In: Chemistry
Draw the internuclear potential as a function of nuclear separation for the ground and excited states of a molecule where the equilibrium separation of the (bound) excited state is smaller than that in the ground state. Draw in the vibrational levels for each state, the electronic transition from the ground vibrational state, and the dissociation energies for each electronic state. What should the spectrum look like?
The Franck-Condon Principle describes the intensities of vibronic transitions, or the absorption or emission of a photon. It states that when a molecule is undergoing an electronic transition, such as ionization, the nuclear configuration of the molecule experiences no significant change. This is due in fact that nuclei are much more massive than electrons and the electronic transition takes place faster than the nuclei can respond. When the nucleus realigns itself with with the new electronic configuration, the theory states that it must undergo a vibration.
Since electronic transitions are very fast compared with nuclear motions, vibrational levels are favored when they correspond to a minimal change in the nuclear coordinates.