Positron annihilation lifetime in mesoporous silica MCM-41 at different vacuum levels

Abstract

Positron annihilation lifetime spectra of MCM-41 and zeolite Y were measured at different vacuum levels. When the experiments were carried out in air, a very long lifetime component (τ4 = 35-45 ns, I4 = 15-20%) was observed for MCM-41, while the longest lifetime for zeolite Y was only 2-4 ns with an intensity of 15-25%. However, when the experiments were carried out in vacuum, the very long lifetime components could be observed for both samples, although with different intensities, ~30% for MCM-41 and ~10% for zeolite Y. For MCM-41 in air, the longest lifetime (τ4) is ~42 ns, corresponding to the ortho-positronium (o-Ps) annihilation lifetime in MCM-41 cavities. This value is slightly longer but very close to the value of 39 ns, which was estimated by using a bouncing quantum particle model. These peculiar positron annihilation characteristics were explained by air quenching mechanism of o-Ps annihilation in MCM-41. It was suggested that because of the existence of the very long lifetime component even in air, positron lifetime spectroscopy could be a very useful tool for nondestructive measurement of the cavity size of mesoporous solids such as MCM-41.