Selective Production of Rydberg-Stark States of Positronium

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

Production of 2(3) S-1 positronium atoms by single-photon excitation in an electric field

We report experiments in which positronium (Ps) atoms are produced in the 2 3 S1 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 3 S1 levels, whose presence was detected via the time dependence of their annihilation γ radiation. The observed ≈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

Resonant shifts of positronium energy levels in MgO powder

We report measurements of shifts in the frequencies of 13S1→23PJ and 23PJ→n3D/n3S transitions optically driven in positronium atoms while they are inside the open volumes of MgO smoke powder. The observed intervals are larger than the corresponding vacuum excitations, but, surprisingly, the transitions to Rydberg states are less strongly affected, and the energy shifts exhibit no dependence on the principal quantum number n of the final state. We attribute these shifts to resonant interactions between Ps atoms and MgO surfaces, mediated via spectrally overlapping MgO ultra violet (UV) photo-luminescent absorption bands. Since many insulating materials suitable for Ps confinement exhibit similar broadband UV absorption characteristics, the observed phenomena have implications for optical diagnostics and laser cooling schemes of relevance to studies of high-density Ps ensembles in insulating cavities, including the production of a Ps Bose-Einstein condensate

Electrostatically Guided Rydberg Positronium

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

Positronium decay from n=2 states in electric and magnetic fields

We report measurements and the results of calculations demonstrating that the annihilation dynamics of positronium (Ps) atoms can be controlled by Stark and Zeeman mixing of optically excited states. In the experiments a trap-based pulsed positron beam was employed to generate a dilute Ps gas with a density of ∼107 cm−3 using a porous silica target. These atoms were excited via 13S1→23PJ transitions in parallel electric and magnetic fields using a nanosecond pulsed dye laser, and Ps annihilation was measured using single-shot lifetime spectroscopy. The composition of the excited n=2 sublevels was controlled by varying the polarization of the excitation laser radiation and the strength of the electric and magnetic fields in the excitation region. The overall decay rates of the excited states can vary by a large amount, owing to the enormous differences between the annihilation and florescence lifetimes of the accessible field-free states. The energy-level structure, spectral intensities, and florescence and annihilation lifetimes in the presence of the fields were determined from the eigenvalues and eigenvectors of the complete n=2 Hamiltonian matrix in an |nSℓJMJ⟩ basis. Using these data as the input to a Monte Carlo model yielded calculated values which could be compared with experimentally measured quantities; qualitative agreement with the measurements was found. Varying the electric field in the presence of a weak parallel magnetic field provides control over the amount of level mixing that occurs, making it possible to increase or decrease the Ps lifetime. Field-controlled Ps decay can be used as an ionization-free detection method. Conversely, increasing the excited-state lifetime can potentially be exploited to optimize multistep excitation processes using mixed intermediate states. This will be useful either in minimizing losses through intermediate-state decay during excitation or by making it possible to separate excitation laser pulses in time. In addition, the adiabatic extraction of appropriate eigenstates from the electric field in which they are excited can, in principle, be used to prepare pure 23S1 atoms. The availability of atoms in these states produced via single-photon excitation will facilitate high-resolution microwave spectroscopy of the Ps n=2 fine structure

Measurement of Rydberg positronium fluorescence lifetimes

We report measurements of the fluorescence lifetimes of positronium (Ps) atoms with principal quantum numbers n=10–19. Ps atoms in Rydberg-Stark states were produced via a two-color two-step 1S3→2P3→nS3/nD3 excitation scheme and subsequently detected after traveling 1.2 m. The measured time-of-flight distributions were used to determine the mean lifetimes of the Rydberg levels, yielding values ranging from 3μs to 26μs. Our data are in accord with the expected radiative lifetimes of Rydberg-Stark states of Ps

Microwave spectroscopy of positronium atoms in free space

We report measurements of the positronium (Ps) 2 3 S 1 → 2 3 P 2 interval in which atoms traveling in free space were irradiated with microwave radiation generated using a horn antenna. Previous measurements of this transition, performed using atoms in waveguides, exhibited asymmetric and shifted line shapes. In the free-space measurements we report here, much smaller line-shape asymmetry was observed, but with large frequency shifts that varied with the orientation of the horn antenna. Our observations are supported by line-shape simulations and demonstrate that variations in the microwave radiation field distribution can perturb measured line shapes and give rise to apparent frequency shifts without necessarily causing large asymmetries; this effect can explain previous measurements in which an apparent discrepancy with predictions from quantum electrodynamics was ob- served [L. Gurung, T. J. Babij, S. D. Hogan, and D. B. Cassidy, Phys. Rev. Lett. 125, 073002 (2020)]

Observation of asymmetric line shapes in precision microwave spectroscopy of the positronium 2S13→2PJ3 ( J=1,2 ) fine-structure intervals

We report new measurements of the positronium (Ps) 2 3 S 1 → 2 3 P J fine-structure intervals, ν J ( J = 0 , 1 , 2 ). In the experiments, Ps atoms, optically excited to the radiatively metastable 2 3 S 1 level, flew through microwave radiation fields tuned to drive transitions to the short-lived 2 3 P J levels, which were detected via the time spectrum of subsequent ground-state Ps annihilation radiation. Both the ν 1 and ν 2 line shapes were found to be asymmetric, which, in the absence of a complete line-shape model, prevents accurate determination of these fine-structure intervals. Conversely, the ν 0 line shape did not exhibit any significant asymmetry; the observed interval, however, was found to disagree with QED theory by 4.2 standard deviations

Multiring electrostatic guide for Rydberg positronium

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

State-selective electric-field ionization of Rydberg positronium

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