R aman Scattering, Fundamentals

Abstract An introduction of the fundamentals of linear and nonlinear Raman spectroscopy is given. The Raman effect is the result of inelastic light scattering. A small amount of the photon energy of the incident light wave is modulated by the molecular scattering system. An energy transfer occurs as a result of the coupling between the incident radiation and the quantized states of the scattering system. Depending on the coupling, the incident photons either gain or lose energy. The light, which has less energy than the incident laser light, is named Stokes–Raman scattering, and the radiation, which has more energy, is referred to as anti‐Stokes–Raman scattering. In the case of the coupling between strong laser fields and molecular vibrations the observation of nonlinear Raman effects such as hyper‐Raman scattering, stimulated Raman scattering (SRS), coherent anti‐Stokes–Raman spectroscopy, the Raman gain spectroscopy, etc., is possible. Apart from theoretical aspects of Raman spectroscopy an introduction into the instrumentation of linear and nonlinear Raman techniques is provided. For linear Raman spectroscopy two alternate approaches are described: dispersive Raman and Fourier transform Raman (FT‐Raman) spectroscopy. Special Raman techniques such as micro‐Raman spectroscopy and difference Raman spectroscopy are discussed. In addition, a review of the instrumentation of several nonlinear Raman methods which are based on the second‐order (χ(2)) and the third‐order nonlinear susceptibility (χ(3)) is given. These methods include coherent anti‐Stokes–Raman scattering (CARS), stimulated Raman gain spectroscopy (SRGS), inverse Raman scattering (IRS), photoacoustic Raman spectroscopy (PARS), and ionization‐detected stimulated Raman spectroscopy (IDSRS).