Low-density polyethylene, either cross-linked or not, was oxidized and its absorption spectra were measured in the terahertz (THz) range and infrared range. The absorption was increased by the oxidation in the whole THz range. In accord with this, infrared absorption due to carbonyl groups appears. Although these results indicate that the increase in absorption is induced by oxidation, its attribution to resonance or relaxation is unclear. To clarify this point, the vibrational frequencies of three-dimensional polyethylene models with and without carbonyl groups were quantum chemically calculated. As a result, it was clarified that optically inactive skeletal vibrations in polyethylene become active upon oxidation. Furthermore, several absorption peaks due to vibrational resonances are induced by oxidation at wavenumbers from 20 to 100 cm %1 . If these absorption peaks are broadened and are superimposed on each other, the absorption spectrum observed experimentally can be reproduced.Therefore, the absorption is ascribable to resonance.

Optical absorption spectra and photoluminescence spectra were obtained for eight kinds of polyolefin sheet samples using photons in a range from visible to vacuum ultraviolet. Almost all samples were found to exhibit an absorption peak at around 6.5 eV and a luminescence band at around 4.3 eV. The luminescence was found to be induced by the absorption. Furthermore, it was found that successive absorption of ultraviolet photons weakened the luminescence intensity. It is assumed from these results that α, β -unsaturated carbonyls are luminous and that the carbonyls are decomposed through the Norrish type II reaction by absorbing ultraviolet photons. Quantum chemical calculations were carried out using polyethylene models with and without an unsaturated carbonyl to verify the above-mentioned assumption. As a result, the model with an unsaturated carbonyl was found to have localized electronic states in the forbidden band. One of the differential energies between the states is close to the photon energy, by which the luminescence is induced. The bond length of a double bond, which is next to the carbonyl, was found to be longer at the excited singlet state than at the ground state. These results obtained by computation support the above-mentioned assumption of the luminescence center and its decomposition.

SUMMARYOptical absorption spectra and photoluminescence spectra were obtained for eight kinds of polyolefin sheet samples using photons in a range from visible to vacuum ultraviolet (UV). Almost all samples were found to exhibit an absorption peak at around 6.5 eV and a luminescence band at around 4.3 eV. The luminescence was found to be induced by the absorption. Furthermore, it was found that successive absorption of UV photons weakened the luminescence intensity. It is assumed from these results that α, β-unsaturated carbonyls are luminous and that the carbonyls are decomposed through the Norrish type II reaction by absorbing UV photons. Quantum chemical calculations were carried out using polyethylene models with and without an unsaturated carbonyl to verify the abovementioned assumption. As a result, the model with an unsaturated carbonyl was found to have localized electronic states in the forbidden band. One of the differential energies between the states is close to the photon energy, by which the luminescence is induced. The bond length of a double bond, which is next to the carbonyl, was found to be longer at the excited singlet state than at the ground state. These results obtained by computation support the abovementioned assumption of the luminescence center and its decomposition. C⃝ 2014 Wiley Periodicals, Inc. Electr Eng Jpn, 188(1): 1-8, 2014; Published online in Wiley Online Library (wileyonlinelibrary.com).

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