Measurement of the 13S1→23S1 interval in positronium using field ionization of Rydberg states

We report a 40 parts per 109 measurement of the positronium 13S1→23S1 interval using pulsed two-photon optical spectroscopy. The transition is detected via field ionization of atoms excited from the 2S to the 20P Rydberg state. Precise Monte Carlo line-shape simulations allow for the accounting of effects such as Doppler and ac Stark shifts, while an optical heterodyne measurement of the excitation laser pulse is used to correct for laser frequency chirp. A value of 1233607210.5±49.6MHz is obtained. This scheme allows for the measurement of the velocity distribution of positronium atoms to correct for the second-order Doppler effect. This is a major source of systematic uncertainty expected for future measurements of this transition with a cw laser; thus, our technique paves the way toward a new generation of a high-precision determination of this interval in positronium. Published by the American Physical Society 2025