Study on the effect of atmospheric gases adsorbed in MnFe₂O₄/MCM-41 nanocomposite on ortho-positronium annihilation

In this paper, results of positron annihilation lifetime spectroscopy (PALS) studies of MnFe₂O₄/MCM-41 nanocomposites in N₂ and O₂ atmosphere have been presented. In particular, the influence of manganese ferrite loading and gas filling on pick-off ortho-positronium (o-Ps) annihilation processes in the investigated samples was a point of interest. Disappearance of the longest-lived o-Ps component with τ₅ present in the PAL spectrum of initial MCM-41 mesoporous material in the PAL spectra of MnFe₂O₄-impregnated MCM-41 measured in vacuum is a result of either a strong chemical o-Ps quenching or the Ps inhibition effects. The intensity I₄ of the medium-lived component initially increases, reaching a maximum value for the sample with minimum manganese ferrite content, and then decreases monotonically. Analogous dependence for the intensity I₃ of the shortest-lived component shows a maximum at higher MnFe₂O₄ content. Filling of open pores present in the studied nanocomposites by N₂ or O₂ at ambient pressure causes partial reappearance of the τ₄ and τ₅ components, except a sample with maximum ferrite content. The lifetimes of these components measured in O₂ are shortened in comparison to that observed in N₂ because of paramagnetic quenching. Anti-inhibition and anti-quenching effects of atmospheric gases observed in the MnFe₂O₄/MCM-41 samples are a result of neutralization of some surface active centers acting as inhibitors and weakening of pick-off annihilation mechanism, respectively.