Positron lifetime studies in organic media

Lifetimes of positrons annihilating in a number of organic compounds consisting of aromatic hydrocarbons, substituted benzenes, phenols, anilines and ketones have been measured. Experimentally measured values of tau 2 and I2 are examined to determine whether positron annihilation takes place at a particular site of the molecule or whether it depends on average properties of the molecules. The intensity I2 seems to be related to the width of Ore gap in organic molecules; the tau 2 values in molecular liquids, however, can be interpreted in terms of additive atomic contributions to pick-off quenching. In substituted benzenes the results show changes due to substituent effects and these changes could be correlated with Hammett's sigma -constant. Results indicate that positron annihilation in organic media is determined by microscopic properties of the molecules such as ionization potentials (or width of Ore gap) and electron densities at the annihilation sites.