Fine-Structure Measurement in the First Excited State of Positronium

The 2^3S₁-2^3P₂ energy splitting of positronium has been observed by inducing rf transitions in a weak magnetic field (50 G) in the absence of a buffer gas and observing a resonant increase in the Lyman- emission. We obtain 8628. 42. 8 MHz, which is within 2 standard deviations of the theoretical value computed to order ^3R by Fulton and Martin.