Positrons in cyclohexane. Photoionization of positronium in solid phase?

Highlights: • In the rigid phase the majority of positronium is formed with trapped electrons. • The light of λ < 880 nm shortens o-Ps lifetime in rigid phase by photoionization. • At the lifetimes ≈1 ns the Tao–Eldrup model underestimates the void sizes. - Abstract: The ortho-positronium (o-Ps) lifetime and intensity in cyclohexane was measured as a function of temperature at various wavelengths of illuminating light. Accommodation of excess electrons in traps was observed in whole range of rigid crystalline phase; the trap depth was about 0.75 eV. At the wavelength λ < 880 nm a slight decrease of o-Ps lifetime in rigid phase was observed, which could be explained by photoionization of Ps atoms. The binding energy of hydrogenlike Ps atom confined in a void is smaller than that of free atom. The lifetime of o-Ps in plastic phase corresponds to that expected for positronium located in vacancies. The radii of voids obtained from the popular Tao–Eldrup model seem to be too small for the lifetimes of order of 1 ns. Oxygen from surrounding atmosphere begins to penetrate the cyclohexane in plastic phase already 15 K below the melting point