Positronium hydride in hydrogen-laden thermochemically reduced MgO single crystals.

Abstract

Thermochemical reduction of hydrogen-laden MgO single crystals at {ital T}{approx}2400 K results in a large concentration of both hydride (H{sup {minus}}) ions and anion vacancies ({gt}10{sup 24} m{sup {minus}3}). Positron-lifetime experiments of these crystals provide evidence for bound positronium hydride states also referred to as ({ital e}{sup +}-H{sup {minus}}) 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{sup {minus}} concentration remains intact, two long-lived components appear in the lifetime spectrum. Furthermore, these two components correlate with the presence of the H{sup {minus}}ions. These results suggest the existence of bound ({ital e}{sup +}-H{sup {minus}}) states when positrons are trapped by the H{sup {minus}} ions, and the subsequent formation of positronium (Ps) states by the dissociation of the ({ital e}{sup +}-H{sup {minus}}) states. From the values of the intermediate lifetime component, a value of (570{plus minus}50) ps is obtained for the lifetime of the PsH state located in an anion vacancy in MgO. The longest lifetime component {approx}(1--3) ns is attributed to pick-off annihilation of ortho-Ps states.