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Physical Review A

Production of 2S13 positronium atoms by single-photon excitation in an electric field

A. Alonso, S. D. Hogan, D. B. Cassidy

Published Mar 9, 2017• 28 citations
Publisher

Abstract

We report experiments in which positronium (Ps) atoms are produced in the $2\phantom{\rule{0.16em}{0ex}}^{3}S_{1}$ level by single-photon excitation from the ground state. To accomplish this, Stark-mixed $n=2$ 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 $2\phantom{\rule{0.16em}{0ex}}^{3}S_{1}$ 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.

DOI
10.1103/physreva.95.033408

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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 \ensuremathγ\ensuremath{\gamma}\ensuremathγ radiation. The observed \ensuremath≈4\ensuremath{\approx}4%\ensuremath≈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.
We show that the annihilation dynamics of excited positronium (Ps) atoms can be controlled using parallel electric and magnetic fields. To achieve this, Ps atoms were optically excited to n=2 sublevels in fields that were adjusted to control the amount of short-lived and long-lived character of the resulting mixed states. Inclusion of the former offers a practical approach to detection via annihilation radiation, whereas the increased lifetimes due to the latter can be exploited to optimize resonance-enhanced two-photon excitation processes (e.g., 1^{3}S→2^{3}P→nS/nD), either by minimizing losses through intermediate state decay, or by making it possible to separate the excitation laser pulses in time. In addition, photoexcitation of mixed states with a 2^{3}S_{1} component represents an efficient route to producing long-lived pure 2^{3}S_{1} atoms via single-photon excitation.
We report experiments in which positronium (Ps) atoms were guided using inhomogeneous electric fields. Ps atoms in Rydberg-Stark states with principal quantum number n=10 and electric dipole moments up to 610 D were prepared via two-color two-photon optical excitation in the presence of a 670 V cm^{-1} electric field. The Ps atoms were created at the entrance of a 0.4 m long electrostatic quadrupole guide, and were detected at the end of the guide via annihilation gamma radiation. When the lasers were tuned to excite low-field-seeking Stark states, a fivefold increase in the number of atoms reaching the end of the guide was observed, whereas no signal was detected when high-field-seeking states were produced. The data are consistent with the calculated geometrical guide acceptance.
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 .
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 $1^{3}S_{1}\ensuremath{\rightarrow}2^{3}S_{1}$ transition in positronium has been observed using two-photon Doppler-free excitation. Transitions were induced by a pulsed dye laser, and detected by photoionization of the 23S12^{3}S_{1}23S1​ state. The resonance signal has a linewidth of 1.5 GHz and a signal-to-noise ratio of 20:1. The frequency agrees with theory to 0.5 GHz. The resonance signal has an amplitude proportional to the square of the laser intensity and displays the expected Stark shift to higher frequency under an applied electric field.
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.