Advanced Analytical Chemistry

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Advanced Analytical Chemistry CHM 6157 Y.
CAI Florida International UniversityUpdated on
9/18/2008 Chapter 5 Raman Spectrometry

• Chapter 5 Raman Spectrometry
• Fundamental aspects
• 1.1 An example of Raman Effect

Raman Scattering
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Advanced Analytical Chemistry CHM 6157 Y.
CAI Florida International UniversityUpdated on
9/18/2008 Chapter 5 Raman Spectrometry

• 1.2 Background and brief history
• A beam of monochromatic light incident on a
  sample
• transmitted
• absorbed
• Scattered
• Most of the scattered light has the same
  wavelength as the incident light.
• A small fraction of the scattered light is
  shifted in wavelength by the molecular vibrations
  and rotations of the molecular in the sample. The
  shifts in wavelength depend upon the chemical
  structure of the molecular responsible for the
  scattering. The spectrum of this
  wavelength-shifted light is called a Raman
  spectrum.
• Many sharp bands that are characteristics of the
  specific molecules in the sample for
  qualitative analysis
• The intensity of a Raman spectrum is proportional
  to concentration for quantitative analysis

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Advanced Analytical Chemistry CHM 6157 Y.
CAI Florida International UniversityUpdated on
9/26/2006 Chapter 5 Raman Spectrometry

• This scattering phenomena was discovered by the
  Indian scientist C.V. Raman 1928
• C.V. Raman and K.S. Krishnan, A new type of
  secondary radiation, Nature, 1928, 121, 501

1888-1970
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Advanced Analytical Chemistry CHM 6157 Y.
CAI Florida International UniversityUpdated on
9/28/2006 Chapter 5 Raman Spectrometry

• 1.3 Raman effect

• A photon raises the energy of the molecule from
  the ground state to a virtual state.
• The increase in energy is equal to the energy of
  photon h?.
• The process is not quantized, just a distortion
  of the electron distribution of a covalent bond.
• Depending upon the frequency of the radiation
  from the source, the energy of the molecule can
  assume any of an infinite number of values,
  virtual states
• The molecule immediately relaxes back to the
  original electronic state by emitting a photon.
  

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Advanced Analytical Chemistry CHM 6157 Y.
CAI Florida International UniversityUpdated on
9/26/2006 Chapter 5 Raman Spectrometry
Rayleigh, John William Strutt 1842-1919

• Rayleigh scattering
• The molecule returns to the vibrational energy
  levels from which it started. No energy is
  transferred to the molecule.
• Raman Scattering
• Stokes Raman Scattering
• E h? - ?E
• Anti-Stokes Raman Scattering
• E h? - ?E

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Advanced Analytical Chemistry CHM 6157 Y.
CAI Florida International UniversityUpdated on
9/26/2006 Chapter 5 Raman Spectrometry

• Light Intensities

At thermal equilibrium, the fraction of the
molecules in one vibrational energy level
relative to another is given by the Bolzmann
distribution equation
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Advanced Analytical Chemistry CHM 6157 Y.
CAI Florida International UniversityUpdated on
9/28/2006 Chapter 5 Raman Spectrometry

• 1.4 What is light scattering
• The re-emission of light from the light-induced
  oscillation in the electron cloud is called
  scattering.

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Advanced Analytical Chemistry CHM 6157 Y.
CAI Florida International UniversityUpdated on
9/28/2006 Chapter 5 Raman Spectrometry

• 1.5 Raman scattering and infrared absorption
• Infrared absorption requires that a vibrational
  mode of the molecule have a change in dipole
  moment or charge distribution associated with it.
  Only then can radiation of the same frequency
  interact with the molecule and promote it an
  excited vibrational state.
• Scattering involves a momentary distortion of the
  electrons distributed around a bond in a
  molecule, followed by reemission of the radiation
  as the bond returns to its normal state. It is
  important to note that Raman scattering requires
  that the polarizability of a bond varies as a
  function of distance of the internuclear
  separation (r).

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Advanced Analytical Chemistry CHM 6157 Y.
CAI Florida International UniversityUpdated on
9/28/2006 Chapter 5 Raman Spectrometry

• IR measures the frequency of the absorbed light
• Raman measures the frequency shift between
  incident and emitted light
• Raman shift does not changes with incident light!

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Advanced Analytical Chemistry CHM 6157 Y.
CAI Florida International UniversityUpdated on
9/28/2006 Chapter 5 Raman Spectrometry

• Instrumentation
• 2.1 Sources
• High intensity of light sources are needed.
• Laser sources are normally employed.

Table Some Common Laser Sources for Raman
Spectroscopy
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Advanced Analytical Chemistry CHM 6157 Y.
CAI Florida International UniversityUpdated on
9/28/2006 Chapter 5 Raman Spectrometry

• Two advantages with near-infrared radiation
  sources
• Can operated at much higher power without causing
  photodecomposition of the sample.
• Reduce fluorescence interference because they are
  not energetic enough to populate a significant
  number of fluorescence producing excited
  electronic energy states in most molecules.
• One must choose the one that is not absorbed by
  the sample and solvent (except with resonance
  Raman measurement).

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Advanced Analytical Chemistry CHM 6157 Y.
CAI Florida International UniversityUpdated on
9/28/2006 Chapter 5 Raman Spectrometry