Understanding Raman Spectroscopy Techniques

Dec 17, 2024

Raman Spectroscopy: An Overview

Introduction

  • Analytical technique used to understand molecular structure and composition.
  • Applicable in both industry and academic laboratories.
  • Used for analyzing liquid, slurry, paste, or solid phase.
  • Often compared to IR (infrared) spectroscopy.

Comparison with IR Spectroscopy

  • Both provide structural and property information through vibrational transitions.
  • Raman Spectroscopy:
    • Scattering phenomenon.
    • Inelastic photon scattering (Raman scattering).
  • Infrared Spectroscopy:
    • Absorption technique.

Types of Scattering

  • Rayleigh Scattering:
    • Elastic scattering.
    • Scattered photon frequency is the same as excitation photon frequency.
    • Non-informational about chemical makeup.
  • Raman Scattering:
    • Inelastic scattering.
    • Informational about chemical composition.

Process of Raman Scattering

  1. Excitation source photons excite the molecule into a virtual energy state.
  2. Molecule relaxes by releasing scattered photons to a ground state.
  • Rare occurrence: 1 in 10 million photons.
  • Technological advancements (solid-state lasers, gratings, CCD detectors) have increased its utility.

Types of Raman Scattering

  • Stokes Raman Scattering:
    • Scattered photons have longer frequencies than excitation photons.
    • Most commonly measured.
  • Anti-Stokes Scattering:
    • Scattered photons have shorter frequencies than excitation photons.

Raman Shift and Spectrum

  • Frequency of Raman scattered photons depends on bond types and interactions.
  • Acts as a molecular fingerprint.
  • Spectrum is a function of Raman shift vs. intensity.
  • Raman shift indicates the change in frequency from the excitation source.
  • Intensity is proportional to the number of specific chemical bonds.

Quantitative Analysis

  • Calibration allows quantitative measurements.

Real-time Monitoring

  • Probes coupled to Raman systems enable real-time measurements.
  • Useful for understanding:
    • Reaction kinetics.
    • Initiation and endpoint.
    • Intermediates and crystal form.
    • Molecular backbone and mechanistic information.

Additional Information