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Physical Review Letters

Excitation of the Positronium 1S13→2S13 Two-Photon Transition

Steven Chu, Allen Paine Mills

Published May 10, 1982• 103 citations
Publisher

Abstract

The $1^{3}S_{1}\ensuremath{\rightarrow}2^{3}S_{1}$ transition in positronium has been observed using two-photon Doppler-free excitation. Transitions were induced by a pulsed dye laser, and detected by photoionization of the $2^{3}S_{1}$ state. The resonance signal has a linewidth of 1.5 GHz and a signal-to-noise ratio of 20:1. The frequency agrees with theory to 0.5 GHz. The resonance signal has an amplitude proportional to the square of the laser intensity and displays the expected Stark shift to higher frequency under an applied electric field.

DOI
10.1103/physrevlett.48.1333

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We have used two-photon absorption from counterpropagating laser beams of different frequencies to measure Stark shifts and splittings of the 62S6^{2}S62S, 72S7^{2}S72S, 82S8^{2}S82S, 52D5^{2}D52D, and 62D6^{2}D62D states in sodium. Beams from a fixed-frequency, single-mode Ar+{\mathrm{Ar}}^{+}Ar+ laser and a tunable, single-frequency cw dye laser were focussed into a coarse sodium atomic beam and the fluorescence was monitored. The collimation of the atomic beam reduced the residual Doppler width inherent in using two different frequencies. Using the two lasers allowed two-photon transitions to be observed at frequencies unattainable with the dye laser alone, and enhanced the transition rates due to the near resonance of the dye laser with the strong Na $3^{2}S\ensuremath{\rightarrow}3^{2}P$ transition. Our experimental results for the scalar and tensor polarizabilities, ${\ensuremath{\alpha}}_{0}$ and ${\ensuremath{\alpha}}_{2}$, agree with calculations in the Bates and Damgaard Coulomb approximation within 2% for ${\ensuremath{\alpha}}_{0}$ and 9% for ${\ensuremath{\alpha}}_{2}$. As a by-product the hyperfine-interaction constants aaa for the SSS states and the fine-structure intervals $\ensuremath{\delta}\ensuremath{\nu}=\frac{({E}_{\frac{3}{2}}\ensuremath{-}{E}_{\frac{5}{2}})}{h}$ for the DDD states were obtained.
Fine structure transitions in the first excited state of positronium (Ps) have been measured using 'Backscatter Ps' production on a Mo surface by observation of a change in the emitted Lyman- alpha intensity under resonant microwave irradiation. Production, fine structure transitions and Lyman- alpha decay of the Ps atoms took place inside a waveguide designed to transmit the microwave frequencies of 8.6, 13.0 and 18.5 GHz for the transitions from the 2 3 S 1 state to the 2 3 PJ, J=2, 1, 0, states, respectively. In the presence of a magnetic field, all transitions observed show a shift to higher frequencies, compared with earlier calculations and measurements in zero magnetic field. The deviations exceed the expected Zeeman shift significantly but may be explained by assuming a motional Stark effect as predicted by Lewis and Hughes for Ps with kinetic energies of several eV.
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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.
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