Molecular Spectroscopy and UV-Vis Spectroscopy Notes

Molecular Spectroscopy Overview

  • Molecular Spectroscopy: Study of interaction between molecules and electromagnetic radiation.
  • UV-Vis Spectroscopy: Focuses on ultraviolet (UV) and visible (Vis) light ranges.
    • When radiant energy hits a solution:
    • Absorbed
    • Transmitted
    • Reflected
    • Refracted

Energy Transition and Absorption

  • Electronic Transition: Absorption of UV-Vis radiation promotes outer (valence) electrons to higher energy levels, indicating bonding characteristics of molecules.
  • Energy Level Diagram relates energy states to electronic transitions.
    • Ground State: The lowest energy configuration of electrons in an atom.
    • Excited States: Higher energy configurations following absorption of energy.

Instrumentation in UV-Vis Spectroscopy

1. Light Source
  • Characteristics of an ideal light source:
    • Stable output
    • Sufficient intensity for detection
    • Continuous radiation across wavelength range with low noise
  • Commonly used radiation sources:
    • Deuterium Arc Lamp: Good for UV region
    • Hydrogen Lamp: Lower intensity, less expensive
    • Tungsten-Halogen Lamp: Good for UV and visible range
    • Xenon Lamp: Provides continuum across UV and visible regions
2. Monochromatic System
  • Purpose: Isolates a specific wavelength from a continuous spectrum.
  • Components:
    • Entrance Slit: Isolates a single wavelength
    • Prism: Disperses light by wavelength; sensitive to temperature
    • Holographic Grating: More stable than prisms, reflecting light at varied angles based on wavelength
3. Sample Cells
  • Properties:
    • Should be clean, free of scratches, and handled with care.
    • Must not have air bubbles, and solution level must cover light beam.
  • Types of Sample Cells:
    • Quartz/Silica: For UV range
    • Glass/Plastic: For visible range
  • Importance of chemical properties of solvents considered in choice:
    • Example solvents: Water (180 nm) and Acetone (330 nm+)
4. Detectors
  • Converts the optical signal into an electrical signal with ideal characteristics:
    • Linear response, low noise, high sensitivity
  • Types of detectors:
    • Photomultiplier Tube Detector: Amplifies signals through multiple stages.
    • Photodiode Detector: Measures light intensity; charges connected capacitors, with the charge amount proportional to intensity.

Future Topics

  • Discussion of single and double beam spectrophotometer differences, similarities, and operational principles in upcoming sessions.