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Journal of Physics B Atomic Molecular and Optical Physics

Optical saturation of the 1 3 S-2 3 P transition in positronium

Klaus P. Ziock, CHARLES D. DERMER, Richard H. Howell, F. Magnotta, K. M. Jones

Published Jan 28, 1990• 34 citations
Publisher

Abstract

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.

DOI
10.1088/0953-4075/23/2/015

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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.
We report optical excitation of the 1{sup 3}S-2{sup 3}P transition in positronium, and a second excitation from n=2 to higher n states. The experiment used light from two pulsed dye lasers. Changes in the positronium annihilation rate during and after the laser pulse were used to deduce the excited state populations. We found that we could saturate the n=2 level and excite a substantial fraction of n=2 positronium to higher levels. Preliminary spectroscopic measurements were performed on n=14 and n=15 positronium. 14 refs., 7 figs.
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 .
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.
Zeeman mixing of singlet and triplet 2P states of positronium (Ps) atoms, followed by decay back to the ground state, can effectively turn a long-lived triplet atom into a short-lived singlet state, which would seem to preclude laser cooling of Ps in a magnetic field. Here we report experiments which show that, in fact, because of the large splitting of the n=2 states in a high magnetic field (the Paschen-Back regime), the amount of such mixing diminishes approximately exponentially with an increasing magnetic field >0.01 T and is essentially eliminated above ∼2 T. Thus, laser cooling of Ps should be feasible at high fields, which will facilitate the production of a Ps Bose-Einstein condensate.
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 on laser cooling of a large fraction of positronium (Ps) in free flight by strongly saturating the 1^{3}S-2^{3}P transition with a broadband, long-pulsed 243 nm alexandrite laser. The ground state Ps cloud is produced in a magnetic and electric field-free environment. We observe two different laser-induced effects. The first effect is an increase in the number of atoms in the ground state after the time Ps has spent in the long-lived 2^{3}P states. The second effect is one-dimensional Doppler cooling of Ps, reducing the cloud's temperature from 380(20) to 170(20) K. We demonstrate a 58(9)% increase in the fraction of Ps atoms with v_{1D}<3.7×10^{4} ms^{-1}.