Raman scattering: Investigation of nematic and smectic ordering

Abstract

Quantitative measurements of vibrational Raman depolarization ratios have been used to study molecular orientational order in nematic and for the first time, in smectic liquid crystals. Temperature dependence of the depolarization ratios of the terminal C≡N and the central biphenyl link vibrational bands of two alkylcyanobiphenyl compounds (CBn) and a cyclohexane derivative (PCH7) has been investigated extensively. Two experimental technics have been used: a conventional triple monochromator and the Raman microprobe to overcome the spurious depolarization due to the long wave director fluctuations. The orientational order parameters <P2≳ and <P4≳ were then calculated. The results have been discussed assuming repulsive forces are dominant for the CBn compounds and using the Maïer–Saupe theory in the case of PCH7.

Computer simulation study of liquid crystal formation in a semi-flexible system of linked hard spheres

“…However, small fluctuations in local ordering always occur, and the finite size of our simulation box guarantees that the diagonalization of Q~ always produces a small non-zero value for S. An alternative approach suggested by Eppenga and Frenkel [15] is to define S as Both (P2(cos 0)) and (P4(cos 0)) are readily amenable to experimental measurement [18]. P2 may be obtained from a range of experimental methods [19] including nuclear magnetic resonance, magnetic susceptibility and refractive index measurements, while P4 values are available from optical [22][23][24] (Raman scattering and two-photon dichroism) and X-ray techniques [25]. Optical measurements have suggested that (P4(cos0)) values become negative at temperatures just below the phase transition contrary to the predictions of both mean field [20] and Onsager theories [21,22].…”

On the determination of orientational order parameters in liquid crystals from Raman scattering: CB9 revisited

“…Constant and Decoster [8], using a Raman microprobe, reported spurious effects on the incident and scattered light polarizations due to these fluctuations for thicknesses larger than SO pm. O n thc contrary, Jen et ul.…”

Probing thick uniaxial birefringent medium in confined geometry: A polarised confocal micro-Raman approach

“…Since, we are interested in the propagation along oz, the global Ram p symmetric 3 Â 3 tensor is reduced to the 2 Â 2 tensor referring to the ox and oy axes. This general formulation (11) can be inserted in the intensity (10) depending on the mode under study. For the sake of simplicity and to go on with our calculation, we restrict our study to stretching mode of bond practically parallel to the molecule.…”

“…The collected Raman intensity can therefore be calculated from (9) and (10), inserting the correct expression of the Raman operator, either (11) in the very general case or (14) for studies of a stretching mode. These intensities can be plot with respect to either the angle f cell (polar plots) or the height position z 0 (zscan plots).…”

“…A simple experimental procedure is proposed to discriminate between homogeneous and heterogeneous birefringent slabs, to extract from experiments some pertinent parameters, such as transverse polarisability coefficients. Somehow, we extend the already known polarised Raman spectrometry technique, which allows estimating the order parameter P2 [11] of a birefringent sample. A set of backscattered spectra is collected and analysed.…”