GITAM, Department of Engineering Physics

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The processes of spontaneous emission and spontaneous absorption

• If energy is absorbed by an atom, the electrons are excited into vacant energy shells

  • the absorbed energy is equal in magnitude to the difference in energy between shells

    
    W = |Wu-Wl|      ABSORBED ENERGY
    

    where W_l is the energy of the initial (lower) state and W_u is the energy of the final (higher) state

 

 

• The excited atom can SPONTANEOUSLY (randomly) de-excite to a lower level, emitting a photon 

  
  W = |Wu-Wl |     EMITTED ENERGY
  

• But the photon energy has also been given as

  
  Wph = W = h = hc/       PHOTON ENERGY
  

• We clearly see that the energy of a photon is defined by

  • the energy emitted when an electron drops from an excited energy shell to one of lower energy

  • this energy is, in turn, related to and defines the frequency (or wavelength) of the emitted photon

 

Stimulated emission

• Lasers are unique sources of light

  • they do not behave like conventional thermal light sources

   

• For atomic systems in thermal equilibrium with their surroundings, emission of light is the result of two main processes:

  • ABSORPTION of energy

  • SPONTANEOUS EMISSION of energy

   

• A third mechanism also exists:

  • although not a dominant process in thermal systems at room temperatures, it is crucial to the formation of LASER action

  • this process is known as STIMULATED EMISSION

 

 

 

 

 

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