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Franck -Condon Principle

The distribution of intensity among the bonds of on electronic bond system varies greatly from one molecules to other and from one bond system to another of the some molecule.


Let us take three molecules O, CO, I2 molecules:

  • In case of O2 the (O,O) bond is maximum intense while the successive bonds of progression appears rapidly decresaing.
  • In the specturm of CO the intensity of bonds along the progessional first increase rapidly to a maximum and the gradually decreases.
  • In case of I2 molecules a long progression is observed the intensity rises gradually and the maximum reaches at very high value sometimes in the continumm.
Fig. Spectrum of O2, CO & I2

The different types of intensity distribution are explained by Franck-Condon Principle 

An electronic transition in a molecules takes place so rapidly compared to the vibrational motion of the nuclie that the instanteneous inter-nuclear distance and the velocity of the nucleus can be considered remaining unchanged during the electronic transition.

Another important factor to be considered is the distribution of probability density in various vibrational levels of each electronic state.
The most probable vibrational transition are those in which one of the two turning points of a vibrational level of the one electronic state lies of approximately the same internuclear distance as one of the two turning point of a levels of the other electronic states except in case of V =0  level for which the mid point is representing maxima.

Fig. Various vibrational levels 

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