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Visible and ultraviolet spectroscopy (Read 1692 times)
Gerrit-Jan Linker
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Visible and ultraviolet spectroscopy
13.09.09 at 12:23:42
 
Visible and ultraviolet spectroscopy
 
Energy range: 10E+4 to 10E+5 cm-1. (1 kJ/mol = 12cm-1)
Near infrared through to the visible to the ultraviolet.
 
Various types of electronic transition occur at these energies. Consider two atoms A and B, neighbouring atoms in some kind of a solid (e.g. anion/cation in an ionic crystal).  
Four basic types of transition:
 

  • 1. Promotion of an electron from a localised orbital on one atom to a higher energy  localised orbital on the same atom.
    Transitions include d-d or f-f transitions.
     
  • 2. Promotion of an electron from a localised orbital on one atom to a higher energy localised orbital on an adjecent atom. Charge Transfer.
     
  • 3. Promotion of an electron from a localised orbital on one atom to a delocalised energy band, the conduction band which is characteristic of the entire solid. Photoconduction.
     
  • 4. Promotion of an electron from one energy band (the valence band) to another band of higher energy (the conduction band). Band gap determination.

 
A typical UV/VIS absorption spectrum shows 2 principal features.  
 
Absorption edge:
Above a certain energy or frequency known as the absorption edge, intense absorption occurs decreasing the tranmittance of the sample to essentially zero.  
Transitions of types 2 and 3 are generally responsible for the absorption edge.
 
Reflectance techniques must be used to investigate the sample at these energies.
 
Broad adsorption bands:
The second principal feature is the appearance of broad absorption peaks or bands at frequencies below that of the cut-off. These are generally associated with type 1 transitions.
 
 
Source:
Basic solid state chemistry, Anthony R. West
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« Last Edit: 13.09.09 at 12:51:46 by Gerrit-Jan Linker »  

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