A short introduction to Raman Spectroscopy
Now, Raman wondered whether this was possible, but with visible light instead of X-rays. Through experiments, Raman observed that when light interacts with a molecule, a small fraction of the scattered light changes in frequency due to interaction with the vibrational modes of the molecule. The scattered frequencies provide a unique fingerprint of the molecular structure, enabling identification and analysis of chemical compounds (Smith & Dent, 2019). Raman received the 1930 Nobel Prize in Physics for his discovery, and was the first Asian to do so for any branch of science. Since then, the technique has evolved into a powerful analytic technique for identifying chemical compounds across various scientific fields, including Earth Science.
In the 1950s and 1960s, Raman spectroscopy was commonly used in physics and chemistry, after the development of laser source photon detectors. However, only after the development of commercially available Raman equipment in the 1980s, Raman spectroscopy became popular in Earth Science. Despite this, Earth scientists quickly embraced the technique and now plays a crucial role in mineralogy, geochemistry, and environmental science (Pasteris & Beyssac, 2020). Its non-destructive nature allows Raman spectroscopy for in-situ analysis of geological samples, aiding in the identification of minerals and their crystalline structures. This makes Raman spectroscopy particularly valuable for multiple fields of research, such as analysis of fluid inclusions for traces of life in the oldest terrestrial rocks or extraterrestrial materials.
For example, Mars rovers like NASA’s Curiosity are equipped with Raman instruments to analyze the composition of rocks and soil on Mars. Additionally, Raman spectroscopy is used to aid carbon capture research and environmental monitoring by identifying reaction products from CO2 sequestration reactions, and contaminants in air, water, and soil samples. Overall, Raman spectroscopy continues to be a powerful tool in Earth scientific research, providing valuable insights into the composition and behavior of natural materials.
How it works
Raman spectroscopy is a spectroscopic technique that relies on the interaction of photons with vibrational modes of molecules. Typically, when a molecule interacts with photons, the vibrational energy state of the molecule is exited to a virtual energy level. Typically, the excited molecule shifts back to the ground state scattering photons at the wavelength corresponding to the incident light, also known as Reyleigh scattering. Occasionally, the final state is higher or lower in energy than the initial state.
In this case, the scattered photons shift in frequency, also known as Stokes-Raman scattering. The Raman scattered light is collected by photon sensitive detectors and translated in to a Raman spectrum. Because vibrational frequencies are specific to a molecule’s chemical bonds and symmetry, Raman spectroscopy provides a fingerprint to identify molecules and different polymorphs. Modern Raman spectroscopy systems use lasers as excitation source. Generally, shorter wavelength lasers produce stronger Raman scattering, but might result in sample degradation (Smith & Dent, 2019).
References
Compton, A. H. (1923). A quantum theory of the scattering of X-rays by light elements. Physical review, 21(5), 483.
Mosca, S., Conti, C., Stone, N., & Matousek, P. (2021). Spatially offset Raman spectroscopy. Nature Reviews Methods Primers, 1(1), 21.
Raman, C. V., & Krishnan, K. S. (1928). The negative absorption of radiation. Nature, 122(3062), 12-13.
Smith, E., & Dent, G. (2019). Modern Raman spectroscopy: a practical approach. John Wiley & Sons.
Pasteris, J. D., & Beyssac, O. (2020). Welcome to Raman spectroscopy: successes, challenges, and pitfalls. Elements, 16(2), 87-92
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