Poley absorption

Although long-time Debye relaxation proceeds exponentially, short-time deviations are detectable which represent inertial effects (free rotation between collisions) as well as interparticle interaction during collisions. In Debye s limit the spectra have already collapsed and their Lorentzian centre has a width proportional to the rotational diffusion coefficient. In fact this result is model-independent. Only shape analysis of the far wings can discriminate between different models of molecular reorientation and explain the high-frequency pecularities of IR and FIR spectra (like Poley absorption). In the conclusion of Chapter 2 we attract the readers attention to the solution of the inverse problem which is the extraction of the angular momentum correlation function from optical spectra of liquids. 

It clearly shows that perturbation theory is valid and the Mori approach is identical to it in this limit. However, the impact approximation is unacceptable since the collision time tc exceeds ij. This is a fact of principal importance in understanding Poley absorption and related phenomena. At such a condition, the libration maxima appear in spectra... 

Hence, the problem is reduced to whether g(co) has its maximum on the wings or not. Any model able to demonstrate that such a maximum exists for some reason can explain the Poley absorption as well. An example was given recently [77] in the frame of a modified impact theory, which considers instantaneous collisions as a non-Poissonian random process [76]. Under definite conditions discussed at the end of Chapter 1 the negative loop in Kj(t) behaviour at long times is obtained, which is reflected by a maximum in its spectrum. Insofar as this maximum appears in g(co), it is exhibited in IR and FIR spectra as well. Other reasons for their appearance are not excluded. Complex formation, changing hindered rotation of diatomic species to libration, is one of the most reasonable. 

Fano theory, FIR spectra of NH3 128 far infrared (FIR) spectra 62 Fano theory 128 MD calculations 47 Poley absorption 74, 82 fast modulation limit, perturbation theory 182... 

Collision-induced Absorption in Liquids. High-frequency studies of liquids have shown the Poley absorption, indicative of constrained reorientation," for polar substances in the sub-millimetre band. A broad absorption band in the same region was discovered - for non-dlpolar substances such as cyclohexane, benzene, carbon disulphide, and carbon tetrachloride. The order of magnitude of the absorption is compatible with the induction of the fluctuating dipoles by the distribution of the atomic charge on neighbouring molecules. 

The optical absorption 01(1/) is superfidally not unlike a Poley absorption, though much less in intensity. This function of wavenumber must, however, be multiplied by... 

Hie motion of side-chains at low frequencies will be much dependent on the nature of their coimection to the backbone. At higher frequencies, particularly in solution, the damping will be largely characteristic of interaction with the solvent, while bonding to the mmn chain will be approximated by an additional elastic constraint. Something like a Poley absorption has therefore to be sougfrt, and Chantry and co-workers have recently demonstrated its presence in copolymers of ethylene and vinyl acetate, even in the absence of a solvent. 

We remark that a very high-frequency band is visible in all the figures. This band is due to the fast inertial librations of the dipoles in the potential wells. This band corresponds to the THz (far-infrared) range of frequencies and is usually associated with the Poley absorption [24]. For cv 3> 1, the characteristic... 

The far-infrared (FIR) absorption spectrum of low-viscosity liquids contains a broad peak of resonant character with a resonant frequency and intensity which decreases with increasing temperature [14,15]. Phis phenomenon is known as the Poley absorption. It takes its name from the work of Poley [16], who observed that the difference between the high-frequency dielectric permittivity... 

Johari. G.P. Molecular inertial effects in liquids Poley absorption, collision-induced absorption, low-frequency Raman spectrum and Boson peaks. J. Non-Cryst. Solids 2002. 307-310. 114. 

In a simple model proposed by Brot et al. [52,57], the Poley absorption is considered in terms of librational motion of a polar molecule with a moment of inertia (/) within a potential well of depth (F) and semiangular aperture ( ), due to its nearest neighboins. It is of the form ... 

It has been noted that the conditions for observation of this type of absorption are much more favourable in solid polymers than in liquids. The point is that the relaxation times of polar liquids are of the order of magnitude of 10 s this means that the peak of Debye relaxation process will occur in the 1-10 cm" region and the broad absorption will extend through the region in which the Poley band is found. For polymers, however, the relaxation times are commonly much longer, typically ca 10" s, and the peak frequency of the Debye process is moved to a lower frequency therefore, the probability of resolving it from the Poley absorption is much higher. 

The shape of the Poley absorption bwd in PMS, which is mainly due to librational motion of the benzene rings, contains two bands at 75an and... 

The high-frequency absorption appears to correspond to resonant Poley-type alKorption (y-process following the notation [Wl] - the counterpart of -relaxation in polymers) and it is due to short time and small-angle torsional oscillations of a dipoles within a one well. The shape and posititm of the Poley absorption seems to be independent of the number of carbon atoms in the chain. 

In the FIR spectrum of PS, this peak corresponds to the Poley absorption band which is mainly due to libration of the phenyl side groups [31, 75, 98]. 

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