Abstract

Positronium is the quasi-stable bound system consisting of an electron and its anti-particle, the positron. Its energy levels can be explained to a high degree of accuracy by the electromagnetic interaction, affording an ideal test of the quantum electrodynamic (QED) theory of bound systems. In this pursuit, we have measured the 1³S₁–2³S₁ interval in positronium by Doppler-free two-photon spectroscopy to be (1 233 607 216.4 ± 3.2 MHz). We use continuous-wave (cw) excitation to eliminate the problems inherent in pulsed laser measurements of nonlinear transitions. Positronium atoms (Ps) generated in vacuum are excited to the 25 state by using cw laser light built up to 2.5 kW circulating power in a resonant Fabry-Perot cavity. The excited- state atoms are photoionized by using a pulsed laser at 532 nm, and the liberated positrons are counted as the cw laser is tuned relative to a reference line in telluriun (Te₂) molecular vapor.

© 1993 Optical Society of America