Annihilation of positronium atoms confined in mesoporous and macroporous SiO$_2$ films

Abstract : We report experiments in which positronium (Ps) atoms were created in thin, porous silica films containing isolated voids with diameters ranging from 5 to 75 nm. Ps lifetimes in the pore structures were measured directly via time-delayed laser excitation of 1$^3$S$_1$→2$^3$P$_J$ transitions. In a film containing 5-nm pores Ps was predominantly emitted into vacuum, with a small component of confined Ps with a lifetime of 75 ns also observed. In films with larger pores Ps atoms were not emitted into vacuum except from the film surface, and confined Ps lifetimes of ≈90 ns were measured with no dependence on the pore size. However, for such large pores, extended Tao-Eldrup (ETE)-type models predict Ps lifetimes close to the 142-ns vacuum value. Moreover, 1$^3$S$_1$→2$^3$P$_J$ excitation of Ps atoms inside the pores was found to result in annihilation and exhibited an extremely broad (≈10 THz) linewidth. We attribute these observations to a process in which nonthermal Ps atoms in the isolated voids become temporarily trapped in a series of surface states that dissociate following excitation. The occurrence of this mechanism is not necessarily apparent from ground-state Ps decay rates without some prior knowledge of the sample structure, and it precludes the application of ETE-type models as they do not take into account surface interactions other than pickoff annihilation.
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Article dans une revue
Phys.Rev.B, 2018, 97, pp.205302. 〈10.1103/PhysRevB.97.205302〉
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https://hal.archives-ouvertes.fr/hal-01815109
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Soumis le : mercredi 13 juin 2018 - 18:23:26
Dernière modification le : vendredi 15 juin 2018 - 01:12:37

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B.S. Cooper, J.P. Boilot, C. Corbel, F. Guillemot, L. Gurung, et al.. Annihilation of positronium atoms confined in mesoporous and macroporous SiO$_2$ films. Phys.Rev.B, 2018, 97, pp.205302. 〈10.1103/PhysRevB.97.205302〉. 〈hal-01815109〉

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