Photomagnetic positronium-spin conversion in phosphorescent solids

Illumination affects the annihilation characteristics of positrons in positronium (Ps) forming phosphorescent substances, including photosynthetic materials in vivo. The excited triplet states $T$ of the phosphors can induce Ps-spin conversion. The cross-section ratio of para-to-ortho Ps conversion and ortho-to-para Ps conversion is measured to be 9 ± 2. Two conversion processes which conserve total spin are considered. Spin catalysis converts Ps spin by electron exchange, leaving $T$ unchanged. Spin flip converts Ps spin under the concomitant exothermic conversion of $T$ to the singlet ground state. The experiments decide that spin flip dominates photomagnetic Ps-spin conversion.