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AIP conference proceedings

Production of long-lived positronium states via laser excitation to 33P level

(The AEgIS collaboration)

Published Jan 1, 2019• 1 citation
Publisher

Abstract

The 33P state of positronium is an intermediate level suitable for producing long-lived positronium states. On one hand, it can be used in a two-step laser excitation scheme from the ground state to Rydberg levels. On the other hand, excitation of positronium to 33P level is a simple pathway for producing metastable 23S positronium atoms by spontaneous radiactive decay. In this work, experiments showing the production of such long-lived levels, using the 33P state as intermediate state, are presented. The characteristics of the two long-lived levels, in view of experiments of deflectometry/interferometry with positronium, are discussed.

DOI
10.1063/1.5135827

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The 33P state of positronium is an intermediate level suitable for producing long-lived positronium states. On one hand, it can be used in a two-step laser excitation scheme from the ground state to Rydberg levels. On the other hand, excitation of positronium to 33P level is a simple pathway for producing metastable 23S positronium atoms by spontaneous radiactive decay. In this work, experiments showing the production of such long-lived levels, using the 33P state as intermediate state, are presented. The characteristics of the two long-lived levels, in view of experiments of deflectometry/interferometry with positronium, are discussed.
Positronium in the 2(3)S 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, 2(3)S positronium is produced, in the absence of an electric field, via spontaneous radiative decay from the 3(3)P 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. 2(3)S positronium atoms with velocity tuned between 7 x 10(4) ms(-1) and 10 x 10(4) ms(-1) were thus produced. Depending on the selected velocity, a 2(3)S production efficiency ranging from similar to 0.8% to similar to 1.7%, with respect to the total amount of emitted positronium, was obtained. The observed results give a branching ratio for the 3(3)P-2(3)S spontaneous decay of (9.7 +/- 2.7)%. The present velocity selection technique could allow one to produce an almost monochromatic beam of similar to 1 x 10(3) 2(3)S atoms with a velocity spread of <10(4) ms(-1) and an angular divergence of similar to 50 mrad.
We demonstrate experimentally the production of Rydberg positronium (Ps) atoms in a two-step process, comprising incoherent laser excitation, first to the 2(3)P state and then to states with principal quantum numbers ranging from 10 to 25. We find that excitation of 2(3)P atoms to Rydberg levels occurs very efficiently (~90%) and that the ~25% overall efficiency of the production of Rydberg atoms is determined almost entirely by the spectral overlap of the primary excitation laser and the Doppler broadened width of the 1 (3)S-2(3)P transition. The observed efficiency of Rydberg Ps production can be explained if stimulated emission back to the 2P states is suppressed, for example, by intermixing of the Rydberg state Stark sublevels. The efficient production of long-lived Rydberg Ps in a high magnetic field may make it possible to perform direct measurements of the gravitational free fall of Ps.
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.
Rydberg positronium (Ps) atoms have been prepared in selected Stark states via two-step (1s→2p→nd/ns) optical excitation. Two methods have been used to achieve Stark-state selection: a field ionization filter that transmits the outermost states with positive Stark shifts, and state-selected photoexcitation in a strong electric field. The former is demonstrated for n=17 and 18 while the latter is performed for n=11 in a homogeneous electric field of 1.9 kV/cm. The observed spectral intensities and their dependence on the polarization of the laser radiation are in agreement with calculations that include the perturbations of the intermediate n=2 manifold. Our results pave the way for the generation of Rydberg Ps atoms with large electric dipole moments that are required for the realization of schemes to control their motion using inhomogeneous electric fields, an essential feature of some proposed Ps free-fall measurements requiring focused beams of long-lived atoms.
The production of long-lived Rydberg positronium (Ps) and correction for Doppler shifts in the excitation laser frequencies are crucial elements of proposed measurements of the gravitational freefall of antimatter and for precision measurements of the optical spectrum of Ps. Using a two-step optical transition via 2P2P2P levels, we have prepared Ps atoms in Rydberg states up to the term limit. The spectra are corrected for the first-order Doppler shift using measured velocities, and the Balmer transitions are resolved for $15\ensuremath{\le}n\ensuremath{\le}31$. The excitation signal amplitude begins to decrease for n>50n>50n>50, consistent with the onset of motional electric field ionization in the 3.5-mT magnetic field at the Ps formation target.
We report experiments in which positronium (Ps) atoms are produced in the 23S12\phantom{\rule{0.16em}{0ex}}^{3}S_{1}23S1​ level by single-photon excitation from the ground state. To accomplish this, Stark-mixed n=2n=2n=2 states were optically excited in electric fields. By adiabatically switching off the electric field after laser excitation, some of the mixed states evolved into pure 23S12\phantom{\rule{0.16em}{0ex}}^{3}S_{1}23S1​ levels, whose presence was detected via the time dependence of their annihilation $\ensuremath{\gamma}$ radiation. The observed $\ensuremath{\approx}4%$ production efficiency relative to that of Rydberg Ps states is consistent with a Monte Carlo simulation that takes into account the rate at which the electric fields were switched off.