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Europhysics Letters (EPL)

Measurement of the annihilation decay rate of 2 3 S 1 positronium

R. E. Sheldon, Tamara J. Babij, Brendan Devlin-Hill, L. Gurung, D. B. Cassidy

Published Nov 3, 2020• 3 citations
Publisher

Abstract

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 .

DOI
10.1209/0295-5075/132/13001

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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 .
The fluorescence decay rate of 2 P J 3 positronium (Ps) atoms [ Γ fl ] has been experimentally determined using a Stark-mixing technique. Ground-state Ps atoms were optically excited in an electric field to states containing both 2 P J 3 and 2 3 S 1 components. The relative state populations were controlled via an applied mixing electric field, and a larger, pulsed, electric field was used to induce rapid quenching of the system after a fixed time delay. The dependence of the quenching signal on the mixing field strength was used to determine the relative and populations of the mixed states, whose decay rates are determined by the intrinsic self-annihilation and fluorescence decay rates. The measurements were compared to a trajectory simulation and Stark-mixing model, using the fluorescence decay rate as a free parameter. By finding the best agreement between the data and simulations we obtain Γ fl = 299 ± 25 µ s − 1 , which is consistent with the mean theoretical value of 313.343 ± 0.003 µ s − 1 . Published by the American Physical Society 2024
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 $\ensuremath{\gamma}$ 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.
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 measurements of the fluorescence lifetimes of positronium (Ps) atoms with principal quantum numbers n=10−−19n=10--19n=10−−19. Ps atoms in Rydberg-Stark states were produced via a two-color two-step $1\phantom{\rule{0.16em}{0ex}}^{3}S\ensuremath{\rightarrow}2\phantom{\rule{0.16em}{0ex}}^{3}P\ensuremath{\rightarrow}n\phantom{\rule{0.16em}{0ex}}^{3}S/n\phantom{\rule{0.16em}{0ex}}^{3}D$ 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 $3\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{s}$ to $26\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{s}$. Our data are in accord with the expected radiative lifetimes of Rydberg-Stark states of Ps.
The 301.5 nm electric-field-induced quench radiation on a thermal beam of neutral Bi atoms in the metastable 6p3 2Ρ3/2 state for transition to the ground 6p 3 4 S3 /2 state was observed.The measurement of the Stark-enhanced intensity of the radiation as a function of the electric field yields a relationship between the field-free-decay rates for transitions between the metastable 6p 3 2P3/2 state and the nearby states that are mixed in by the field.Using the known decay rates, the measurement sets an approximate transition rate of A(6p3 2 Ρ3/2 → 6p2 7s 4 Ρ112 ) (0.50 ± 0.05) s -1 .
The authors report optical saturation of the 1 3 S-2 3 P transition in positronium using ultraviolet light from a frequency-doubled, pulsed dye laser. The increase in ground-state annihilation radiation due to magnetic triplet-singlet mixing in the excited state is used to monitor the transition. From the variation of the annihilation rate as a function of magnetic field, laser power, wavelength and polarisation, the authors verify that the transition is optically saturated, in agreement with model calculations of the system.