Optical saturation of the 1³S-2³P transition in positronium

The authors report optical saturation of the 1³S-2³P transition in positronium using ultraviolet light from a frequency-doubled, pulsed dye laser. The increase in ground-state annihilation radiation due to magnetic triplet-singlet mixing in the excited state is used to monitor the transition. From the variation of the annihilation rate as a function of magnetic field, laser power, wavelength and polarisation, the authors verify that the transition is optically saturated, in agreement with model calculations of the system.