Module 6 - Updated

Module Overview

  • Module 6: Spectroscopic, Diffraction and Microscopic Techniques

    • Areas of focus:

      • Fundamental concepts in spectroscopic and instrumental techniques

      • UV-Visible Spectroscopy: Principle and applications

      • X-Ray Diffraction (XRD): Principle and applications (including numerical)

      • Scanning Electron Microscopy (SEM): Overview of various techniques

Introduction to Spectroscopy and Fundamental Concepts

  • Spectroscopy: The study of the interaction between matter and electromagnetic radiation (EMR).

  • Electromagnetic Radiation (EMR):

    • Radiant energy exhibiting particle and wave properties.

    • Different types of EMR make up the Electromagnetic Spectrum (EMS).

    • Visible Light: The portion of EMR that can be seen by the human eye.

The Electromagnetic Spectrum

  • Increasing energy and decreasing wavelength includes categories such as:

    • Gamma rays

    • X-rays

    • UV light (causes sunburn)

    • Visible light (400 nm - 700 nm)

    • Infrared (heat sensation)

    • Radio waves (lowest energy, used in TVs, NMR, and MRI)

Properties of Electromagnetic Radiation

  • Components:

    • Electric and magnetic fields at right angles to each other.

  • Travel: EM waves can propagate through a vacuum, unlike sound or water waves which require a medium.

  • Key Terms:

    • Frequency (n): Number of wave crests passing a point per second (measured in Hz).

    • Wavelength (\u03BB): Distance between two adjacent crests (in meters, often nm).

    • Velocity (c): Speed of EM waves in a vacuum is constant (c = 2.998 x 10^8 m/s).

    • Relationships:

      • c = \u03BBn

Photon Energy

  • Quantum Nature: Light is quantized; treated as particles called photons.

  • Energy equations:

    • E = hn

    • E = hc/\u03BB

    • Energy is proportional to frequency and inversely proportional to wavelength.

    • Common units for energy include Joules (J, and often kJ/mol).

UV-Visible Spectroscopy

  • Setup:

    • Light source, monochromator, sample cell, and detector.

    • The cuvette must be quartz for UV light to avoid absorption by glass/plastic.

  • Absorbance Factors:

    • Sample thickness, concentration, and nature of the absorbing species determine absorbance.

  • Beer-Lambert Law:

    • Relationship: A = \u03B5cl (A --> absorbance, c --> concentration, l --> path length, \u03B5 --> molar absorptivity).

    • Transmission equations and calculations of absorbance can be done using I = I0 10^(-\u03B5lc) and A = -log10(T).

X-Ray Diffraction (XRD)

  • Diffraction:

  • Occurs when a wave encounters an obstacle or opening, leading to constructive or destructive interference.

  • Constructive Interference: Peaks in intensity when waves are in phase.

  • Destructive Interference: Reduction in amplitude when waves are out of phase.

  • X-ray diffraction patterns are produced by the periodic arrangement of crystalline atoms.

    • Diffraction occurs at specific angles related to the crystal structure.

    • Bragg's Law: n\u03BB = 2dsin(\u03B8) used to relate wavelength, angle, and distance between planes.

    • The Scherrer equation allows for calculation of average crystallite size from diffraction peak broadening.

Scanning Electron Microscopy (SEM)

  • Basic Principle:

    • Utilizes electrons instead of light; captures secondary and backscattered electrons to create images.

  • Equipment Components:

    • Electron gun, lenses, specimen stage, and detectors for signal.

  • Resolution and Magnification:

    • SEM can achieve magnifications up to 3,000,000x and resolution down to 1 nm, with performance influenced by multiple factors.

  • Specimen handling: Orientation and distance from detectors are critical for image quality.

Summary

  • These techniques provide powerful means to analyze and characterize materials at microscopic levels, offering insights into their structure, composition, and properties.