Raman 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).

Soil Chemical Insights Provided through Vibrational Spectroscopy

“…This type of scattering arises from approximately 10 −4 of incident photons and is thus more intense (Smith and Dent, 2005) relative to the inelastic scattering of 10 −8 of incident photons in Raman scattering (Petry et al, 2003). Inelastic scattering can result from (1) excitation of molecules in the ground state (v 0 ) to a higher energy vibrational state (Stokes) and (2) return of molecules in an excited vibrational state to the ground state (anti-Stokes) (Popp and Kiefer, 2006). The different transition schemes are illustrated in Figure 1.1.…”

“…The energy of emitted photons is relative to the incident light and hence, although plotted like IR spectra, Raman spectra display the wavenumbers shift in the energy of the incident radiation. For a more comprehensive explanation of the principle, theory and instrumentation of Raman spectroscopy the reader is directed to several resources (Colthup et al, 1990;Ferraro, 2003;Lewis and Edwards, 2001;Long, 1977Long, , 2002Lyon et al, 1998;Pelletier, 1999;Popp and Kiefer, 2006;. The change in polarizability of a molecular bond measured by Raman spectroscopy is more intense in pi bonds of symmetric molecules (e.g.…”

Evaluating Lignocellulosic Biomass, Its Derivatives, and Downstream Products with Raman Spectroscopy

“…The scattering produced can have an identical (elastic), higher (inelastic), or lower (inelastic) frequency than that of the excitation source [Figure 1 ; Lupoi ( 2012 )]. These types of scattering are named Rayleigh, Stokes, and anti-Stokes, respectively (Carey, 1982 ; McCreery, 2000 ; Smith and Dent, 2005 ; Popp, 2006 ). Rayleigh scattering is the most intense, and needs to be thoroughly removed from the optical beam path using specialized optics such as holographic notch filters (HNFs) (Smith and Dent, 2005 ; Dao, 2006 ).…”

A compact laser platform for nonlinear Raman microspectroscopy: multimodality through broad chirp tunability

“…High‐resolution spectroscopic analysis and ultrafast, short‐pulse interrogation are the two main modalities of optical technologies based on nonlinear Raman scattering . While high‐resolution Raman spectroscopy requires narrowband probes, capable of resolving individual molecular or atomic features in often crowded Raman spectra of complex chemical systems, broadband, short‐pulse drivers offer a vast arsenal of tools for time‐resolved studies of ultrafast dynamics in physical, chemical, and biological systems and open a whole new world of chemically specific, high‐speed nonlinear microscopy . The requirements on laser fields needed to implement these modalities are seemingly incompatible.…”