Positronium states in the pores of silica gel

The lifetime spectra of positrons annihilating in silica gels with pore radii from 2 to 10 nm were measured, and the results compared with the predictions made by the Tao-Eldrup model extended to include the annihilation of positronium from the excited states in the potential well. The long-lived parts of the spectra were analysed as a sum of 1-3 components with continuous lambda distributions (gaussian in the logarithmic scale). The Ip state population in Si40 is demonstrated; the decay constant of ortho-Ps in it agrees quite well with the model estimate and the growth of intensity with temperature resembles that expected from the Boltzmann distribution. In one-component fit the lifetimes at the maximum of lambda distribution at a room temperature for various pore radii were found also to be very close to the calculated lifetime averaged over all states of the positronium particle in the rectangular potential well. (C) 1998 Elsevier Science B.V. AII rights reserved.