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Vibrational spectroscopy (Read 2436 times)
Gerrit-Jan Linker
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Vibrational spectroscopy
26.03.09 at 10:50:30
Vibrational spectroscopy
Vibrational spectroscopy provides detailed information on both the structure and the dynamics of molecules.  
Frequency range: 10–4000 cm–1 depending on the strength of the bond and the reduced mass of the vibrational mode.
Infrared and Raman spectroscopy each probe vibrational motion, but respond to a different manifestation of it.
In both IR and Raman spectroscopy the fundamental interaction is between a dipole moment and an electromagnetic field. Ultimately, the two can only couple with each other if they have the same frequency, otherwise the time average of their interaction energy is zero.  
Commonly used methods:

  • Infrared (IR) spectroscopy
    Infrared spectroscopy is sensitive to a change in the dipole moment as a function of the vibrational motion.
    The most important consideration for a vibration to be IR active is that its dipole moment changes upon vibration.
  • Raman spectroscopy
    Raman spectroscopy probes the change in polarizability as the molecule undergoes vibrations.
    Resonance Raman spectroscopy also couples to excited electronic states, and can yield further information regarding the identity of the vibration.
    The primary criterion for a Raman active mode is that the polarizability of the molecule changes as a function of vibrational coordinate. An atom or molecule placed in an electric field will acquire a dipole moment that is proportional to the size of the applied field and the ease at which the charge distribution redistributes, that is, the polarizability.

Less commonly used methods:

  • inelastic neutron scattering (INS)
  • helium atom scattering
  • electron energy loss spectroscopy (EELS)
  • photoelectron spectroscopy
  • others...  

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« Last Edit: 26.03.09 at 12:46:50 by Gerrit-Jan Linker »  

Gerrit-Jan Linker
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