Urbach Rule in the Red-Shifted Absorption Edge of PET Films Irradiated with Swift Heavy Ions

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 electron-enriched 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.

Enhancement of Luminescence of PET Films after Swift Heavy Ion Irradiation

“…In [ 17 ] we found that for incident photon energies in the region ~ 2.5–3.8 eV the shape of the red shifted absorption edge corresponds well to the Urbach rule. This Urbach behaviour is the result of passive interaction between the film and the incident light and is associated with the steady growth of extended conjugated systems in PET films under increasing SHI fluences.…”

“…The results we obtained in [ 17 ] can provide an illustration. As is well-known, the red shift of the absorption edge in PET films under SHI irradiation occurs in the UV/vis region.…”

“…We demonstrated that the degree of ordering depended on both the SHI fluence and on the initial ion charge and involved both intra- and inter-molecular ordering induced by the residual electric field in the SHI latent tracks. This dependence on the ion charge, together with our further investigations [ 16 , 17 ], cannot be understood by considering only the swift δ -electrons ejected from the core of the latent track, but can be by considering also the slow electrons drawn into the core by the field of the passing ion [ 18 ].…”

“…In addition, the observed luminescence photon energy range coincides with that of the red shift of the PET absorption edge under SHI irradiation [ 16 ], suggesting that these two processes may be connected. The experimental transmission spectra of SHI-irradiated PET [ 16 ] together with our analysis in [ 17 ] provide an opportunity to test this conjecture.…”

“…We have previously noted that in “absorbance” mode, our spectrophotometer actually measures “non-transmittance” (or overall attenuation of the intensity of the incident illuminating light) [ 17 ]. It does so by measuring transmitted light at all frequencies above the sensitivity of the photodiode, assuming the sample is not optically active and thus that all the light transmitted is still of the incident frequency.…”