Intro to Spectroscopy and Infrared

Measuring electromagnetic radiation abasorbed / emitted due to an energy state transition


  • vibrations are quantised
  • IR can cause transitions between vibrational energy states 
  • need oscillating dipole moment 
  • Coupling of vibrations and E field ocurs
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FIR Spectroscopy

  • Antisymmetric and bending are IR active
  • Use harmonic theory and mechanical models to describe the energy needed for bond deformation

Increases wavenumber

  • Multiple bonds, smaller mass, stretching compared to benfing, assymetry compared to symmetry, polar bonds 

Each bond corresponds to a wavenumber. Wavenumber is not affected by the presence of other atoms/ bonds


  • 4000-1000 cm-1 --> Functional group
  • 1300 - 1000 cm-1 --> Polymer fingerprint
  • 1000-650 cm-1--> Ring region
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FIR Quantitative measurements

Can use Infrared to make qunatitative measurements about concentrations

  • using Beer Lambert law: It is the relation of absorabcnec to molar concentration
  • Subtract absorbance of the sample with the background, by measuring the Absorbacne of the solvent
  • Can use law to get relative concentrations [keep path length the same]
  • The law is only valid for low concentrations
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Dispersive IR

  • Polychromatic light reflected by mirror hits sample, sent through monochromator 
  • use diffraction grating at specidic position to satisfy Bragg's equation to choose 1 wavelength to be detected
  • See how that wavelengdth has been absorbed through sample
  • Cheap, slow
  • Labour intensive --> need to move grateing
  • Can't identify compounds
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Fourier Treransform IR

  • Use superposition to turn polychromatic light into monochromatic light
  • Use half-transparent mirror to get transmitted and reflected beam
  • Have fixed mirror and movable mirror
    • Which gnerates a path difference between the beams 
  • Only 1 wavelength causes constructive interference at a ceratin position of the mirror
  • Resolution --> inverse of the maximum displacement of the mirror
  • The position of the mirror is known using a laser (Ne laser)
    • fast , high signal to noise
  • To read data quickly must use a higher noise detector ( pyroelectric blometer)
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Sample preparation and H-bonding in FIR

Sample preparation

  • Support, container and solvent should be transparent to IR e.g. NaCl and KBr
  • Liquids : use NaCl plates, aqueous: use H2O insoluble material
  • Solids: powder into mull (using oil, which may interfere), then compress in disc, dissolve in voltaile solvent

Effects of Hydrogen bonding

  • Change wavenumber that certain bonds appear ( C =O and N-H)
  • once bonds are broken ( at high temperature), the wavenumber changes back
  • Can use Synchrotron to produce finely focused IR imaging
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Intro to Ramen

  • Complementary to IR as weak absorption in IR is strong in Ramen ( and vice versa)
  • light scattering occurs


  • E-feld creates dipole between electron and nuclei
    • source of EM, emits scattered light
  • Intensity dependent on the polarisability of a bond
  • Symmetric bonds give strong signal has they have high polarisability
  • Rayleigh scattering: electrons excited to virtual energy then drops back
  • Stokes Ramen: excited to virtual state then back down to vibrational state
  • Anti-stoke: from thermal state (not ground) to virtual state and then back to ground
  • The energy transition in ramen and IR for a particular bond type is the same
  • Can get fluoresene interferring with the signal
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Ramen: Fluorescence, lasers, Sample Preparation


  • Exitation to real energy sate, then to lower E state
  • Photon emission from this can overtake small Raman signals

Laser: need highly monochromatic light to isolate wavenumbers


  • Laser shine on sample , use monochromator (mirrors)  perpendicular to the sample to detect one wavelength

Sample preparation

  • Liquids and aqeous solutions: focus laser on small spot, use capillaries, quartz cells, light piping
  • Solids: collect back scattered light; signal from top surface 
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