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Physical Review A

Doppler-corrected Balmer spectroscopy of Rydberg positronium

A. C. L. Jones, T. H. Hisakado, H. J. Goldman, Harry W. K. Tom, Allen Paine Mills, D. B. Cassidy

Published Jul 3, 2014• 34 citations
Publisher

Abstract

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 $2P$ 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 $n>50$, consistent with the onset of motional electric field ionization in the 3.5-mT magnetic field at the Ps formation target.

DOI
10.1103/physreva.90.012503

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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≤n\ensuremath≤3115\ensuremath{\le}n\ensuremath{\le}3115\ensuremath≤n\ensuremath≤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 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.
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.
We characterized the pulsed Rydberg-positronium production inside the Antimatter Experiment: Gravity, Interferometry, Spectroscopy (AEg‾IS\mathrm{AE}\overline{\text{g}}\mathrm{IS}AEg​IS) apparatus in view of antihydrogen formation by means of a charge exchange reaction between cold antiprotons and slow Rydberg-positronium atoms. Velocity measurements on the positronium along two axes in a cryogenic environment ($\ensuremath{\approx}10\phantom{\rule{0.16em}{0ex}}\mathrm{K}$) and in 1T1\phantom{\rule{0.16em}{0ex}}\mathrm{T}1T magnetic field were performed. The velocimetry was done by microchannel-plate (MCP) imaging of a photoionized positronium previously excited to the n=3n=3n=3 state. One direction of velocity was measured via Doppler scan of this n=3n=3n=3 line, another direction perpendicular to the former by delaying the exciting laser pulses in a time-of-flight measurement. Self-ionization in the magnetic field due to the motional Stark effect was also quantified by using the same MCP-imaging technique for Rydberg positronium with an effective principal quantum number neff{n}_{\mathrm{eff}}neff​ ranging between 14 and 22. We conclude with a discussion about the optimization of our experimental parameters for creating Rydberg positronium in preparation for an efficient pulsed production of antihydrogen.
A Surko-type positron trap has been used to produce intense positron pulses, and thus a dilute positronium (Ps) gas in vacuum with an initial density of 10^7 cm-3. This gas was excited with lasers for a variety of experiments. n = 2 atoms were prepared using a single-photon excitation scheme (1S to 2P) in controlled electric and magnetic fields for studies of Stark and Zeeman effects, measurements of Ps cooling via Doppler spectroscopy, and for the produciton of ensembles of atoms in pure 2S states. The first measurements of Ps excitation to specific Rydberg-Stark states were performed via a two-color two-photon excitation scheme ( 1S to 2P to nS/nD). Ps atoms in low-field–seeking Rydberg Stark states were prepared at the entrance of an electrostatic quadrupole guide which was used to control the trajectories and velocity distributions of the Ps atoms. These advances, together with the results from numerical simulations, suggest that these methods could be used in to perform Stark deceleration and trapping of Rydberg Ps, high-precision Ps spectroscopy, and to create positron-atom bound states.
Abstract We produce Rydberg Ps by a two-step laser excitation from and from to states of principal quantum level that are Stark split by a motionally induced electric field. Our measurements are largely free of first-order Doppler shifts such that we are able to investigate the impact of laser polarization on the population of the closely spaced Stark levels. We find a variation in the distribution that is primarily dependent on the IR laser polarization with respect to the direction of the motionally induced electric field. With the IR light polarized parallel to the electric field F , the ratio of excitation probability to the levels of maximal Stark splitting compared to that of excitation to the states of minimal Stark splitting is found to be 3.37 ± 0.51, whereas with the IR light polarized perpendicular to F , the excitation ratio is 0.87 ± 0.64. Our results agree with those of Wall et al (2015 Phys. Rev. Lett. 114 173001 ) obtained with n = 11 and will be useful in the preparation of high- n states of Ps for a variety of experiments, including measuring the interaction of Ps with gravity, in precision time-of-flight (TOF) energy spectroscopy, and precision optical spectroscopy of Ps.
Calculations of the photoexcitation spectra of ortho-positronium Rydberg states with principal quantum numbers between 10 and 30 are presented. The effects of Doppler broadening and saturation of the corresponding electric-dipole transitions are studied, together with the role of static and motionally induced electric fields. This is done in the context of recent measurements reported by Cassidy et al. [Phys. Rev. Lett. 108, 043401 (2012)], and with regard to experiments involving the production of antihydrogen by charge-exchange between Rydberg positronium and cold antiprotons.