Raman Spectroscopy
Raman Spectroscopy is an analytical technique used to observe vibrational, rotational, and other low-frequency modes in a system. It relies on Raman Scattering of light, which was named after the Indian physicist Sir C.V. Raman, who along with his student K.S. Krishnan, discovered this phenomenon in 1928.
History
- The discovery of Raman Scattering was made by Sir C.V. Raman on February 28, 1928, which is now celebrated as National Science Day in India.
- The first observations of Raman scattering were made using sunlight as the excitation source, but it was quickly realized that more intense, monochromatic light sources like mercury vapor lamps could enhance the effect.
- The development of lasers in the 1960s significantly improved the sensitivity and application of Raman Spectroscopy, making it a widely used analytical tool.
Basic Principle
The fundamental principle behind Raman Spectroscopy involves the interaction of light with molecular vibrations. Here's how it works:
- When a beam of monochromatic light (typically from a laser) strikes a sample, most of the photons are elastically scattered at the same energy as the incident photons, known as Rayleigh scattering.
- However, a small fraction of the light (approximately 1 in 107 photons) undergoes inelastic scattering, where the photon either loses or gains energy. This inelastic scattering is known as Raman Scattering.
- The energy difference between the incident photon and the scattered photon corresponds to the vibrational energy levels of the molecules in the sample, providing a molecular fingerprint.
Applications
Raman Spectroscopy has a wide range of applications:
- Material Science: It is used to identify the structure of materials, study phase transitions, and analyze stress and strain.
- Chemistry: It helps in the identification of chemical species, monitoring chemical reactions in real-time, and studying the interaction between molecules.
- Biology and Medicine: It's applied in diagnosing diseases, analyzing biological samples, and in drug discovery processes.
- Forensics: Raman can be used to analyze drugs, explosives, and other materials at crime scenes without destroying the evidence.
- Art Conservation: For analyzing pigments in paintings to authenticate artworks or understand their degradation processes.
Advantages and Limitations
Advantages:
- Non-destructive analysis.
- Requires minimal sample preparation.
- Can be used for both solid, liquid, and gas samples.
Limitations:
- The intensity of Raman scattering is relatively weak, requiring sensitive detectors and powerful light sources.
- Fluorescence can often interfere with the Raman signal, although techniques like Surface-Enhanced Raman Spectroscopy (SERS) can mitigate this issue.
- The technique is less effective for materials with very dark colors or those that strongly absorb at the laser wavelength.
Recent Advances
- Surface-Enhanced Raman Spectroscopy (SERS): Increases the sensitivity of Raman scattering by using nanostructures.
- Tip-Enhanced Raman Spectroscopy (TERS): Provides nanoscale spatial resolution by combining Raman with scanning probe microscopy techniques.
- Resonance Raman Spectroscopy: Enhances the Raman effect by matching the excitation laser wavelength with an electronic transition of the sample.
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