Raman Spectroscopy Overview

Raman Spectroscopy Overview

• Raman spectroscopy is a technique to study the frequency change of light interacting with matter.
• It provides information about molecular vibrations, useful for sample identification and quantification.

Light Scattering

• Light scattering is a process where molecules interact with electromagnetic radiation, redirecting it from its original path.
• Scattering can be elastic (Rayleigh scattering) or inelastic (Raman scattering).
  • Rayleigh scattering accounts for 99.999% of scattered light, with scattered photons having the same energy as the incident photons.
  • Raman scattering accounts for <0.001% of scattered light with shifted photon frequencies different from the incident photons.

Raman Effect

• Discovered by Sir C.V. Raman in 1928.
• A small fraction of scattered light has a different wavelength than the incident light, the shift in frequency depending on the molecule's structure.
• The scattering with frequency change is called Raman scattering.

Raman Scattering (Quantum Mechanical View)

• Quantum mechanically, Raman scattering corresponds to excitation to a virtual state, lower in energy compared to an electronic transition (real state), with nearly coincident de-excitation and vibrational energy change.
• Polarization changes are needed to form the virtual state required in Raman scattering.

Stokes and Anti-Stokes Radiation

• Stokes Radiation: Photons collide with a molecule in the ground state, causing energy loss (hν_in - ΔE) during scattering; this is more intense.
• Anti-Stokes Radiation: Photons collide with a molecule in an excited state, causing energy gain (hν_in + ΔE) during scattering; this is less intense.
• Both Stokes and anti-Stokes radiation produce similar spectra but have different intensities.

Raman Effect Mechanism

• Raman effect arises when a photon interacts with a molecule's electric dipole.
• Monochromatic light scattered by gas, liquid, or solid molecules are studied using lasers in the NIR, Visible or UV range.
• Raman spectrum is a plot of the shifted radiation intensity versus frequency (wavenumber).
• Raman spectra are plotted relative to the laser frequency, using a scale where the Rayleigh band lies at zero.

Raman Spectrum Analysis

• Band positions in the Raman spectrum correlate with energy levels of functional group vibrations.
• Compounds with strong IR bands correspond to weak Raman bands and vice versa; this difference is due to the electrical nature of the vibration.
• Polar bonds (C-O, N-O, O-H) result in a strong IR absorption band but a weak Raman scatter.
• Relatively neutral bonds (C-C, C-H, C=C) result in weak IR but strong Raman scatter.

Selection Rules for Raman Activity

• For a vibration to be Raman active, the molecule's polarizability must change during the vibration.
• Change in polarizability can be either a change in magnitude or direction of the polarizability ellipsoid.
• Using group theory and character tables, the symmetry labels (x, y, z or products of x, y, z) of the normal modes can predict if a mode is active in IR or Raman spectroscopies.

Vibrational Raman Spectra (CO₂)

• Symmetric stretch (v1): IR inactive, Raman active.
• Asymmetric stretch (v2): IR active, Raman inactive.
• Bending (v3): IR active, Raman inactive.

Raman Spectrometer Operation

• A sample (solid, liquid, or gas) is illuminated with a laser.

• Some light is scattered by molecules in the medium.

• Most scattered light has the same frequency as the incident light (Rayleigh scattering).

• The shifted light (Raman scattered light) is separated from the Rayleigh scattered light using a wavelength selector and detected.

• The basic Raman spectrometer design uses a dispersive element to separate the Raman light from the incident light.

• FT-Raman spectrometers utilize Michelson interferometers for Raman spectral acquisition.

Raman Spectroscopy Applications

• Raman spectroscopy provides insights into molecular shape and structure (linear/non-linear, symmetric/asymmetrical).
• Ideal for studying aqueous phase samples, as the water's absorption is less intense in the Raman spectrum.
• Useful, but not limited to, identifying or quantifying samples.

Advantages of Raman Spectroscopy

• Water doesn't interfere and can be used as a solvent.
• Can study aqueous solutions

Limitations of Raman Spectroscopy

• Instrument cost is high.

Mutual Exclusion Rule

• A molecule with a center of symmetry cannot display modes that are both IR and Raman active.

Comparison of Raman and IR Methods

• | Property | Raman | IR | |---|---|---| |Interaction mechanism| Scattering | Absorption | |Polarizability change requirement| necessary| necessary| |Dipole change requirement | Not necessary | Necessary | |Solvent use | Water (often) | Water (not often)| |Cost| Expensive | Relatively inexpensive|

Explore the fundamentals of Raman spectroscopy, a crucial technique for studying molecular interactions with light. Understand the principles of light scattering, including Rayleigh and Raman scattering, and the significance of the Raman effect as discovered by Sir C.V. Raman in 1928.