Fine-Structure Measurement in the First Excited State of Positronium

The 2³S₁-2³P₂ 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-α emission. We obtain 8628.4±{}2.8 MHz, which is within 2 standard deviations of the theoretical value computed to order α³R by Fulton and Martin.