1​ state alone provides insufficient information to determine its lifetime.