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« SPECTROSCOPY


a possible enantioselective technique. Quite the contrary, many textbooks and research articles explicitly state that Raman spectroscopy is inherently incapable of discriminating between the enantiomers of a chiral substance. However, the recent introduction of enantioselective Raman (esR) spectroscopy has proven these statements wrong.1


The esR technique takes advantage of the optical activity of a sample containing chiral molecules. On the one hand, this optical activity results in a rotation of the polarization of the laser light as it travels through the sample. On the other hand, it also rotates the polarized and depolarized signal components. The latter is utilized for the enantioselective detection. However, before this is possible, the inherent symmetry of the scattered light in terms of polarization rotation has to be broken. This symmetry arises from the different enantiomers rotating the polarization plane by exactly the same angle but with opposite sign. Consequently, looking at the vertically and horizontally polarized signal components leads to identical Raman spectra for both enantiomers.


This problem can be overcome by inserting a simple optical component into the signal detection path before the vertically and horizontally polarized parts are separated from each other. This magic component is an achromatic half-wave plate, which rotates the signals of the two enantiomers by different angles and thus breaks the symmetry. The schematic experimental setup is illustrated in Figure 1. It is a conventional Raman spectroscopy setup with polarization- resolved signal detection in direction perpendicular to the laser beam. The only additional component is the half-wave plate inserted before the polarizing beam splitter. The principle of breaking the symmetry is shown in the inset polarization diagrams before and after the half-wave plate. It becomes clear that the polarization directions of the Raman signals are asymmetric with respect to the 0° axis, and thus they can be distinguished from each other.


Opportunities and Challenges


A key benefit of the esR technique over a classical determination of the sole rotation angle is that it provides the same rich structural information as conventional


Raman spectroscopy. Hence, the signal can be used for a qualitative analysis of the sample enabling the identification of all molecular species in the measurement volume. In addition, Raman spectroscopy is a quantitative tool as the signal intensity scales with the concentration of the molecules. Consequently, esR is technically capable of


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topography


topographic Raman i mage


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