Chapter 15 Raman Spectroscopy - Tagged

Raman Spectroscopy Nano 3015

Overview of Raman Scattering

  • C. V. Raman: Discovered Raman scattering in 1928, receiving the 1930 Nobel Prize in Physics.

    • Shortest time from discovery to Nobel Prize recognition.

  • Rayleigh Scattering:

    • Named after Lord Rayleigh.

    • Scattering of light or electromagnetic radiation by particles smaller than the light's wavelength.

    • Prominently occurs in gases, also in transparent solids and liquids.

    • Nobel Prize awarded in 1904 (physics).

Rayleigh Scattering

  • Elastic Effect: Light does not gain or lose energy, remains at the same wavelength.

  • Scattering Dependency:

    • Strongly dependent on wavelength, proportional to λ^(-4).

    • Example: More intense scattering of blue light compared to red.

Raman Scattering

  • A small fraction (about 1 in 1 million photons) is inelastically scattered.

    • Scattered photons usually have a different and often lower frequency.

Molecular Energy States

  • Involves molecular vibrational, rotational, or electronic energy.

Rayleigh vs. Raman Scattering

Rayleigh Scattering Process

  1. Photon interacts with a molecule, polarizing the electron cloud, elevating to a “virtual” energy state.

  2. This state is short-lived (~10^(-14) seconds); the molecule drops to ground state, releasing a photon.

  3. Photon released can scatter in any direction, maintaining the same wavelength.

Raman Effect Process

  1. Incident photon excites an electron to a virtual state.

  2. In spontaneous Raman effect, energy state transitions occur:

    • Stokes Raman Scattering: Molecule from ground state to a vibrational excited state.

    • Anti-Stokes Scattering: Photon scatters with more energy if the molecule transitions from a vibrational state to ground state.

  3. Required: Changes in molecular polarizability concerning vibrational coordinates for the Raman effect.

Raman Spectrum Characteristics

  • Primarily utilizes the Stokes half due to its greater intensity.

  • Wavelengths near the laser line filtered out before collecting light.

Historical Experiment by C. V. Raman

  • Used sunlight focused through a telescope and monochromatic filter.

  • Passes through various liquids leading to scattering observed through a crossed filter, demonstrating wavelength change.

Raman Spectroscopy Instrumentation

  • Light Source: Generally a laser to provide necessary intensity; visible sources preferred for their intensity.

    • Raman scattering is typically only 0.001% of the light source.

    • Allows usage of glass/quartz sample cells and optics; linked to UV/Vis detectors.

Advantages of Raman Spectroscopy

  1. Applicable to solids, liquids, and gases.

  2. No sample preparation required.

  3. Unique spectra for materials for conclusive identification.

  4. Non-destructive method.

  5. No vacuum necessary, reducing equipment costs.

  6. Rapid spectra acquisition.

  7. Compatible with aqueous solutions where infrared spectroscopy has limitations.

  8. Can utilize glass vials without interference.

  9. Allows for remote sampling via fiber optics.

  10. Offers fast analysis capabilities.

Disadvantages of Raman Spectroscopy

  1. Not suitable for metals or alloys.

  2. Weak Raman effect leads to low sensitivity for trace amounts.

  • Can be improved using techniques like Resonance Raman.

  1. Susceptible to fluorescence interference.

  2. Color samples may absorb laser light and risk damage.

  3. High cost of equipment.

Raman vs. IR Spectroscopy

  • Complementary Techniques:

    • Both involve scattering radiation at particular frequencies and observe shifts related to molecular vibrations.

    • IR Spectroscopy: Measures changes in dipole moments; emphasizes polar functional groups.

    • Raman Spectroscopy: Measures changes in polarizability; emphasizes aromatic and carbon backbone.

Active Raman Vibrations

  • Requires change in the polarizability of the molecule during vibration (related to electron cloud distribution).

    • Example: O=C=O is IR inactive but Raman active, and vice versa for certain vibrations.

  • Selectivity: Some molecules exhibit Raman activity but not IR

Molecular Dipole Moments

  • Many molecules exhibit a permanent dipole moment, indicating polar character.

    • Example: Water has a significant dipole due to asymmetry.

  • Polarizability is enhanced through applied electric fields.

Applications of Raman Spectroscopy

  • Stress Measurement: Can monitor stress in materials through bond tension affecting frequency.

  • Forensics: Compact spectrometers detect drugs and explosives in the field.

  • Process Monitoring: Non-destructive and real-time analysis, applicable at a distance via fiber optics.

  • Art Restoration: Analyzing ancient artifacts’ composition and techniques.

  • Life Detection on Mars: Miniaturized Raman devices included in space missions to analyze Martian conditions.

  • Carbon Nanotubes: Utilized to assess structural properties through resonance effects.