Identification

• Electromagnetic radiation is emitted or absorbed in discrete packets called photons. The energy of an individual photon is given by the formula E = hv, where "E" is energy, "h" is Planck's constant and "v" is the frequency of the radiation. Infrared radiation has a lower frequency than visible light, and hence less energy. The energy of infrared photons is typically insufficient to excite electrons but can cause vibrational excitation of atoms and/or groups in molecules.
  
  

Function

• The frequencies of infrared radiation absorbed corresponds to the kind of vibrational motions characteristic of their structure and composition. Since molecules with different structures will absorb different frequencies of infrared light, by bouncing infrared light off of molecules on a surface and checking to see which frequencies are absorbed chemists can identify a compound and deduce certain facts about its structure. This technique is called IR reflection absorption spectroscopy.
  
  

Considerations

• Reflection absorption spectroscopy is used to study compounds that are adsorbed on a surface. The infrared light is generally shone on the surface so that it only grazes it. The signal is generally weak but some instruments are sensitive enough to make accurate measurements with this technique.