**1. Introduction**

Raman spectroscopy is a fast, non-destructive and molecule-specific technique, which requires no or very little sample preparation. Raman spectroscopy is therefore attractive as an experimental technique for on-site investigations of molecular samples of very different

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© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

nature. A vibrational Raman spectrum contains the unique and highly resolved vibrational signature of the molecule and it is obtained by illuminating the sample with polarized laser light with wavenumbers in either the near-infrared (NIR), visible or ultraviolet (UV) regions and monitoring the backscattered light as a function of wavenumber. The main challenge of Raman spectroscopy has always been the low Raman cross-section, where typically 10<sup>8</sup> laser photons only generate a single Raman photon. The result is that the intensity in Raman spectra is generally low. Another challenge particular in Raman investigations of biomolecules is that the excitation of the Raman process is followed by a simultaneous excitation of fluorescence. Since the fluorescence cross-section is generally several orders of magnitude higher than the Raman cross-section, the Raman signal may be partly or completely hidden behind the fluorescence background.

However, because of the technical improvements in Raman instrumentation, the problems originating from the low Raman cross-section have largely been overcome so that the potential of Raman scattering can be utilized, and even though fluorescence may still be a problem in some cases and requires advanced signal processing, vibrational Raman spectroscopy is now applied as a standard technique in many areas such as medical, food and environmental analysis.

In most practical applications, only the positions and intensities of the Raman bands are analysed, i.e. the Raman technique is applied similarly to infrared (IR) and NIR spectroscopy. Although the polarization properties of Raman scattering have been known since the early days of Raman theory, see e.g. [1], and although Raman dispersion spectroscopy (including polarization) introduced by Mortensen [2] has been applied for many years to explore conformational perturbations in metallo-porphyrins and various proteins, see [3–7] and references therein, the advantage of applying polarization resolved Raman scattering is not yet common knowledge among the increasing group of practically working scientists and laboratory technicians representing very different areas, who apply vibrational Raman spectroscopy as one out of a large number of experimental techniques available for the characterization of molecular samples. Besides, polarization analysis of vibrational Raman data is not a standard option in most commercial Raman instruments.

A unique property of the Raman process is that the polarization of the scattered light is generally different from the polarization of the incident laser light. This holds for molecular solids, i.e. oriented molecules and (perhaps more surprisingly) also for powders and solutions, where the molecules are randomly oriented. In vibrational Raman scattering, the polarization change is found to be specific for each vibrational Raman mode and for excitations near an UV/Visible absorption in the molecule, i.e. in resonance or near-resonance Raman scattering, the change depends in general on the wavenumber difference between the excitation and the absorption as well as on the molecular configuration in the electronically excited state.

The goal of this chapter is to demonstrate why, how and when the application of polarized resolved Raman spectroscopy may increase the outcome of a Raman experiment. This goal is achieved through a discussion of the basic properties of Raman scattering with special focus on the polarization followed by a discussion of two illustrative case studies: Case study 1: Aggregation of haemoglobin in red blood cells (RBC); Case study 2: *In vitro* polarization resolved RRS study of dye-sensitized solar cells (DSC).
