Positronium Formation and Diffusion in a Molecular Solid Studied with Variable-Energy Positrons

Crystalline and amorphous ice were studied with monoenergetic positrons with incident energies $0<E<\mathrm{}5\mathrm{}$ keV. Positronium (Ps) forms in the ice and diffuses as a neutral particle until annihilation or escape from the surface. Measurement of the fraction of $o$-Ps leaving the surface reveals two distinct contributions to the total Ps-formation probability (both Ore- and spur-type) as well as the Ps diffusion coefficient, 0.17±0.09 ${\mathrm{cm}}^2$/sec in crystalline ice. In addition, positronium trapping is observed in sputtered (crystalline) ice and voids (≳ 17 Å diameter) in amorphous ice.