Normal and anomalous positronium states in ionic and molecular solids investigated via magnetic field effects

Abstract

An external magnetic field (B) is applied in positron lifetime spectroscopy and Doppler broadening (DBARL) experiments to derive information on the positronium (Ps) state in solid matrices. The solids investigated are thorium phosphate and a molecular matrix, p-terphenyl, doped with either 0.5% anthracene or 1.5% Chrysene to produce extrinsic defects of sufficient concentration and size to allow Ps formation. In thorium phosphate at zero field, the triplet Ps (o-Ps) pick-off lifetime is short (700 ps), reflecting annihilation in rather small (0.105 nm) free volumes of the lattice. This lifetime rises to 1160 and 1440 ps in p-terphenyl doped with pyrene and anthracene, respectively, reflecting annihilation in microvoids of 0.19–0.23 nm radius, which corresponds roughly to the size of a naphthalene molecule. The variations of the parameter R and of the o-Ps (m= 0) magnetic substrate lifetime with B in thorium phosphate are ‘anomalous’, requiring the use of two fitting parameters. The first parameter is the contact density affecting the hyperfine splitting, with a low value (η= 0.22) denoting an expanded Ps wavefunction. For the second fitting parameter, two possibilities exist; either a contact density affecting the Ps singlet state (p-Ps) lifetime, η′= 0.36, distinct from η, or a p-Ps pick-off lifetime (390 ps) different from that of o-Ps (700 ps). Analysis of the DBARL data shows that the second hypothesis is unlikely. By contrast, the magnetic field effects in doped p-terphenyl are ‘normal’, with a single fitting parameter, η= 0.80 and 0.83, for anthracene and pyrene as dopants, respectively.