Positronium formation and diffusion in the rare-gas solids

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Abstract
The energy distributions of positronium emitted by the solid rare gases Ar, Kr and Xe following positron implantation have been measured. The positrons were implanted with a monoenergetic pulsed beam of incident energies ranging from 0.1 to 2 keV. The energy distributions of the positronium emitted from Ar and Kr are asymmetric, with high-energy tails. At high incident positron energies the high-energy tail is reduced and the authors use a model of hot positronium losing energy as it diffuses to the surface. The emitted distributions are largely well described by the Ore formation model, although the form of the energy distribution from Xe is not as easily interpreted. From Xe, a distribution having roughly the shape predicted by the Ore model is accompanied by a considerable amount of emission at energies below the positronium work function. Increased positronium energy loss with increasing incident positron energy is observed for all the samples, and comparison of the observed positronium emission energy spectra with an energy-dependent diffusion model yields an estimate of the deformation potential parameter for the interaction of positronium atoms with the phonons of the solid rare gases.