T. Goworek

Positronium states in the pores of silica gel

The lifetime spectra of positrons annihilating in silica gels with pore radii from 2 to 10 nm were measured, and the results compared with the predictions made by the Tao–Eldrup model extended to include the annihilation of positronium from the excited states in the potential well. The long-lived parts of the spectra were analysed as a sum of 1–3 components with continuous λ distributions (gaussian in the logarithmic scale). The 1p state population in Si40 is demonstrated; the decay constant of ortho-Ps in it agrees quite well with the model estimate and the growth of intensity with temperature resembles that expected from the Boltzmann distribution. In one-component fit the lifetimes at the maximum of λ distribution at a room temperature for various pore radii were found also to be very close to the calculated lifetime averaged over all states of the positronium particle in the rectangular potential well.

ORTHO-POSITRONIUM LIFETIMES IN NONSPHERICAL VOIDS

Experimental values of o-Ps lifetime were compared with model calculations, approximating the voids in a solid by spheres or cuboids. In the case of resorcinol the model of spherical voids is well consistent with the experimental data. For channel-like free volumes the formula, analogous to that for spheres, gives systematically the values of o-Ps lifetime too large. No positronium is found in anthracene, although the radius of free volume is larger than 0.97 Å at which the Ps binding energy should be zero.

Positronium lifetimes in porous Vycor glass

Abstract The ortho-positronium lifetimes in porous Vycor glass were measured in a wide range of pore radii. A good correlation of the most probable lifetime with the model accounting for the population of upper levels of a particle in the potential well was observed. For channel-like voids (capillaries in porous media) a new value of the empirical parameter, Δ R , entering in the semi-empirical Eldrup equation, is proposed: Δ R =(0.191±0.007) nm; it explains some of the discrepancies previously found in crystalline solids.

Positronium in large voids. Silicagel

Abstract The lifetime spectra of positrons annihilating in a porous medium (silicagel) were measured a function of temperature. In the low temperature limit the lifetime of o-Ps in the pores agrees rather well with the popular Tao-Eldrup formula. With the rise of temperature the second long-lived component grows; that component was identified as o-Ps annihilation from the 1p-like state in the potential well. The lifetimes of o-Ps in various states of the spherical well have been calculated. The experimental value of the lifetime for the 1p state is in good agreement withe model calculation.

Vacuum removal of the template in MCM-41 silica studied by the positron annihilation method.

Positron annihilation lifetime spectra were measured for ordered silica before template (octadecyltrimethyl ammonium) removal. An intermediate lifetime component, well pronounced in the spectra, indicates the presence of structural defects in the template being the size of a surfactant molecule. At the temperature near 420 K a radical change of template structure occurs, consisting in appearance of disorder and the degradation of template; evaporation of the fragments begins. It seems that moderate heating (up to about 500 K only) in vacuum for several hours is an effective and simple method of template removal, leaving no pure carbon residue in the pore structure. The results are compared with the data from the liquid nitrogen adsorption/desorption method and elemental analysis, confirming the positron annihilation results.

Temperature variations of average o-Ps lifetime in porous media

Abstract Modification of the Tao–Eldrup model is proposed in order to extend its usefulness to the case of porous media. The modification consists in the transition from spherical to capillary geometry and in inclusion of pick-off annihilation from the excited states of a particle in the well. Approximated equations for pick-off constant in these states are given. The model was tested by observing the temperature dependences of o-Ps lifetime in various media. In the case of silica gels and Vycor glass with narrow pores, the model seems to work well, while for larger pores in Vycor unexpectedly long lifetimes appear in the range of lowest temperatures.

On possible deviations of experimental PALS data from positronium pick-off model estimates

Abstract The sources of possible inconsistency between the models of trapped positronium annihilation and experimental data are discussed. There are two such sources: excessive simplifications in the models of pick-off process and appearance of other processes not accounted in the models. In the case of the smallest traps, the penetration depth of ortho-positronium (o-Ps) wavefunction into the bulk can be larger than usually assumed. In traps of sizes exceeding 1 nm the population of excited levels in the well has to be taken into account. For such large voids the sources of divergence of the experiment and model predictions can be: escape of o-Ps from open pores to outside, chemical interaction of o-Ps with pore surfaces (adsorbate layers), the presence of impurities inside free volumes.

Temperature changes of the template structure in MCM-41 type materials; positron annihilation studies

Abstract The lifetime spectra of positrons annihilating in MCM-41 ordered silica were measured as a function of temperature. It was found that in alkyltrimethylammonium templates degradation in vacuum begins at 380–400 K, that consists in breaking the long micelle cylinders, rise of gaps between the fragments and finally pore evacuation. The latter process is not completed and particularly in C12TAB the spectrum component belonging to fragmented template preserves its high intensity up to 520 K (the highest temperature in our experiment). The pore surface area over 1000 m 2 /g is achieved already at temperatures about 490 K. The lifetime of the longest-lived component indicates that practically all ortho -positronium atoms leave the pores before annihilation.

Positron probing of the micellar template interior in MCM-41

The properties of positronium annihilating in the micellar template in MCM-41 materials were investigated. For alkyltrimethylammonium bromide surfactants (Cn=14,16,18) the template interior is liquid-like; the bubble model of positronium, elaborated for bulk liquids, can be applied. The equivalent surface tension is linearly dependent on temperature, like in classic liquids. A very long-lived component in o-Ps lifetime spectrum (≈20ns) can be ascribed to o-Ps in cracks in the elongated template structure.

Temperature effects in positronium formation: Solid aromatic hydrocarbons

Abstract The temperature dependence of the probability of positronium formation was investigated in a series of solid aromatic hydrocarbons. The results are dis