Raman Spectroscopy

  • Introduction

    Raman spectroscopy is used to identify different chemicals and to analyze how much of a particular substance is present in a sample. Each chemical has a distinctive fingerprint, which is matched to a known database for instant identification and quantification. This method is applicable to solids, liquids or powders. Without physically contacting or destroying them, samples from microliter range to large objects can be tested in situ, in vitro and in vivo. Even underwater examinations are possible. Raman can also distinguishes between isomers and requires no sample preparation. 

  • The Science

    Radiations be scattered in all directions during interaction with a sample. Molecules begin to move and vibrate when irradiated by monochromatic radiation like laser light. This light interacts with molecular vibrations resulting in the energy of the laser photons being shifted up or down. Capturing this difference produces a distinctive fingerprint and pattern that helps to identify the particular chemical. In a Raman spectrum especially nonpolar bonds shows intensive bands (C=C, N=N, O-O, S-S, P-P).

    The Raman effect , named after the Indian physicist C. V. Raman, is based on molecular deformations in electric field E determined by molecular polarizability α. The laser beam can be considered as an oscillating electromagnetic wave with electrical vector E. Upon interaction with the sample it induces electric dipole moment P = αE which deforms molecules. Because of periodical deformation, molecules start vibrating with characteristic frequency υm.

    Amplitude of vibration is called a nuclear displacement. In other words, monochromatic laser light with frequency υ0 excites molecules and transforms them into oscillating dipoles.  

    Rayleigh scattering

    A molecule with no Raman-active modes absorbs a photon with the frequency υ0. The excited molecule returns back to the same basic vibrational state and emits light with the same frequency υ0 as an excitation source. This type of interaction is called Rayleigh scattering.  

    Raman scattering

    A photon with frequency υ0 is absorbed by Raman-active molecule, which, at the time of interaction, is in the basic vibrational state. Part of the photon’s energy is transferred to the Raman-active mode with frequency υm and the resulting frequency of scattered light is reduced to υ0 - υm. This type of interaction is called Stokes scattering

    If a molecule is already in the excited vibrational state, excessive energy of excited Raman-active mode is released, the molecule returns to the basic vibrational state and the resulting frequency of scattered light goes up to υ0 + υm. This type of interaction is called anti-Stokes scattering.

  • Examples of use
    • characterize a material’s composition, and fast identification of unknown materials, e.g. in food or textiles
    • Probes subtle chemical effects, such as crystallinity, polymorphism, phase, intrinsic stress or strain, protein folding and hydrogen bonding
    • Analysis samples such as historic pigments or vital forensic evidence
    • Analysis of layered samples, inclusions as those found in glass and minerals, thin samples on a substrate and materials in glass or plastic containers.