Temperature-activated transition of positronium from self-trapped to delocalized state in Ca F2

Temperature dependence of the momentum distribution of positronium (Ps) in Ca F2 single crystal has been investigated in a temperature range from 10 to 297 K. Temperature activated transition of Ps from the stable self-trapped state to the metastable delocalized state has been observed. The self-trapping barrier height and the under-barrier tunnel self-trapping rate have been determined to be 0.17 eV and 2.9× 1010 s-1, respectively, and an important role of the under-barrier tunneling has been demonstrated. The bottom of the delocalized Ps band is 6.6 meV higher in energy than the ground self-trapped state. The observed effective mass of the delocalized Ps is considerably larger than the free Ps mass in vacuum.