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Physical Review A

Multiring electrostatic guide for Rydberg positronium

M. H. Rayment, L. Gurung, R. E. Sheldon, S. D. Hogan, D. B. Cassidy

Published Jul 15, 2019• 10 citations
Publisher

Abstract

We report the results of experiments in which positronium (Ps) atoms, optically excited to Rydberg-Stark states with principal quantum numbers ranging from $n=13$ to 19, were transported along the axis of a multiring electrode structure. By applying alternate positive and negative potentials to the ring electrodes, inhomogeneous electric fields suitable for guiding low-field-seeking atoms along the guide axis were generated. The multiring configuration used has the advantage that once the atoms are confined within it appropriate time-varying fields can be generated for deceleration and trapping. However, in this type of structure the possibility of nonadiabatic transitions of the fast (100 km/s) Ps atoms to unconfined high-field-seeking states exists. We show that for typical guiding fields this is not a significant loss mechanism and that efficient Ps transport can be achieved. Our data are in accordance with a Landau-Zener analysis of adiabatic transport through the field minima and Monte Carlo simulations that take into account Ps velocity distributions, electric dipole moments, and lifetimes, as well as the electric-field distributions in the guide.

DOI
10.1103/physreva.100.013410

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We report the results of experiments in which positronium (Ps) atoms, optically excited to Rydberg-Stark states with principal quantum numbers ranging from n=13n=13n=13 to 19, were transported along the axis of a multiring electrode structure. By applying alternate positive and negative potentials to the ring electrodes, inhomogeneous electric fields suitable for guiding low-field-seeking atoms along the guide axis were generated. The multiring configuration used has the advantage that once the atoms are confined within it appropriate time-varying fields can be generated for deceleration and trapping. However, in this type of structure the possibility of nonadiabatic transitions of the fast (100 km/s) Ps atoms to unconfined high-field-seeking states exists. We show that for typical guiding fields this is not a significant loss mechanism and that efficient Ps transport can be achieved. Our data are in accordance with a Landau-Zener analysis of adiabatic transport through the field minima and Monte Carlo simulations that take into account Ps velocity distributions, electric dipole moments, and lifetimes, as well as the electric-field distributions in the guide.
We report experiments in which a slow Rydberg positronium (Ps) beam was produced by velocity selection using a curved electrostatic quadrupole guide. Ps atoms in Rydberg-Stark states with principal quantum number n=14n=14n=14 were prepared by a two-color optical excitation process in a uniform electric field. Low-field-seeking Stark states were produced at the entrance of a 0.6-m-long quadrupole guide that includes a ${45}^{\ensuremath{\circ}}$ bend, and were detected at the end of the guide via their annihilation $\ensuremath{\gamma}$ radiation. The mean speed (kinetic energy) of atoms entering the guide was estimated to be $\ensuremath{\approx}180$ km ${\mathrm{s}}^{\ensuremath{-}1}$ (185 meV), whereas the mean longitudinal speed of guided atoms was measured via time of flight and found to be $\ensuremath{\approx}90$ km ${\mathrm{s}}^{\ensuremath{-}1}$, equivalent to a kinetic energy of 45 meV. The measured transport data are in broad agreement with Monte Carlo simulations, which are also used to establish the efficacy with which the arrangement we describe could be used to perform Ps-atom scattering measurements.
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.
Helium atoms in high- and low-field-seeking Rydberg states with linear and quadratic Stark shifts have been confined in two dimensions and guided over a distance of 150 mm using time-varying inhomogeneous electric fields. This was achieved with an electrode structure composed of four parallel cylindrical rods to which voltages were applied to form oscillating and rotating saddle-point fields. These two modes of operation result in time-averaged pseudopotentials that confine samples in high- and low-field-seeking states about the axis of the device. The experimental data have been compared to the results of numerical particle trajectory calculations that include effects of blackbody radiation and electric field ionization. The results highlight important contributions from single-photon blackbody-induced transitions that cause large changes in the principal quantum number of the Rydberg atoms.
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.
Argon atoms in a pulsed supersonic expansion are prepared in selected Stark components of Rydberg states with effective principal quantum number in the range n * = 15–25. When traversing regions of inhomogeneous electric fields, these atoms get accelerated or decelerated depending on whether the Stark states are low- or high-field seeking states. Using a compact electrode design, which enables the application of highly inhomogeneous and time-dependent electric fields, the Rydberg atoms experience kinetic energy changes of up to 1.2 × 10 −21 J (i.e. 60 cm −1 in spectroscopic units) in a single acceleration/deceleration stage of 3 mm length. The resulting differences in the velocities of the low- and high-field seeking states are large enough that the corresponding distributions of times of flight to the Rydberg particle detector are fully separated. As a result, efficient spectral searches of the Rydberg states best suited for acceleration/deceleration experiments are possible. Numerical simulations of the particle trajectories are used to analyse the time-of-flight distributions and to optimize the time dependence of the inhomogeneous electric fields. The decay of the Rydberg states by fluorescence, collisions and transitions induced by black-body radiation takes place on a timescale long enough not to interfere significantly with the deceleration during the first ∼5 µs.
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.