Positronium hydride in hydrogen-laden thermochemically reduced MgO single crystals

Abstract

Thermochemical reduction of hydrogen-laden MgO single crystals at T\ensuremath{\sim}2400 K results in a large concentration of both hydride (${\mathrm{H}}^{\mathrm{\ensuremath{-}}}$) ions and anion vacancies (>${10}^{24}$ ${\mathrm{m}}^{\mathrm{\ensuremath{-}}3}$). Positron-lifetime experiments of these crystals provide evidence for bound positronium hydride states also referred to as [${\mathit{e}}^{+}$-${\mathrm{H}}^{\mathrm{\ensuremath{-}}}$] or PsH states. The presence of the anion vacancies was found to inhibit the formation of these states. After thermally annealing out these vacancies, such that ${\mathrm{H}}^{\mathrm{\ensuremath{-}}}$ concentration remains intact, two long-lived components appear in the lifetime spectrum. Furthermore, these two components correlate with the presence of the ${\mathrm{H}}^{\mathrm{\ensuremath{-}}}$ions. These results suggest the existence of bound [${\mathit{e}}^{+}$-${\mathrm{H}}^{\mathrm{\ensuremath{-}}}$] states when positrons are trapped by the ${\mathrm{H}}^{\mathrm{\ensuremath{-}}}$ ions, and the subsequent formation of positronium (Ps) states by the dissociation of the [${\mathit{e}}^{+}$-${\mathrm{H}}^{\mathrm{\ensuremath{-}}}$] states. From the values of the intermediate lifetime component, a value of (570\ifmmode\pm\else\textpm\fi{}50) ps is obtained for the lifetime of the PsH state located in an anion vacancy in MgO. The longest lifetime component \ensuremath{\sim}(1--3) ns is attributed to pick-off annihilation of ortho-Ps states.