Poley Absorption Research Articles

Relationship Between the Molecular Dynamics of Polystyrene and Its Modifications and Parameters of Poley Absorption in Far-infrared Spectra

The far-infrared spectra of polystyrene (PS) and its modifications, obtained by methyl group, chlorine and/or bromine atom substitutions for hydrogen atoms in the benzene ring, was measured in the frequency range from 20 to 450 cm−1. All these spectra exhibited abnormally broad asymmetric absorption bands with the position of maximum in the range from 40 to 80 cm−1, depending on the nature of the substituent, which can be assigned to the absorption associated with libration (rotational vibrations) of the phenyl ring of the monomer units, i.e., to the absorption occurring through the Poley mechanism. The relationship between the spectral parameters of the Poley absorption and the molecular characteristics of the studied polymers make it possible to directly analyze the role of molecular structure and intermolecular forces in the dynamics of macromolecules. The heights of the potential barriers of the librating monomer units, QL , estimated based on analysis of this absorption, were found to be close to the...

The extended Debye model analysis of Poley absorption in glass-forming toluene

The extended Debye model is proposed to explain the high frequency (Poley) absorption peak of polar liquids. It is shown that this model satisfactorily describes the experimental data of toluene obtained in the far-infrared region by J. Wiedersich et. al. [J. Wiedersich, N.V. Surovtsev, E. Rössler, J. Chem. Phys. 113 (2000) 1143]. From analysis of spectra by the extended Debye model two resonance modes were found at low temperature, suggesting the presence of two different molecular structures on a microscopic scale in supercooled and glassy toluene.

Anomalous dielectric relaxation in a double-well potential

The fractional Klein–Kramers equation describing the fractal time dynamics of an assembly of fixed axis dipoles rotating in a double-well cosine potential representing the internal field due to neighboring molecules is solved using matrix continued fractions. The solution includes both inertial and strong internal field effects which in combination produce a strong resonance peak (Poley absorption) at high frequencies due to librations of the dipoles in the potential, an anomalous relaxation band at low-frequencies mainly arising from overbarrier relaxation and a weaker relaxation band at mid frequencies due to the fast intrawell modes. The high frequency behavior is controlled by the inertia of a dipole, so that the Gordon sum rule for dipolar absorption is satisfied ensuring a return to optical transparency at very high frequencies. Application of the model to the broad-band (0-THz) dielectric loss spectrum of a dilute solution of the probe dipolar molecule CH 2Cl 2 in glassy decalin is demonstrated.

Dielectric Studies of Bicyclohexylcarbonitrile Nematogens with Large Negative Dielectric Anisotropy

Molecular dynamics of a homologous series of trans, trans-4,4′-dialkyl-(1α,1′-bicyclohexyl)-4β-carbonitrile(CCNs) with a lateral cyano group has been studied by dielectric relaxation spectroscopy. The high frequency relaxation process connected with the molecular reorientation around the long molecular axis was investigated in the isotropic and nematic phases. Due to the negligible value of the longitudinal dipole moment, the relaxation process around the short axis is inactive in dielectric spectroscopy. The dielectric relaxation time changes smoothly at the phase transition isotropic/nematic. The activation energy obtained (EA ∼ 30 kJ mol−1) is typical for a relaxation process around the molecular long axis. For planar alignment the high frequency limit of the dielectric permittivity ˜⊥∞ is distinctly higher than no2. The difference ˜⊥∞−no2 ≈ 1.2 is most probably connected with Poley absorption due to librational motion of molecules around the long axis.

Anomalous diffusion and dielectric relaxation in anN-fold cosine potential

The fractional Klein-Kramers (Fokker-Planck) equation describing the fractal time dynamics of an assembly of fixed axis dipoles rotating in an N-fold cosine potential representing the internal field due to neighboring molecules is solved using matrix continued fractions. The result can be considered as a generalization of the solution for the normal Brownian motion in a cosine periodic potential to fractional dynamics (giving rise to anomalous diffusion) and also represents a generalization of Fröhlich's model of relaxation over a potential barrier. The solution includes both inertial and strong internal field effects, which in combination produce a strong resonance peak (Poley absorption) at high frequencies due to librations of the dipoles in the potential, an anomalous relaxation band at low frequencies mainly arising from overbarrier relaxation, and a weaker relaxation band at midfrequencies due to the fast intrawell modes. The high-frequency behavior is controlled by the inertia of the dipole, so that the Gordon sum rule for dipolar absorption is satisfied, ensuring a return to optical transparency at very high frequencies. Application of the model to the broadband (0-THz) dielectric loss spectrum of a dilute solution of the probe dipolar molecule CH2Cl2 in glassy decalin is demonstrated.

Molecular inertial effects in liquids: Poley absorption, collision-induced absorption, low-frequency Raman spectrum and Boson peaks

The far-infrared absorption spectra of low-viscosity liquids contains a broad peak whose frequency decreases with increase in temperature and height increases. This is known as the Poley absorption. In the theory of itinerant oscillations of molecules in a liquid, the frequency of the Poley absorption peak is inversely proportional to the square root of a molecule's moment of inertia. Dipolar molecules confined to symmetrical cages in the structure of an ice clathrate crystal also show a peak in the far-infrared region. This has also been interpreted in terms of rotational oscillations of the dipolar molecule with its frequency being inversely proportional to the square root of the molecule's moment of inertia. The low-frequency Raman and the far-infrared absorption spectra (or the dielectric loss spectra in the THz frequency range) of a glass and a supercooled liquid also show a peak and several other features depending on the manner of presenting the Raman scattering and infrared absorption data. Data on these features are collected from the literature and compared here. It is shown that despite the differences arising from molecule-specific effects, these three features are remarkably similar, thus indicating that the vibrational effects attributed to the Boson peak and to other features of the low-frequency Raman scattering are likely to admit to the same underlying mechanisms as the Poley absorption. There is also a collision-induced absorption in liquids in the far-infrared region. Its equivalence in the Raman-scattering studies is yet to be recognized.

Dielectric response and a phenomenon of a narrow band absorption for a classical rotor in a double well potential

The theory of dielectric relaxation in a planar ensemble of polar molecules is presented for a model where dipoles may rotate in a conservative double well potential (DWP) U=U0 sin2 ϑ. The evolution of wideband dielectric spectra as a function of the potential well depth U0 is studied; both isotropic and anisotropic media being taken as examples. The spectra comprise the Debye relaxation and the quasiresonant Poley absorption regions. The exact theory is compared with a simple quasielastic bond/extended diffusion approximation. The latter is valid for a quantitative description for any value of the field parameter p=[U0/(kBT)]1/2. For p≫1 the spectrum comprises one narrow absorption band, while the Debye relaxation region is markedly shifted to low frequencies. It is shown that in the long lifetime τ limit there exists a minimum absorption band Δν0(p). The quantity Δν0 becomes small for values of parameter p≫1. The dielectric relaxation in ice 1 is discussed in relation to this phenomenon.

The relationship between the poley absorption and fractional power law relaxation in the cluster model

The response function derived for the cluster model possesses an analytical form over the whole time range form zero to infinity. Previous work has concentrated upon its behaviour in the non-inertial (relaxation) regime where it predicts a non-Debye decay consisting in general of two power laws, namely t −n at t < 1/ω p and t −(m+1) for t > 1/ω p with ω p the loss peak frequency. Here it is shown that the exact response function of this model gives an absorption peak in the inertial region (Poley absorption) as well as a relaxation loss peak. Furthermore the relative magnitude of the predicted Poley absorption is shown to decrease with the value of the exponent n (<1) appropriate to the relaxation peak, leading to an isolated Debye peak for n=0. A comparison with the experimental data will be made and the relationship between the Cluster Model and other models for the Poley absorption discussed.

Non-vanishing molecular collision times and dielectric relaxation for linear molecules

The long-standing problem of Poley absorption in the theory of dielectric relaxation was studied by Burshtein and McConnell, and it was found that some progress in solving the problem could be obtained by allowing the possibility of nonvanishing collision times. The implications of such a possibility are further examined in the present paper, the central idea of which is to regard molecular collisions as being equivalent to reorientation in a cos θ- or a cos 2θ-potential well.

Molecular theory of underdamped dielectric relaxation: understanding collective effects in dipolar liquids

A molecular theory of underdamped dielectric relaxation of a dense dipolar liquid is presented. This theory properly takes into account the collective effects that are present (due to strong intermolecular correlations) in a dipolar liquid. For small rigid molecules, the theory again leads to a three-variable description which, however, is somewhat different from the traditional version. In particular, two of the three parameters are collective in nature and are determined by the orientational pair correlation function. A detailed comparison between the theory and the computer simulation results of Neria and Nitzan is performed and an excellent agreement is obtained without the use of any adjustable or free parameter - the calculation is fully microscopic. The theory can also provide a systematic description of the Poley absorption often observed in dipolar liquids in the high-frequency regime.

Molecular mechanisms of dielectric relaxation of highly polar liquids

The dielectric spectra of highly absorbing simple liquids CH3F, CH3Cl, CH3l CHF3 caused by reorientations of polar molecules have been computed. The calculations were performed over a wide temperature range for the confined rotator (CR) model and for the hybrid confined rotator/extended diffusion model (HM). The mechanisms of absorption and dielectric losses result from periodic librations of molecules and from their orientation in the alternating local electric field Ec(t), respectively.The calculated dielectric spectra agree with experiment over the frequency range 0 < ν/cm–1 < 200, covering the Debye relaxation and Poley absorption regions of the spectra. With increasing temperature T, the libration amplitude β increases while the libration time τ(the correlation time of the angular velocity |Ω|) and the barrier height UΩ decrease. The maximum discrepancy between theory and experiment occurs at temperatures close to the critical value; in this case the HM gives a better description of the Debye frequency spectrum than the CR model. Estimates based on application of the resonance condition, the liquid cell model and the hard sphere model are proposed.The self-diffusion coefficient D(T) has been calculated; in this case the collision frequency of molecules 1/τv(V) was estimated for the hard-sphere model and it was assumed that the self-diffusion is caused by molecules whose translational velocity v is sufficient to surmount the barrier Uv which is dependent on T. It was shown that the barrier Uv is essentially less than UΩ and for a first approximation, is proportional to UΩ.

Impact description of the poley absorption

The impact theory is used for spectral band shape description for the liquid phase, the multiplication of averaged rotational frequency and angular momentum correlation time being small. The physical difference between liquid and gas interactions is taken into account by introducing the non-Poisson statistics for the time succession of collisional events and preferable reflection for rotational changes by impact. On this basis, it proved to be possible to propose a quite simple unified explanation for the Poley absorption in dielectric losses and far infrared spectra along with the R(ν) representation of the depolarized Rayleigh scattering.

The Poley absorption problem

Abstract The Gordon J-diffusion model is generalized by allowing the presence of non-instantaneous molecular collisions. The implications of the generalized model for absorption in the far infared region are studied with special reference to Poley absorption.

Far infrared spectra of octyl and nonyl cyanobiphenyl liquid crystals

Abstract Far infrared absorption of 4-n-octyl-4-cyanobiphenyl (8CB) and 4-n-nonyl-4-cyanobiphenyl (9CB) liquid crystals has been measured in the wavenumber range 15–220 cm −1 in their crystalline, smectic A, nematic and isotropic phases. Fine structure in the spectra reported previously on 8CB has not been observed. The dependence of the intensity and frequency of the various bands on the phase has been studied, and the bands are being assigned. The conventional Poley absorption is observed to arise in the 15 – 50 cm −1 wavenumber range whereas the absorption in the 95 – 110 cm −1 range seems to arise from a combination of the rattling motion of the molecules within the imaginary tubes in which the molecules may be situated and the molecular torsions around the long molecular axes. Lattice modes in 60 – 90 cm −1 wavenumber range which appear in the crystalline state are found to disappear in the liquid crystalline phases.

On the quantitative theory of dielectric spectra in polar liquids

The complex dielectric permittivity ϵ*(ν)=ϵ′+iϵ″ and the absorption coefficient α(ν) caused by reorientations of polar molecules are calculated for two analytic molecular models - the confined rotator (CR) model and the hybrid confined rotator/extended diffusion model (HM). In the case of liquid fluorometane CH3F the agreement of theoretical spectra with the experimental ones was shown to be satisfactory for a wide temperature range (133 < T < 293 K) and for a broad band (0<ν <200 cm−1) containing the relaxation and the Poley absorption regions. In the first (CR) model the dipole is supposed to librate in a rectangular potentiall well with infinite walls. In the second model (HM) it is supposed that slow moleculles (ε <U ) librate as in confined rotator model, while fast molecules (ε > U ) rotate freely during lifetime τR between collisions, ε is the kinetic energy of rotation. The frequences νD and νL of maximum dielectric loss (ϵ″D) and absorption (αL) depend strongly on the potential barrier UΩ which decreases with the increase of temperature T. The self-diffusion coefficient D(T) of a liquid is calculated together with the dielectric spectra. It has been supposed that self-diffusion is caused by fast molecules overcoming the potential barrier Uv (mv2/2 > Uv). Uv is the barrier for translation motion, v is the velocity and m is the mass of the molecule.

Submillimetre wave spectroscopy of 4-n-alkyl-4′-cyano biphenyl liquid crystals

Abstract The absorption coefficient of the 4-n-alkyl-4′-cyano biphenyls has been studied in the submillimetre wave range 50–220 cm−1. Molecular processes which give rise to superimposed but distinct absorption bands are reported. These processes have been analysed using a multi-oscillator model for the observed absorption and are assigned to the Poley absorption as well as to intermolecular and intramolecular mechanisms. Spectra at temperatures in the range 20–70°C and of solutions in cyclohexane support the proposed assignments.

A new far-infrared broadband absorption in non-rigid polar molecules

A new far-infrared broadband absorption has been observed for non-rigid ketone molecules in the frequency range 220 to 340 cm −1. It has been found that the amplitude of this absorption increases with an increase in the flexibility of the molecule. Such an increase in the amplitude of this absorption is accompanied by a corresponding reduction in that of the Poley absorption.

Far infrared study of molecular reorientation in some alkoxyazoxybenzenes

Abstract Far infrared spectra in the frequency range 20 to 200 cm−1 for the 4,4δ-di-n-alkoxyazoxybenzene homologous series are presented for the isotropic and nematic phases and for 0.1 M solution in benzene. In addition some partially deuteriated samples were studied. The spectra consist generally of two broad bands. By comparison of the spectra for different deuteriated compounds it is possible to interpret the higher frequency band as being connected with the intramolecular torsional vibration of the end groups. The band at lower frequencies we associate with the Poley absorption of librating molecules. The confined rotator model calculations performed for this band, for all compounds studied, give the rotational correlation time which is in good agreement with the correlation times as derived from quasi-elastic neutron scattering experiments.

The Poley absorption in liquid crystals

The far infrared torsional oscillatory absorption of a nematic liquid crystal has been isolated for the first time free of higher frequency proper modes. This has been achieved using a molecule specially synthesised for this purpose with a dipole moment engineered perpendicular to the long axis so that torsional oscillation about this axis modulates this component only. The far infra-red power absorption of the liquid crystal in the isotropic, nematic, solid and supercooled conditions is therefore a Poley absorption, i.e. the torsional oscillation about the rigid long axis of the liquid crystal molecule itself. The effect of phase changes on a macroscopic level is to sharpen and intensify this absorption. This is clear evidence to the effect that phase changes in liquid crystals are cooperative phenomena, the molecular librational dynamics about the long axis are little different from those in a regular molecular liquid.

Cooperative behaviour in liquids

Evidence is presented for the existence of strong cooperative behaviour in the liquid state. It is suggested that the far-infrared spectrum of CH3CN is composite in nature and that only a small portion may be attributed to the so-called ‘Poley absorption’ supposedly characteristic of polar liquids in the far-infrared. This is in accord with a recent conclusion of other authors in a quite independent investigation.