PALS Investigation of an Unaged Epoxy Film

In the case of implantation depths below approx. 50 nm, Iq.ps is reduced - the lower the positron implantation depth, the lower is Iq-ps- This reduction of io Ps can happen if fewer free electrons are available for the ortho-positronium formation. This is the case for the shorter spur of the positron due to a lower kinetic energy or if free electrons diffuse to the surface. Additionally, this Io.p reduction can be interpreted in terms of an out-diffusion of ortho-positronium [19]. In addition to these arguments, which are specific to the PALS method itself, depth-de-pendent properties in the top 10 nm of the epoxy, such as different densities of electron acceptor groups, could also play their part in the observed decHne in Io.ps. 

Regarding the average lifetime of ortho-positronium to.ps for the unaged epoxy film on Au substrate in Fig. 29.2, Tq-ps is constant within the experimental scatter for positron implantation depths above approx. 20 nm. This reflects constant free-volume void sizes, independent of the sample depth. A shallow maximum around 150 nm and a decrease in x. p above 400 nm are not significant. On the other hand, for very low depths up to 20 nm, a significant increase in To-Ps is observed. This can be interpreted as the effect of a surface region in the order of less than 20 nm wide where the sizes of the free-volume voids increase [19]. However, a diffusion of ortho-positronium to the epoxy surface and into vacuum or an impediment of the positronium formation due to a reduced 

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