Delocalized positronium in alkali-halide crystals: analysis of possible lattice-scattering processes

A theoretical analysis of possible lattice-scattering processes for delocal ized positronium atoms (Ps) in alkali-halide crystals is presented. Optic d eformation-potential scattering, unless forbidden by selection rules, is sh own to renormalize the acoustic deformation-potential constant of Ps thereb y dramatically broadening its momentum distribution with increasing tempera ture. This explains the anomalous temperature broadening of the momentum di stribution of Ps in crystalline MgF2 and its absence in alpha -SiO2 as rece ntly observed. An effect analogous to that in MgF2 is predicted for beta -S iO2 above the alpha-beta phase transition at 846 K. (C) 2001 Published by E lsevier Science B.V.