Positron Lifetime Spectra in Molecular Gases

Positron lifetime spectra have been measured in a wide variety of molecular gases at pressures up to about thirty atmospheres. The spectra were analyzed to yield the free-position annihilation rate (λₑ⁺) and triplet-positronium quenching rate (λq) at unit pressure. Attachment of the free positrons occurs in many of the gases. For the others, a strong correlation between λₑ⁺ and α, the electric-dipole polarizability at optical frequencies, is found, leading to a general relationship (for both molecular and atomic gases) between these two quantities. This is apparently the first experimental evidence that long-range dipole distortion is the predominant mechanism in low-energy positron scattering.