Raman Spectroscopy

1
Raman Spectroscopy

• Physical Biochemistry, November 2006
• Dr Ardan Patwardhan, a.patwardhan_at_imperial.ac.uk,
  Faculty of Natural Sciences, Imperial College
  London

2
Polarizability in an electric field

• An electric field will distort the molecular
  orbital
• This is a weak effect that grows with the square
  of the intensity

3
Induced electric dipole moment

• An electric field can distort the electron cloud
  of a molecule, thereby creating an induced
  electric dipole moment
• The oscillating electric field associated with EM
  radiation will therefore create an oscillating
  induced electric dipole moment which in turn will
  emit, i.e. scatter, EM radiation

4
Raman scattering
Molecule
Scattered photon hnsc
Excitation photon hnex

• Rayleigh scattering elastic interaction, no
  non-kinetic transfer of energy between molecule
  and photon, nsc ? nex
• Raman scattering inelastic interaction, transfer
  of energy between molecule and photon, nsc ? nex
• Stokes lines Energy of molecule increases, nsc lt
  nex
• Anti-stokes lines Energy of photon increases,
  nsc gt nex

5
Energy level diagram

• Virtual energy levels do not correspond to an
  actual absorption

6
Raman spectra

• Transitions between vibrational/rotational levels
  will lead to spectral lines on either side of the
  excitation line
• The spectra on the stokes side will be more
  intense than the spectra on the anti-stokes side
  due to a significant difference in population
  between vibrational levels
• The difference in energy between these lines and
  the central excitation line corresponds to
  energies for pure transitions

Stokes
Anti-stokes
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Raman versus IR

• Aqueous solutions can be used
• Any wavelength can be used
• Variation in polarization is a requirement rather
  than variation in dipole moment ? some normal
  modes that do not show up in IR spectra may show
  up in Raman
• Good examples are homonuclear diatomic molecules,
  e.g. oxygen !!
• Some normal modes are invisible to both IR and
  Raman spectroscopy!!!

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A comparison of spectra
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A Raman Spectrometer

• The required high intensities require laser
  illumination that is focused onto the specimen
• Light from the illuminated spot is collected
  using a lens and scanned using a monochromator

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Main Points

• A vibration must involve a variation in the
  polarization of the molecule for it to be visible
  in Raman spectroscopy
• The vibrational spectra is centred at the
  wavelength of the source
• Aqueous solutions can be studied
• Can provide information complementary to IR
  spectroscopy