Raman spectroscopy is a spectroscopic technique that utilizes scattered light to probe the vibrational energy modes present within a sample. It is named after the eminent Indian physicist C. V. Raman who pioneered its observation in 1928 alongside his collaborator K. S. Krishnan. Originally, the Raman effect was observed using sunlight and mercury arc lamps. However, modern Raman spectroscopy employs laser light, which interacts with a molecule, engaging with the electron cloud and the bonds within that molecule. This spectroscopic technique is based on inelastic scattering that occurs during this interaction.
This method has evolved into a powerful tool for obtaining both chemical and structural insights, offering a unique 'fingerprint' for substance identification. Each molecule produces a distinct Raman spectrum, making it possible to identify and characterize different substances. Applications of Raman spectroscopy span various fields, including chemistry, materials science, biology, and medicine, where it is used for studying molecular composition, detecting pathogens, and diagnosing diseases, among other uses.
Principles of Raman Spectroscopy
When monochromatic radiation is directed at a sample, the majority of the light is scattered without any change in energy, a process known as Rayleigh scattering. However, a very small fraction of the photons, again about 1 in 106 - 1 in 108, undergoes inelastic scattering, where energy is exchanged between the photons and the molecules, resulting in what is termed the Raman effect. This effect occurs because of molecular vibrations, specific to the molecular composition and structure, allowing Raman spectroscopy to function as a chemical fingerprinting technique.
Scattering interactions can be categorized into two types: elastic and inelastic.
Raman scattering is crucial because it provides insight into the vibrational energy levels of the molecules within the medium. These vibrational energies are unique to the molecular structure and composition, making Raman spectroscopy a powerful tool for identifying chemical substances and analyzing molecular properties. Advanced detectors and spectrographs can produce detailed spectra that reveal sharp peaks corresponding to specific vibrational modes. These peaks can be used to determine the identity, concentration, phase, morphology, and other characteristics of the sample.
Additionally, rotational Raman spectra can also be observed, particularly in diatomic and linear molecules, although they are less common than vibrational spectra. Rotational transitions typically involve much lower energy changes and are often observed in the far-infrared or microwave regions. By analyzing the energy shifts, or Raman shifts, in the scattered light relative to the incident light, Raman spectroscopy provides valuable information about the vibrational and rotational modes, as well as the molecular structure of the sample.
The spectrum is typically measured with the laser line serving as a reference point. Consequently, the peaks are assessed based on their deviation from the laser line. These peak positions in the above figure are dictated by the vibrational energies inherent in the molecular bonds constituting the sample. As a result, the spectrum often bears a striking resemblance to an infrared (IR) spectrum and is interpreted in a similar fashion.
Experimental Setup for Raman Spectroscopy
The instrumental setup for modern Raman spectroscopy comprises three components: a laser source, a sample illumination system, and a suitable spectrometer. Laser sources are utilized due to their high intensity, which is necessary to generate Raman scattering with sufficient intensity for measurement with a reasonable signal-to-noise ratio. Because the intensity of Raman scattering is proportional to λ-4, where λ represents laser wavelength. Consequently, laser sources such as argon (488 nm or 514.5 nm), krypton ion (530.9 nm or 647.1 nm), certain diode lasers (e.g., 785 nm or 830 nm), and helium-neon (632.8 nm) lasers, which emit within specific wavelength ranges, are advantageous over other sources.
Consequently, as the laser wavelength increases, Raman scattering intensity decreases. When comparing UV lasers to near-infrared lasers, spectra obtained from near-infrared lasers can be up to 15 times less intense. As a result, UV and visible lasers necessitate shorter accumulation times and can operate at lower laser power levels compared to near-infrared lasers.
The light reflected from the laser source passes through the lens to the sample. An edge filter directs the laser beam onto the sample, while a notch filter selectively transmits Raman-scattered light to the grating. This grating then reflects the light to the detector.
The detector records the Raman-scattered light, which is then diffracted into a narrow band of wavelengths by a monochromator. Finally, the measured data is processed and analyzed using dedicated software, allowing for comprehensive spectral interpretation.
Types of Raman Spectroscopy
Raman Spectroscopy can be further divided into four prominent types such as: Resonance Raman Spectroscopy, Surface-enhanced Raman Spectroscopy (SERS), Micro-Raman Spectroscopy, and Nonlinear Raman Spectroscopy.
Technique
Principle
Application
Advantages
Limitations
Resonance Raman Spectroscopy
Enhanced Raman scattering due to resonance between incident light and electronic transitions
Studying electronic and vibrational transitions
High sensitivity to electronic transitions
Requires matching excitation wavelength to electronic transitions in the sample, limiting applicability
Surface-enhanced Raman Spectroscopy (SERS)
Amplification of Raman signal via electromagnetic enhancement near nanostructured surfaces
Trace analysis, biosensing, detection of low-concentration analytes
Extremely high sensitivity, detection of single molecules possible
Reproducibility of enhancement, sample preparation issues, signal variation from surface heterogeneity
Micro-Raman Spectroscopy
Spatially resolved Raman spectroscopy with micron-scale resolution
Characterization of materials, mapping chemical composition
High spatial resolution, non-destructive
Limited penetration depth, potential for sample damage at high laser powers, limited sensitivity for trace analysis
Nonlinear Raman Spectroscopy
Utilizes higher-order nonlinear optical processes for signal enhancement
Investigation of molecular dynamics and nonlinear optical phenomena
High sensitivity to molecular dynamics
Complex instrumentation, potential for photodamage, limited availability of commercial systems
Applications of Raman Spectroscopy
Raman spectroscopy finds widespread applications across numerous fields due to its versatility, sensitivity, and non-destructive nature. Some prominent applications of Raman spectroscopy include:
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