The behavior of positrons in crystalline and amorphous ice has been studied with a beam of monoenergetic positrons with incident energies 0–4.5 keV. Positronium (Ps) is formed in the bulk ice and diffuses until it annihilates or escapes from the surface. Measurements were carried out on the fraction of ortho-Ps leaving the surface and of the Doppler broadening of the 511-keV γ annihilation line. For incident energies 0–60 eV the Ps formation probability shows large variations. These variations are associated with Ps formation in the so-called Ore gaps and reflect the electronic structure of ice as demonstrated by Monte Carlo simulations of the positron slowing-down process. At higher energies, up to about 1 keV, the total Ps yield increases from about 50 to 75 %, which is attributed to Ps formation via spur processes. A large difference is found between the Ps diffusion coefficient in crystalline ice (about 0.2 ${\mathrm{cm}}^2$/sec) and in amorphous ice (roughly ${10}^{\mathrm{-}3}$ ${\mathrm{cm}}^2$/sec). From the red shift of the 511-keV annihilation line the Ps work function (affinity) in the crystalline ice is estimated to be -2±1 eV. Evidence for low-energy-positron diffraction in the crystalline ice is found with scattered intensities higher than 25%. Sputtering of the crystalline ice creates surface damage which strongly reduces the yield of Ps escaping the surface. Cavities of average diameter larger than about 17 Å are found in the as-grown amorphous ice. They anneal out at about 100 K, which is below the crystallization temperature of about 135 K.

• Received 2 May 1985

DOI:https://doi.org/10.1103/PhysRevB.32.7048