Positronium Formation as a Probe of Polymer Surfaces and Thin Films

The lifetime, ${\tau }_3$, 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 Å. Thin film measurements indicate that the mean probe depth can be much smaller, given mainly by the average positron implantation distance, $\overline{z}\left(E\right)$. Surface-induced changes in the void size and glass transition temperature of polystyrene are searched for at $\overline{z}\approx 40Å$.