12 Scientific Article The Israel Chemist and Chemical Engineer Issue 9 · January 2023 · Tevet 5783 due to the reflection geometry of the optical set-up, even lack of transparency does not affect the ability to characterize the structural properties of a crystalline material. The only known governing factors for successful characterisation is presence of hydrogen bonds in addition to other low energy modes. The contrast in polarizability of hydrogen bond in the crystal in different directions resulted in characterizability of the structure using LFRS. The variation of polarizability of the hydrogen bonds and resulting spectral contrast is presented in Figures 5–8. A detailed report of this can be found elsewhere [21]. Polarized LFRS for identification of enantiomers An overview of enantiomers and chirality is presented in earlier sections. Enantiomers being molecules with mirror symmetries have right-handed or left-handed symmetry in their structure. This results in antiparallel polarizabilities. In other words, when plane-polarized laser excitation along a particular polarization plane is incident on D- and L-enantiomers, the polarizabilities induced in the molecule are opposite to one another [22]. With LFR modes in general being orientation dependent, the LFR interaction cross-section is therefore dependent on the polarization angle of the excitation laser [23]. This counterintuitively results in different scattering cross sections along different polarization directions. Observation of this contrast is impossible in RS systems that have normal angle of incidence. This work is the realization of a theoretical study of the polarization phenomena with respect to the laser and Raman signal propagation in optically active samples, which was proposed elsewhere [24]. In our work, we engineered an asymmetry into the optical set-up in both excitation and collection geometries and constructed an offaxis excitation collection set-up. In addition to the engineered asymmetry, the excitation and collection geometries were modified to accommodate polarizers to allow capture of orthogonal ly polarized signals. This resulted in enantio-contrasting interaction cross sections along excitation geometry and enantio-selective LFR spectrum in collection geometry. Figure 9 represents the schematics of asymmetry induced into the optical system by virtue of modification and resulting asymmetrical interaction cross sections and induced contrast into the collection signal. Experimental details can be found elsewhere [25]. The study was the first of its kind for identification of enantiomers in their solid form using LFRS. For representation purposes, we present the spectra showing enantioselective contrast between enantiomeric pairs in Figure 10. A detailed explanation of the experimental set-up can be found elsewhere. Figure 9. (a) Schematic representation of conventional Raman excitation and collection and (b) its collected focal area. (c) Schematic representation of the modified Raman setup and (d) its collected focal area. The modified Raman collects different intensities from the polarization planes after excitation of optically activematerials. The schematics are approximate representations directed to understand the processes. The colored squares are a guide to the eye, representing (a) a symmetric interaction cross section for both D and L enantiomers and (b) relative asymmetric interaction cross section for D and L enantiomers*.
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