Raman spectroscopy is a widely used technique that can help identify chemicals or materials by the way in which they scatter light.
Scattered Light
Scattering occurs when the molecules of a sample absorb photons and re-emit them. While most scattering is elastic, which means that the emitted light is the same frequency as the absorbed light; in some cases, the emitted light is shifted in its frequency. Raman spectroscopy therefore measures these shifts in order to identify the chemical structure of the sample.
The Raman Effect
When a molecule absorbs a photon in Raman spectroscopy it begins to vibrate with more energy until it is excited into a higher vibrational mode. There are three ways this can happen, which determine the frequency of the emitted light:
- The molecule is excited from the ground state to a Raman-inactive mode. The light is emitted at the same frequency (ν0), which gives rise to Rayleigh scattering.
- The molecule is excited from the ground state to a Raman-active mode. Some of the photon's energy is transferred to this mode, whereas the remaining energy (ν0 - νm) is emitted. This altered and emitted Raman frequency is known as the Stokes frequency.
- The molecule is already in the excited Raman-active state. The emitted photon has the additional energy caused by relaxation of this state (ν0 + νm). This is the Anti-Stokes frequency.
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Figure 1. Schematic of the energy levels involved in Raman spectroscopy. Image Credits: Wikipedia.
The Raman activity of a particular vibrational mode is determined by the symmetry of that mode.
The Stokes and Anti-Stokes signals are much weaker than the Rayleigh-scattered light. The main challenge of Raman spectroscopy is to eliminate the main scattered light to a sufficient enough level to get a good reading of the Raman frequencies. This is usually achieved using a filter or spectrophotometer.
Applications of Raman Spectroscopy
Raman spectroscopy is a popular technique used to analyze materials in situ on production lines or in the field. Raman spectroscopy is particularly advantageous for its ability to provide real-time information on samples, particularly those that are in smaller quantities, without requiring extensive preparation. Furthermore, Raman spectroscopy is capable of analyzing a wide variety of substances, regardless of how harsh the environmental conditions may be.
Industries and research fields which make extensive use of Raman spectroscopy include:
- Pharmaceuticals
- Materials Research
- Semiconductors
- Nanotechnology
- Life Sciences
- Forensic Science
- Geoscience
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Figure 2. LabRAM HR Raman Spectrometer from HORIBA Scientific.
Currently, the North American market for Raman spectroscopy technology has experienced a significant growth due to the increasing number of government safety regulations that have been put into place for the healthcare, food and beverage industries. Within the chemical sector, applications of Raman spectroscopy can typically be found in fingerprint identification. Additionally, life science applications of Raman spectroscopy have been extended beyond blood sample analysis to the early diagnosis and analysis of various cancers and diseases.
Hybrid Raman Techniques
The sample flexibility of Raman spectroscopy also makes it particularly well-suited for its use in conjunction with other analytical techniques. Some hybrid Raman techniques which have become very popular in the last few years include:
- AFM-Raman: A very powerful technique that allows for the correlation of physical surface topography with chemical composition. The pressure from the AFM probe can also enhance the Raman signal through tip-enhanced Raman spectroscopy (TERS).
- Raman-Photoluminescence Spectroscopy: Photoluminescence spectroscopy, which can determine the electronic state of a molecule, coupled with Raman to examine the vibrational modes, can provide users with a complete analysis of a material's structure in a single technique.
- Raman-Fluorescence Microscopy: As a particularly useful tool for bioscience applications, this technique first utilizes fluorescence microscopy to identify areas of interest in a microscope sample, followed by the use of confocal Raman spectroscopy to then obtain chemical data on the sample.
- Raman-FTIR: Of the vibrational modes in a material which have no Raman activity due to symmetry restrictions, many are active in the IR region. In fact, some modes can be active in both. Combining these two techniques is therefore a natural step to gain a full picture of the vibrational structure of a material.
Hybrid Raman techniques are a very active field, which has produced some fascinating results. Researchers and equipment manufacturers in this area will continue to work together to find more ways of using the advantages of Raman spectroscopy to assist other analytical techniques in the coming years. For example, one recent advancement in the development of Raman hybrids includes a handheld hybrid device that combines both Raman spectroscopy and optical coherence tomography (OCT). This unique hybrid device has shown particularly promising results for real-time in vivo tissue measurements regarding the morphological and biochemical components of the sample.
Sources and Further Reading
This article was updated on the 7th March, 2019.