Positronium reactivities toward NO2 and SO2 molecules adsorbed in porous media

The positronium annihilation lifetime technique has been applied to study the positronium chemical reactivities toward NO2 and SO2 in three porous media, namely XAD4, XAD7, and zeolite Y. Three positron lifetimes have been observed in these systems. Two long-lived components are attributed to positronium annihilation in the microvoids and on the surface of the media and a short-lived component is attributed to positron annihilation in the bulk. The observed chemical reactivities between positronium atoms and gas molecules in microvoids are found to be a few orders of magnitude less than those reported in free gas. The chemical reactivities on the surface between positronium atoms and adsorbed molecules of the surface, which are in anionic forms, are observed for the first time. The observed reactivities are found to be higher than those in the microvoids and comparable to those in solutions. Relationships among positronium reactivities and microproperties of porous materials and of adsorbed molecules are discussed. 28 references, 4 figures, 5 tables