Does the MELT program accurately reveal the lifetime distribution in polymers? III. Effects due to an e+ lifetime distribution

In previous studies we have investigated artefacts which appear in analysed parameters of positron lifetime spectra containing distributed o-Ps lifetimes. Recently, we discovered also that the disagreements between the experimental positronium lifetime parameters and theoretical expectations, I₁/I₃ > I_p-Ps/I_o-Ps = 1/3 and τ₁ > τ_p-Ps, may be well understood by assuming a distribution of e⁺ lifetimes in addition to an o-Ps lifetime distribution. In this paper we give further experimental evidence for these deviations by taking positron lifetime spectra of high statistical accuracy for various amorphous polymers. Furthermore, a systematic study of the influence of the widths of the e⁺ and o-Ps lifetime distributions on the analysed lifetime parameters will be presented. The analysis of computer-generated spectra showed that all of the analysed lifetimes τ₁, τ₂ and τ₃ increases artificially with increasing width of the e⁺ and o-Ps lifetime distributions. The intensity I₁ increases also, while I₂ and I₃ decrease. The deviations of I₁/I₃ and τ₁ from the theoretical expectations are mainly controlled by the width of the e⁺ lifetime distribution. Suggestions for suitable constraints in the discrete-term analysis in order to get reliable lifetime parameters are also made.