Semiempirical Approach to Positron Annihilation in Molecules

Anomalously large annihilation rates of positrons in binary collisions with molecules are interpreted as originating from the formation of virtual positronium accompanied by pick-off annihilation with one of the other molecular electrons. On this basis, a semiempirical ``universal'' formula is obtained which relates the annihilation rate to the molecular ionization energy in a manner similar to that originally discovered by Murphy and Surko [Phys. Rev. Lett. 67, 2954 (1991)]. This approach is found to lead to an explanation of the variation of Zeff over several orders of magnitude and to reproduce the correct general trends found among different molecular families.