On the role of umklapp processes in the scattering of delocalized positronium by acoustic phonons in ionic crystals

The effect of umklapp processes in the scattering of delocalized positroniu m atoms by acoustic phonons in ionic crystals is analyzed. It is shown that at temperatures above which this effect becomes large, umklapp scattering renormalizes the constant of the deformation potential of positronium. This renormalization has recently been observed in experiments measuring the mo mentum distribution of delocalized positronium in a MgF2 crystal. (C) 1999 American Institute of Physics.