Abstract
A beam of positrons is used to study positronium (Ps) formation at the surfaces of clean, single-crystal targets. The Ps formation probability changes with incident energy $E$ as ${(\frac{E}{{E}_{0}}+1)}^{\ensuremath{-}1}$, with ${E}_{0}=915(30), 6000(150), \mathrm{and} 2900(150)$ eV for Si, Ge, and Al at 300 K. Since ${E}_{0}$ is observed to be temperature dependent, it is argued that Ps is not being formed by backscattering from the surface; rather, the energetic positrons thermalize in the bulk and diffuse to the surface where about half of the positrons escape as free Ps at 300 K.
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