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Frank-Condon Principle - Potential Energy Surfaces

  1. Nov 7, 2014 #1
    Theory and my Understanding:

    So I understand how the frank condon principle lets us effect electronic transitions instantaneously, since the motion of nuclei (on the timescale of such electronic transitions) is quite slow.

    Consequently, when a photon of light is absorbed you can have an electron being promoted from the Sx0 to the Sy1 where S represents the singlet state and x and y are vibrational levels (x>y)

    My Question

    What effects how much the upper curve is translated over to the right-hand-side, with respect, to the lower curve ?

    My Interpretation

    When an electron is promoted from S00 to S21, for example, the electron is being put into an anti-bonding orbital consequently weakening bonds and leading to a greater vibrations.

    The more anti-bonding character the S1 state has the greater the amount by which the upper curve is translated over to the right-hand-side and hence the larger the vibrations.

  2. jcsd
  3. Nov 8, 2014 #2


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    Science Advisor

    Yes, your explanation sounds good.
  4. Nov 8, 2014 #3
    Thanks! I just have one last question, if you imagine a porphyrin ring versus simple benzene. It is my belief that the upper curve (see above diagram) is displaced more to the right in benzene.

    This is because a single electronic excitation in a massive porphyrin ring (a multi electron system) is unlikely to change the bonding character of the molecule by that much.
  5. Nov 8, 2014 #4


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    Science Advisor

    Yes, but it depends on the localization of the orbitals that are involved.
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