We demonstrate the laser excitation of the $n=3$ state of positronium (Ps) in vacuum. A combination of a specially designed pulsed slow positron beam and a high-efficiency converter target was used to produce Ps. Its annihilation was recorded by single-shot positronium annihilation lifetime spectroscopy. Pulsed laser excitation of the $n=3$ level at a wavelength $\lambda \approx 205$ nm was monitored via Ps photoionization induced by a second intense laser pulse at $\lambda =1064$ nm. About 15% of the overall positronium emitted into vacuum was excited to $n=3$ and photoionized. Saturation of both the $n=3$ excitation and the following photoionization was observed and explained by a simple rate equation model. The positronium's transverse temperature was extracted by measuring the width of the Doppler-broadened absorption line. Moreover, excitation to Rydberg states $n=15$ and 16 using $n=3$ as the intermediate level was observed, giving an independent confirmation of excitation to the $3{}^{3}P$ state.

• Received 25 September 2015
• Revised 31 May 2016

DOI:https://doi.org/10.1103/PhysRevA.94.012507

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Published by the American Physical Society