Positronium hydride in hydrogen-laden thermochemically reduced MgO single crystals

Thermochemical reduction of hydrogen-laden MgO single crystals at T{~}2400 K results in a large concentration of both hydride (H^-) ions and anion vacancies (>10²⁴ m^-3). Positron-lifetime experiments of these crystals provide evidence for bound positronium hydride states also referred to as [e⁺-H^-] 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 H^- concentration remains intact, two long-lived components appear in the lifetime spectrum. Furthermore, these two components correlate with the presence of the H^-ions. These results suggest the existence of bound [e⁺-H^-] states when positrons are trapped by the H^- ions, and the subsequent formation of positronium (Ps) states by the dissociation of the [e⁺-H^-] states. From the values of the intermediate lifetime component, a value of (570±{}50) ps is obtained for the lifetime of the PsH state located in an anion vacancy in MgO. The longest lifetime component {~}(1--3) ns is attributed to pick-off annihilation of ortho-Ps states.