Correlation of Triplet Positronium Annihilation Parameters with Structural and Electronic Properties of Organic Liquids

The parameters of triplet positronium annihilation in 193 organic liquids have been measured. The annihilation quenching cross sections, ${\mathrm{〈\sigma \upsilon 〉}}_{\mathrm{A\; v}}{\text{\hspace{0.17em}=\hspace{0.17em}(1\hspace{0.17em}/\hspace{0.17em}τ}}_2\mathrm{)\; (M\hspace{0.17em}/\hspace{0.17em}\rho ),}$ is a linear function of the length of the molecule in homologous series of compounds. Partial quenching cross sections, calculated for 27 structural functional groups, are used to calculate annihilation cross sections. The importance of steric effects in the annihilation mechanism is demonstrated in the study of several isomeric systems. Triplet positronium intensities depend upon specific atoms or groups in the molecule, rather than any property of the molecule as a whole. A correlation is also shown between annihilation quenching cross sections and electron polarizabilities as measured from indices of refraction. Several systems which quenched the triplet positronium were examined, e.g., free radicals, quinones, and paramagnetic species.