Annihilation Of Positronium Atoms Confined In Mesoporous And Macroporous Sio2 Films

We report experiments in which positronium (Ps) atoms were created in thin, porous silica films containing isolated voidswith diameters ranging from 5 to 75 nm. Ps lifetimes in the pore structures weremeasured directly via time-delayed laser excitation of 1(3)S(1) -> 2(3)P(J) transitions. In a film containing 5-nm pores Ps was predominantly emitted into vacuum, with a small component of confined Ps with a lifetime of 75 ns also observed. In films with larger pores Ps atoms were not emitted into vacuum except from the film surface, and confined Ps lifetimes of approximate to 90 ns were measured with no dependence on the pore size. However, for such large pores, extended Tao-Eldrup (ETE)-type models predict Ps lifetimes close to the 142-ns vacuum value. Moreover, 1(3)S(1) -> 2(3)P(J) excitation of Ps atoms inside the pores was found to result in annihilation and exhibited an extremely broad (approximate to 10 THz) linewidth. We attribute these observations to a process in which nonthermal Ps atoms in the isolated voids become temporarily trapped in a series of surface states that dissociate following excitation. The occurrence of this mechanism is not necessarily apparent from ground-state Ps decay rates without some prior knowledge of the sample structure, and it precludes the application of ETE-type models as they do not take into account surface interactions other than pickoff annihilation.