Abstract:
This paper presents a new analysis of the experimental transmission spectra of polyethylene
terephthalate (PET) films before and after irradiation with swift heavy ions (SHI) films, as reported
previously by the authors. It is shown that the absorption edge red shift for irradiated films contains
two regions of exponential form, one of which is located in the UV region and the other at lower
energy, mainly in the visible part of the spectrum. The behaviour of the transmission curves under
different irradiating fluences demonstrates that these two regions reflect respectively the electronenriched core of the latent track and its electron-depleted peripheral halo. The focal point method
yields a bandgap energy of 4.1 eV for the electron-enriched core of the latent track, which is similar to
n-doped semiconductors, and a bandgap of about 1.3–1.5 eV for the electron-depleted halo, similar to
p-doped semiconductors. The boundary between the latent track cores and halos corresponds to a
conventional semiconductor p-n junction. The values of the characteristic Urbach energy determined
from experimental data correspond to the nonradiative transition energy between the excited singlet
and triplet levels of benzene-carboxyl complexes in repeat units of the PET chain molecule. A parallel
is drawn between the SHI-induced redistribution of electrons held in structural traps in the PET
film and chemical redox reactions, which involve the redistribution of electrons in chemical bonds.
It is suggested that alkali etching triggers the release of excess electrons in the latent track cores,
which act as a catalyst for the fragmentation of PET chain molecules along the latent tracks of the
SHI irradiation.
