Positronium Formation as a Probe of Polymer Surfaces and Thin Films

Abstract

The lifetime, ${\ensuremath{\tau}}_{3}$, and formation fraction, ${I}_{3}$, of triplet positronium decaying in the void volume near a polymer surface are measured versus the positron implantation energy, $E$. The strong $E$ dependence of ${I}_{3}$ supports a spur-electron capture model of Ps formation with deduced spur sizes ranging from 200 to 660 \AA{}. Thin film measurements indicate that the mean probe depth can be much smaller, given mainly by the average positron implantation distance, $\overline{z}(E)$. Surface-induced changes in the void size and glass transition temperature of polystyrene are searched for at $\overline{z}\ensuremath{\approx}40\AA{}$.