Positron annihilation in small molecules

Positron annihilation in room temperature samples of the molecular gases N2, O2, CO, N2O and CH4 has been studied in the density range below 10 amagat using the positron lifetime technique. Careful analyses of the density dependence of the free positron annihilation rates have been performed that have allowed the annihilation parameter, <Zeff>, to be extracted. We compare our results to those in the literature, and give recommended <Zeff> values from experiment. We have also synthesized the existing data for H2 and derived a recommended value for the annihilation parameter for this molecule