Abstract:
This paper presents the results of a study of polyethylene terephthalate (PET) films irradiated with Ar and Kr ions at both normal orientation and an angle of 40◦
to the normal. Normal
irradiation was performed using Ar8+ and Kr15+ ions with an energy of 1.75 MeV/au and fluences
in the range (2–500) × 1010 cm−2
for Ar8+ ions and (1.6 − 6.5) × 1010 cm−2
for Kr15+ ions. Kr
ions with an energy of 1.2 MeV/au and charges of 13+
, 14+
, and 15+ were used for angled irradiation. For each Kr ion charge value, three fluence values were used: 5 × 1010, 1 × 1011, and
2.5 × 1011 cm−2
. It is well known that irradiation of PET films by swift heavy ions results in a red
shift of the UV-vis transmission spectra absorption edge. The experimental transmission spectra
exhibit well-defined interference fringes, which obscure the underlying transmission response. Using
an existing technique to obtain interference-free transmission curves Tα(λ) for both pristine and
irradiated PET film samples, we found that S, the total radiation-induced absorption of light by the
PET film, is proportional to the logarithm of the fluence F. In addition to this dependence on the
irradiating fluence, we also found that the charge of the irradiating ion has a significant influence on
the position of the absorption edge in the UV-vis spectra. This provides experimentally independent
evidence to confirm our previous results showing that ion charge has an effect on the post-irradiation
state of PET films. We present a physical interpretation of the observed absorption edge red shift in
irradiated PET films as being due to the growth of extended conjugated systems via the formation of
intermolecular helical structures. Our investigations into the stability of irradiation-induced effects
in PET films show that comparison of UV-vis transmission spectra before and after annealing can
provide information about the structure of deep traps in PET.
