Abstract
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 \ifmmode\pm\else\textpm\fi{} 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.
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