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

Positron annihilation lifetime spectra of MCM-41 and zeolite Y were measure d at different vacuum levels. When the experiments were carried out in air, a very long lifetime component (tau (4) = 35-45 ns, I-4 = 15-20%) was obse rved for MCM-41, while the longest lifetime for zeolite Y was only 2-4 ns w ith an intensity of 15-25%. However, when the experiments were carried out in vacuum, the very long lifetime components could be observed for both sam ples, although with different intensities, similar to 30% for MCM-41 and si milar to 10% for zeolite Y. For MCM-41 in air, the longest lifetime (tau (4 )) is similar to 42 ns, corresponding to the ortho-positronium (o-Ps) annih ilation 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 quant um particle model. These peculiar positron annihilation characteristics wer e explained by air quenching mechanism of o-Ps annihilation in MCM-41. It w as suggested that because of the existence of the very long lifetime compon ent even in air, positron lifetime spectroscopy could be a very useful tool for nondestructive measurement of the cavity size of mesoporous solids suc h as MCM-41.