Temperature dependence of positronium lifetime in cylindrical pores

Abstract

Positron Annihilation Lifetime Spectroscopy (PALS) results for ordered mesoporous silica sieves (MCM-41 and its fibre form MSF) are used to verify predictions of the Extended Tao-Eldrup (ETE) model. Cylindrical pores (R ~ 1-2 nm) in these materials have well defined geometry and size, thus the structure of MCM-41 and MSF reproduces the model assumptions. Positron lifetimes were measured in the range 100-500K. In MCM-41, due to low intensity of pore related ortho-positronium component, it is difficult to determine the component's lifetime precisely. Accuracy of the lifetime fitting is better for MSF, where temperature dependence is in a good agreement with the ETE model results. For comparison, the ETE model predictions fit the lifetime of the pore related component in the Vycor sample only for T > 200 K. Below this value the lifetime exceeds the model predictions by over 30%.1 Introduction The ortho-positronium (o-Ps) lifetime in dielectrics is related to the size of electronfree volume. In the case of mesoporous materials (pore size 2-50 nm) at room temperature, the relation is approximated by the extended version of the Tao-Eldrup model (ETE), which takes into account population of excited levels of Ps trapped in the potential well [1]. Pores are usually described as infinite long cylinders that should be accounted for by assuming proper geometry in the model calculations. The correctness of the pore size estimation based on the ETE model for cylindrical geometry is confirmed by experimental results for Vycor glasses and silica gels at room temperature [2]. In Vycor glasses and silica gels, pores form a complicated and usually interconnected structure. Describing the pore shape as infinite long cylinders in particular provides only a significant approximation. Although, such an assumption is sufficient when distinct differences between o-Ps lifetimes in pores of various sizes are investigated.In addition to the relationship between o-Ps lifetime and pore size, the ETE model predicts positron lifetime dependence on temperature. In order to verify the effects of temperature change on the lifetime, highly porous samples with well-defined pore shape and size are preferred. These requirements are fulfilled by mesoporous silica sieves, designated as M41S [3]. An interesting member of this group is MCM-41, which has a hexagonal arrangement of uni-sized cylindrical pores. Unfortunately, the pores in MCM-41 are short (sizes of the porous grains are tens to hundreds of nanometer), resulting in Ps escape from the pores and low intensity of the pore-related lifetime component [2]. More suitable materials are mesoporous silica fibres (MSF), where similar to MCM-41, a honeycomb-like structure of the pores forms long rods which have a diameter of a few micrometers [4]. Moreover, it is possible to produce

Influence of Atmospheric Gases Present in the Pores of MCM-41 on Lifetime of Ortho-Positronium

“…Presence of processes influencing o-Ps lifetimes and dependent on gas pressure, but not taken into account in the model. Such a process is escape of o-Ps from the primary pores to the interparticle spaces [18]. Its presence in the MCM-41 sample produces an inconsistency in the ratios between total volume of the primary pores and the interparticle spaces R v = V p /V s calculated from the PALS data measured for the sample in vacuum (R v = 0.063 ± 0.001), air (R v = 0.285 ± 0.004) and oxygen (R v = 1.14 ± 0.01).…”

Positronium Thermalization Process in Nanoporous Materials and its Influence on the Shape of PALS Spectrum

“…The syntheses of MCM-41 and MSF are detailed in paper [7]. The pore radii (R BJH ) were calculated using the Barrett-Joyner-Halenda method.…”