Positron lifetimes in molecular crystals

Abstract

Positron lifetime measurements were carried out on molecular crystals of C60, TTF-TCNQ, and (BEDT-TTF)2Cu(NCS)2 utilizing a pulsed variable-energy positron beam. Each lifetime spectrum is well described by a single component. Their values are 390, 338, and 357 ps, respectively. In addition, positron lifetimes were calculated theoretically. The electronic density was obtained by the superposed-neutral-atom model or quantum-chemical molecular-orbital calculations with various basis sets. The lifetime enhancement due to the positron–electron correlation was taken into account through an enhancement factor as a function of the high-frequency dielectric constant in the local density approximation or a gradient correction in the generalized gradient approximation. The results are compared with the measured lifetimes.