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arXiv:1904.09004 · Physical Review A

Efficient 2S3 positronium production by stimulated decay from the 3P3 level

M. Antonello, A. Belov, G. Bonomi, Roberto Sennen Brusa, M. Caccia, A. Camper, R. Caravita, F. Castelli, G. Cerchiari, D. Comparat, Giovanni Consolati, Andrea Demetrio, L. Di Noto, M. Doser, Mattia Fanì, S. Gerber, A. Gligorova, F. Guatieri, Philip Hackstock, S. Haider, Alexander Hinterberger, A. Kellerbauer, O. Khalidova, D. Krasnický, Vittorio Lagomarsino, P. Lebrun, C. Malbrunot, S. Mariazzi, V. Matveev, S. R. Müller, G. Nebbia, P. Nédélec, Markus K. Oberthaler, Emmanuel Oswald, Davide Pagano, Luca Penasa, V. Petráček, F. Prelz, B. Rienäcker, Jacques Robert, Ole Myren Rohne, Alberto Rotondi, H. Sandaker, R. Santoro, G. Testera, Ingmari C. Tietje, E. Widmann, T. Wolz, P. Yzombard, Christian Zimmer, Nicola Zurlo

Published Dec 9, 2019• 10 citations
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Abstract

We investigate experimentally the possibility of enhancing the production of $2\phantom{\rule{0.16em}{0ex}}^{3}S$ positronium atoms by driving the $1\phantom{\rule{0.16em}{0ex}}^{3}S$--$3\phantom{\rule{0.16em}{0ex}}^{3}P$ and $3\phantom{\rule{0.16em}{0ex}}^{3}P$--$2\phantom{\rule{0.16em}{0ex}}^{3}S$ transitions, overcoming the natural branching ratio limitation of spontaneous decay from $3\phantom{\rule{0.16em}{0ex}}^{3}P$ to $2\phantom{\rule{0.16em}{0ex}}^{3}S$. The decay of $3\phantom{\rule{0.16em}{0ex}}^{3}P$ positronium atoms toward the $2\phantom{\rule{0.16em}{0ex}}^{3}S$ level has been efficiently stimulated by a 1312.2 nm broadband IR laser pulse. The dependence of the stimulating transition efficiency on the intensity of the IR pulse has been measured to find the optimal enhancement conditions. A maximum relative increase of $\ifmmode\times\else\texttimes\fi{}(3.1\ifmmode\pm\else\textpm\fi{}1.0)$ in the $2\phantom{\rule{0.16em}{0ex}}^{3}S$ production efficiency, with respect to the case in which only spontaneous decay is present, was obtained.

DOI
10.1103/physreva.100.063414

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The field of experimental positronium physics has advanced significantly in the last few decades, with new areas of research driven by the development of techniques for trapping and manipulating positrons using Surko-type buffer gas traps. Large numbers of positrons (typically ≥106) accumulated in such a device may be ejected all at once, so as to generate an intense pulse. Standard bunching techniques can produce pulses with ns (mm) temporal (spatial) beam profiles. These pulses can be converted into a dilute Ps gas in vacuum with densities on the order of 107 cm−3 which can be probed by standard ns pulsed laser systems. This allows for the efficient production of excited Ps states, including long-lived Rydberg states, which in turn facilitates numerous experimental programs, such as precision optical and microwave spectroscopy of Ps, the application of Stark deceleration methods to guide, decelerate and focus Rydberg Ps beams, and studies of the interactions of such beams with other atomic and molecular species. These methods are also applicable to antihydrogen production and spectroscopic studies of energy levels and resonances in positronium ions and molecules. A summary of recent progress in this area will be given, with the objective of providing an overview of the field as it currently exists, and a brief discussion of some future directions.

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We investigate experimentally the possibility of enhancing the production of 23S2\phantom{\rule{0.16em}{0ex}}^{3}S23S positronium atoms by driving the 13S1\phantom{\rule{0.16em}{0ex}}^{3}S13S--33P3\phantom{\rule{0.16em}{0ex}}^{3}P33P and 33P3\phantom{\rule{0.16em}{0ex}}^{3}P33P--23S2\phantom{\rule{0.16em}{0ex}}^{3}S23S transitions, overcoming the natural branching ratio limitation of spontaneous decay from 33P3\phantom{\rule{0.16em}{0ex}}^{3}P33P to 23S2\phantom{\rule{0.16em}{0ex}}^{3}S23S. The decay of 33P3\phantom{\rule{0.16em}{0ex}}^{3}P33P positronium atoms toward the 23S2\phantom{\rule{0.16em}{0ex}}^{3}S23S level has been efficiently stimulated by a 1312.2 nm broadband IR laser pulse. The dependence of the stimulating transition efficiency on the intensity of the IR pulse has been measured to find the optimal enhancement conditions. A maximum relative increase of \ifmmode×\else\texttimes\fi(3.1\ifmmode±\else\textpm\fi1.0)\ifmmode\times\else\texttimes\fi{}(3.1\ifmmode\pm\else\textpm\fi{}1.0)\ifmmode×\else\texttimes\fi(3.1\ifmmode±\else\textpm\fi1.0) in the 23S2\phantom{\rule{0.16em}{0ex}}^{3}S23S production efficiency, with respect to the case in which only spontaneous decay is present, was obtained.
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 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 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.
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.
We report on laser cooling of a large fraction of positronium (Ps) in free flight by strongly saturating the 1^{3}S-2^{3}P transition with a broadband, long-pulsed 243 nm alexandrite laser. The ground state Ps cloud is produced in a magnetic and electric field-free environment. We observe two different laser-induced effects. The first effect is an increase in the number of atoms in the ground state after the time Ps has spent in the long-lived 2^{3}P states. The second effect is one-dimensional Doppler cooling of Ps, reducing the cloud's temperature from 380(20) to 170(20) K. We demonstrate a 58(9)% increase in the fraction of Ps atoms with v_{1D}<3.7×10^{4} ms^{-1}.
The field of experimental positronium physics has advanced significantly in the last few decades, with new areas of research driven by the development of techniques for trapping and manipulating positrons using Surko-type buffer gas traps. Large numbers of positrons (typically ≥106) accumulated in such a device may be ejected all at once, so as to generate an intense pulse. Standard bunching techniques can produce pulses with ns (mm) temporal (spatial) beam profiles. These pulses can be converted into a dilute Ps gas in vacuum with densities on the order of 107 cm−3 which can be probed by standard ns pulsed laser systems. This allows for the efficient production of excited Ps states, including long-lived Rydberg states, which in turn facilitates numerous experimental programs, such as precision optical and microwave spectroscopy of Ps, the application of Stark deceleration methods to guide, decelerate and focus Rydberg Ps beams, and studies of the interactions of such beams with other atomic and molecular species. These methods are also applicable to antihydrogen production and spectroscopic studies of energy levels and resonances in positronium ions and molecules. A summary of recent progress in this area will be given, with the objective of providing an overview of the field as it currently exists, and a brief discussion of some future directions.