Polarized Raman Spectroscopy

Polarized Raman spectroscopy is a spectroscopic technique that uses polarized laser light having appropriate excitation wavelength to monitor the polarization dependent vibrational signature of a molecule by detecting the vibrations involving the change in polarizability. Raman spectra collected using this approach can reveal information about the symmetry of vibrational modes, molecular structure, degree of molecular orientation as well as symmetry of highly organized samples such as polar liquids, crystals, polymers, and carbon materials. Polarized Raman spectroscopy has the following advantages over other analytical techniques: (1) determination of anisotropy, (2) qualitative and quantitative determination of molecular orientation, and (3) exploration of amorphicity and crystallinity in molecular system. The main goal of this chapter is to help the reader comprehend the role of incident laser beam polarization and its interaction with isotropic and anisotropic samples. The chapter provides detailed elucidation of polarized Raman measurement and describes the usefulness of this technique to broadly characterize the isotropic and anisotropic samples. In contrast to the isotropic samples, anisotropic samples are very sensitive to polarization of incident beam as they possess axis of symmetry or optical axis orientated relative to the incident laser polarization. The Porto notation, a standard notation used to define the polarization of the incident beam and the collection of scattered beams, has been explained in detail. The polarized Raman spectra of anisotropic samples are distinct in terms of the intensity of Raman peaks when measured in two different relative orientations of polarizer and analyser, resulting in distinct depolarization ratio, which provided an estimate of degree of anisotropy in the sample. Finally, the usefulness of the technique is demonstrated by practical applications including a study of sample morphology, molecular symmetry, anisotropy and non-coincidence effect.