Raman spectroscopy is a versatile spectroscopic technique that is based on the inelastic scattering of photons - an effect of which is known as Raman scattering.[1][2] The technique/effect is named after Sir Chandrasekhara Venkata Raman, an Indian physicist who first observed it.[3]

Theory & General Principle

Rayleigh vs. Raman Scattering

When light is scattered by a Raman active substance, most of the photons experience no change in the energy levels between the absorbtion and re-emittion, meaning that the energy transfer during the process is negligible.[4][5]

This type of elastic scattering is known as Rayleigh scattering, and it is by far the most common scattering interaction.[2] However, approximately for every million elastically scattered photons, one inelastically scattered photon appears. Some of these photons are Raman scattered, and they form the basis for Raman spectroscopy. Fig. 1 compares Rayleigh scattering to Stokes and anti-Stokes scattering, both of which are a form of Raman scattering.

Figure 1: Rayleigh and Raman scattering visualized based on the quantum energy transitions of the absorbed and re-emitted photons. Figure from Wikipedia (License: CC BY-SA 3.0).

It should be noted that, during the scattering process, photons are not really absorbed by a substance, meaning that the energy of the photon is not really used to excite the molecules/atoms. The absorbtionre-emittion period is very short lived[6] (basically inexistent) , during which the molecule rises to a "virtual state" from which it then relaxes[7] (a trick that quantum physicists pull to make their math work). As Fig 1. shows, in Rayleigh scattering the energy of the adsorbed and re-emitted photons (hv0) is the same. However, in Stokes scattering, the molecule first rises to the virtual state (E+hv0) and then, instead of relaxing back to its ground state (E), it is relaxed onto a vibarational state (E+hvm). Some of the energy of the photon (hv- hvm) transforms into the vibrational energy of the molecular bonds and thus the wavelength of the re-emitted photon is longer (lower energy). In anti-Stokes scattering, the molecule is in a vibrational state to begin with, from which it rises onto a virtual level, and finally relaxes back to its ground state, making the re-emitted photon's wavelength shorter (higher energy).

Raman Activity of a Molecule

Raman activity arises from the periodic change in the polarizability of a molecule with respect to its vibrational motion. Every molecule has a fingerprint of characteristic vibrational states it can adopt - some of which have an effect on its polarizability. The energy required to set a molecule on these polarizability-changing vibrational states corresponds to the energy shift in the Raman scattered photons. In Raman spectroscopy, the irradiating light source is monochromatic (usually a laser), meaning that the wavelength of the Rayleigh scattered photons is well known (same as that of the laser). By detecting the scattered photons that are more or less energetic than the incident beam, a Raman active molecule can be characterized.[2]

However, not all molecules are Raman active. These molecules can only achieve vibrational states that have no effect on the polarizability of the molecule. A good rule-of-thumb is that a symmetric vibration is Raman active, while an asymmetric vibration is Raman inactive.[5]

Raman Spectrometer

Figure 9. Raman spectrometer. License:[8]

Method of Characterization


Type of Radiative Energy



Near infrared-Near UV[4]



Nature of Interaction



Inelastic / Raman[2]

Materials Characterized

Molecules / Crystals[3]

Raman & IR Spectroscopy

Usually, a molecule cannot be fully characterized based solely on its Raman fingerprint. This due to the vast number of asymmetric vibrations that Raman doesn't detect. For this reason, infrared spectroscopy is often used as a complimentary tool to carry out an accurate characterization of a sample.[5] It can be noted that not all vibrational modes are restrictively only Raman or IR active - some can be both. The nature of the spectroscopical activity arises from the symmetry of the vibrating molecule. Molecules with central symmetry (centrosymmetric) follow the so called rule of mutual exclusion, meaning that a Raman active mode is not detectable with IR, and vice versa. Examples of such molecules are benzene, acetylene, and CO2. However, non-centrosymmetric molecules can adopt vibrational states that are detectable with both techniques. Examples of such molecules are water, ethanol, and acetic acid.[9]

Fig. 2 and Fig. 3 show examples of different CO2 vibrational modes that can only be detected by IR or Raman - but not both.

Figure 2: The symmetric stretching of CO2 at wavenumber 1480 cm-1. In this illustration, the same molecule is depicted in two distinctive moments in time.[9] (License: CC BY-NC-SA 2.0 UK)

As Fig. 2 shows, the symmetric streching of the oxygen atoms has an effect on the polarizability (with respect to the equilibrium position) of the molecule, while the dipole moment remains constant. This makes this vibrational mode Raman active and, due to mutual exclusion, IR inactive.[9]

Figure 3: The asymmetric bending of CO2 at wavenumber 526 cm-1. In this illustration, the same molecule is depicted in two distinctive moments in time.[9] (License: CC BY-NC-SA 2.0 UK)

As Fig. 3 shows, the asymmetric bending of the oxygen atoms has an effect on the dipole moment of the molecule, while the polarizability (with respect to the equilibrium position) remains constant. This makes this vibrational mode Raman inactive and, due to mutual exclusion, IR active.[9]

Raman Instrumentation

Raman Sampling

Raman doesn't require any high-end sampling prior to the measurements, which makes it an easy and relatively quick characterization method. Also, many different types of samples can be characterized, including gases, liquids (also solutions), and solids.[1][2][5]

Raman Spectrometer

Fig. 4 shows a schematic drawing of the working principle of a Raman spectrometer. As Fig. 4 shows, the Raman measurement begins as a monochromatic incident beam is created by a laser and guided towards the sample by mirrors. Once the incident beam hits the surface of the sample, it gets scattered and heads back for the detector. Since most of the refracted light has been Rayleigh scattered, filters are used to separate only the Raman scattered photons. These photons are then diffracted into a spectrum via a process called grating. Finally, the spectrum reaches the detector which converts it into electrical signals that can be interpreted by a computer. Throughout the process, a microscope can be used to monitor the sample and to scan the sample surface for areas of interest.[10]

Figure 4: A schematic of the working principle of a Raman spectrometer (Picture by Tommi Rinne, inspiration from[10]).

Examples of Raman Spectra

In Fig. 5, Fig. 6, and Fig. 7 the Raman spectra of eskolaite, diamond, and graphite are shown. Carbon allotropes are an example of minerals that are well characterized with Raman. This can be seen from the sharp characteristic peaks in their Raman spectra (graphite ca. 1580 cm-1, diamond ca. 1330 cm-1).

Figure 5: Raman spectrum of eskolaite - the two data sets correspond to two different frequencies of the incident beam. (License: Public Domain)[11]

 Figure 6: The Raman spectrum of diamond  - the different data sets correspond to different incident beam polarization. (License: Public Domain)[12]

Figure 7: The Raman spectrum of graphite - the different data sets correspond to different incident beam polarization. (License: Public Domain)[13]

Summary: Pros and Cons

Advantages of Raman Spectroscopy

  • Wide range of analyzable samples (gases, liquids, solids, solutions)
  • No extensive sample preparation and short measuring time
  • Not vulnerable to interference with glass containers or water (aqueous solutions can be analyzed)
  • Measurements don't require a vacuum
  • Non-destructive[14]

Drawbacks of Raman Spectroscopy

  • The inability to characterize metals/alloys[1]
  • Vulnerable to sample fluorescence[4]


1. 1 2 3 4 5

Vandenabeele, Peter. Practical Raman spectroscopy: an introduction. John Wiley & Sons, 2013.

2. 1 2 3 4 5 6

Vandenabeele, Peter. Practical Raman spectroscopy: an introduction. John Wiley & Sons, 2013.

3. 1 2

Smith, Ewen, and Geoffrey Dent. Modern Raman spectroscopy: a practical approach. John Wiley & Sons, 2013.

4. 1 2 3

Ferraro, John R. Introductory raman spectroscopy. Academic press, 2003.

5. 1 2 3 4 5

Larkin, Peter. Infrared and Raman spectroscopy: principles and spectral interpretation. Elsevier, 2017.

6. 1

National Research Council (US). Conference on Glossary of Terms in Nuclear Science. A glossary of terms in nuclear science and technology. No. 110. National Academies, 1957.

7. 1

Dissemination of IT for the Promotion of Materials Science (DoITPoMS-Project). University of Cambridge. Access: https://www.doitpoms.ac.uk/tlplib/raman/comparison.php. Cited: 06.04.2018.

8. 1

Work sponsored by a contractor of the U.S. Government under contract DE-AC05-76RL01830. Accordingly, the U.S. Government retains a nonexclusive, royalty-free license to publish or reproduce these documents, or to allow others to do so, for U.S. Government purposes. These documents may be freely distributed and used for non-commercial, scientific and educational purposes (https://www.pnnl.gov/notices.asp).

9. 1 2 3 4 5

Dissemination of IT for the Promotion of Materials Science (DoITPoMS-Project). University of Cambridge. Access: https://www.doitpoms.ac.uk/tlplib/raman/active_modes.php. Cited: 06.04.2018.

10. 1 2

Dissemination of IT for the Promotion of Materials Science (DoITPoMS-Project). University of Cambridge. Access: https://www.doitpoms.ac.uk/tlplib/raman/raman_microspectroscopy.php. Cited: 06.04.2018.

11. 1

The RRUFF Database: Eskolite. Access: http://rruff.info/eskolaite/display=default/. Cited: 06.04.2018.

12. 1

The RRUFF Database: Diamond. Access: http://rruff.info/diamond/display=default/R050205. Cited: 06.04.2018.

13. 1

The RRUFF Database: Graphite. Access: http://rruff.info/graphite/display=default/R050503. Cited: 06.04.2018.

14. 1

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