Radiative electronic transitions (mainly dipolar) in the molecule occur very fast with respect to the re-adjustment time of the inter-atomic distance (R). Then, in spite of similar radial dependance of the potential energy curves for different electronic levels, these curves are shifted with respect to one another in function of the excitation energy.
So, The R values corresponding to a high
probability of presence of the electron in the 
state (first index correspond to the electronic energy state, second index
to the vibrationnal energy level) correspond to a low probability of
presence for the 
state (
).
As the transition probability is proportionnal to the square of the overlap integral between wave functions, transitions between states of same vibrationnal quantum number is highly unprobable.
So, during excitation, a 
electron in the 
state will be excited to a 
(
) and the
absorbed enrergy (
)
will be larger than the energy difference (
)
between the two aenergy levels 
and 
.
After excitation, the electron wil rapidly fall
down to the first vibrationnal level by non-radiative transitions.
after that, the radiative de-excitation will take place between the 
state to the 
state. (
)
The emitted energy (
)
will then be lower than 
.
Finally :
The difference between 
and 
explains the shift between emission and absorption spectra of a molecule
and is called the Stokes shift.
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