Positron Annihilation in the Normal Aliphatic Hydrocarbons

The lifetimes and intensities of triplet positronium have been measured in 15 normal aliphatic hydrocarbons. The lifetime of the triplet state in the deoxygenated samples decreases from 4.63±0.06 nsec for butane, C4H10, to 3.19±0.06 nsec for octadecane, C18H38. The intensities of the triplet positronium components are less variable; butane has the highest intensity, 38.4%±0.8%, and tetradecane, C14H30, the lowest, 29.6%±1.2%. The cross section for triplet-state quenching, 〈σv〉Av=(M/ρN0)(τ2 )−1, is shown to have a linear dependence on the length of the hydrocarbon chain. Partial cross sections for quenching by the component groups of the hydrocarbons, the methyl and methylene groups, have values of 0.895 and 0.965×10−14 cm3/sec, respectively. Back calculations of 〈σv〉Av using partial quenching cross sections lead to an average error of 0.56% in 〈σv〉Av for the 15 hydrocarbons. The largest back-calculated error is 1.92% for butane.