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Physical Review A

State-selective electric-field ionization of Rydberg positronium

A. Alonso, L. Gurung, Benedict Sukra, S. D. Hogan, D. B. Cassidy

Published Nov 15, 2018• 14 citations
Publisher

Abstract

We report experiments in which positronium (Ps) atoms, optically excited to Rydberg states with principal quantum numbers $n$ in the range 18--25, were selectively ionized by both static and pulsed electric fields. The experiments were modeled using Monte Carlo simulations that include tunnel ionization rates calculated for hydrogen and scaled by the Ps reduced mass. Our measurements exhibit a small disagreement with the calculated tunnel ionization rates. Despite this we show that the electric fields in which different Ps states are ionized are sufficiently separated to allow selective field-ionization methods to be used in typical experimental conditions.

DOI
10.1103/physreva.98.053417

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We report experiments in which positronium (Ps) atoms, optically excited to Rydberg states with principal quantum numbers nnn in the range 18--25, were selectively ionized by both static and pulsed electric fields. The experiments were modeled using Monte Carlo simulations that include tunnel ionization rates calculated for hydrogen and scaled by the Ps reduced mass. Our measurements exhibit a small disagreement with the calculated tunnel ionization rates. Despite this we show that the electric fields in which different Ps states are ionized are sufficiently separated to allow selective field-ionization methods to be used in typical experimental conditions.
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.
Field ionization of high Rydberg manifold states (n=112–137)(n=112\text{--}137)(n=112–137) of rubidium-85 under a rotating electric field has been investigated experimentally and theoretically. Applying a rapidly reversed electric field with a small static perpendicular field to rotate the total electric field around zero, we have observed substantial increase of the fraction of the tunneling field-ionization process and a profound broadening of its ionization peak over a wide range of field strengths with increasing transverse electric field. The observed tunneling field-ionization fraction is almost independent of the slew rate of the applied electric field, and also on the principal quantum number nnn over the range investigated. To compare with these experimental results, theoretical calculations have been performed with a successive two-step regime. As the first step we calculated the redistributions of magnetic quantum number ${m}_{\ensuremath{\ell}}$ and parabolic quantum number ${n}_{1}\ensuremath{-}{n}_{2}$ of the states under the rotating electric field. Then the following time evolution of the manifold states with $\ensuremath{\mid}{m}_{\ensuremath{\ell}}\ensuremath{\mid}\ensuremath{\leqslant}3$ was traced on the Stark map in a coherent as well as an incoherent model. The calculated results are generally in good agreement with the experimental ones. The increase of the tunneling field-ionization fraction under a rotating electric field plays an essential role in achieving the high efficiency in the recently proposed selective-field-ionization scheme for high Rydberg atoms.
Rydberg states of atomic helium with principal quantum numbers ranging from n=20n=20n=20 to n=100n=100n=100 have been prepared by non-resonance-enhanced single-color two-photon excitation from the metastable 23S12\phantom{\rule{0.16em}{0ex}}{{}^{3}S}_{1}23S1​ state. Photoexcitation was carried out using linearly and circularly polarized pulsed laser radiation. In the case of excitation with circularly polarized radiation, Rydberg states with azimuthal quantum number $|{m}_{\ensuremath{\ell}}|=2$ were prepared in zero electric field and in homogeneous electric fields oriented parallel to the propagation axis of the laser radiation. In sufficiently strong electric fields, individual Rydberg--Stark states were resolved spectroscopically, highlighting the suitability of non-resonance-enhanced multiphoton excitation schemes for the preparation of long-lived high-$|{m}_{\ensuremath{\ell}}|$ hydrogenic Rydberg states for deceleration and trapping experiments. Applications of similar schemes for Doppler-free excitation of positronium atoms to Rydberg states are also discussed.
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 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.
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.