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.

We measured absorption spectra of unidirectionally stretched poly-ε-caprolactone (PCL) film in a range from 0.3 to 3.6 THz at temperatures from 10 to 300 K. Several absorption peaks were observed, when the electric field of THz waves was set in directions parallel and perpendicular to the stretching direction. The absorption bandwidths became significantly broad at around 200 K and above at least in two specific peaks. This temperature is close to the glass transition temperature of PCL. Further, it is shown by quantum chemical calculations that all the peaks obtained experimentally originate in skeletal vibrations of PCL. Therefore, it has become clear that a specific feature appears in the THz absorption spectrum of PCL associated with its glass transition.

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Perturbation studies of the a-; angular correlation for the (5486a) (59.6~) ($-+-<+)cascade have been carried out in a polycrystalline 241Am sample using the timedifferential technique. The exponential trend of the angular correlation coefficients A,@) as a function of the mean time delay I shows a strong time-dependent behaviour of the interaction phenomena. This is consistent with the theory of Abragam and Pound for polycrystalline sources. The value of the relaxation constant A,, characteristic of the timedependent perturbation, is found to be 1, = (1.81 k0.17) x lo9 s-'.The time-integrated perturbation coefficient obtained is G,(t = 2.4 ns) = 0.19k0.01. Using theexperimental I., value, the spin-relaxation timersfor the Sfelectron shell iscalculated as rS = (1.56k0.63)~ lo-'' s. The validity of the condition U,?, << 1 in the present case strongly suggests that the time-dependent character of the angular correlation is carried primarily by magnetic interactions between the unfilled 5f electron shell of 237Np and the magnetic moment of the nucleus and, secondarily, by electron excitation of the Np atom.