Interactions between positronium atoms in porous silica

Interactions between pairs of positronium (Ps) atoms confined in porous silica films have been directly observed for the first time. Because of selection rules, the nature of such interactions should depend on the structure of the porous medium: if a Ps surface state exists, dipositronium (Ps(2)) molecules may be created, and if there is a continuum of cavity energy levels, spin exchanging collisions may occur. Using two structurally different silica films, we have been able to isolate and study these two processes. Our data indicate that Ps(2) formation occurs primarily via a Langmuir-Hinshelwood-type mechanism on the internal pore surfaces, with an interaction length of the order of 7x10(-8) cm, and that the effective cross section for nonthermalized Ps-Ps spin exchange quenching in porous silica is around 9x10(-15) cm(2).