The quantum mechanical formulation of this principle is that the intensity of a vibronic transition is proportional to the square of the overlap integral between the vibrational wavefunctions of the two states that are involved in the transition. LIGHT ABSORPTION AND FATE OF EXCITATION ENERGY: Franck-Condon Principle (by Govindjee, based on witch and Govindjee, ). Using the Franck-Condon principle, we are able to calculate the intensities of transitions between vibrational states of the electronic energy levels. Due to their.


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This is a largely unsolved question in chemistry, and points franck condon principle a future, more improved set of theories. For the Franck—Condon principle applied to phonon transitions, the label of the horizontal axis of Figure 1 is replaced in Figure 6 with the configurational coordinate for a franck condon principle mode.

Because the energy of single phonons is generally quite small, zero- or few-phonon transitions can only be observed at temperatures below about 40 kelvins.


See Zero-phonon line and phonon sideband for further details and references. Franck—Condon principle in solvation[ edit ] Figure 7.

Energy diagram illustrating the Franck—Condon principle applied to the solvation of chromophores. The parabolic potential curves symbolize the interaction energy between the chromophores and the solvent.

The Gaussian curves represent franck condon principle distribution of this interaction energy. Franck—Condon considerations can also be applied to the electronic transitions of chromophores dissolved in liquids.

What is the Frank-Condon principle? - Quora

In this use of the Franck—Condon metaphor, the vibrational levels of the chromophores, as well as interactions of the chromophores with phonons in the liquid, continue to contribute to the structure franck condon principle the absorption and emission spectra, but these effects are considered separately and independently.

Consider chromophores surrounded by solvent molecules. These surrounding molecules may interact with the chromophores, particularly if the solvent molecules are polar. When light is absorbed, each quantum is taken up by a single franck condon principle or molecule and the whole energy of the quantum is communicated to it.

The absorbing atom or molecule is thus excited, that is, lifted from its normal state of lowest energy highest stability to an excited, energy-rich state. According to Niel Bohr's theory of atomic and molecular structure, an atom or molecule can exist only in a series of discrete states of electronic energy.

This is represented below.

Franck–Condon principle | physics |

Ground state E 0 and two excited states E1, E2 of a molecule vibrational and rotational levels are not shown. The energy levels are indicated, as they usually are in franck condon principle, by properly spaced horizontal lines.

The lowest line represents the ground state, E0, of franck condon principle atom or molecule in which it exists in the absence of external activation. The higher lines E1 and E2 represent excited electronic states--the only kind of excited states possible in an atom.

In a diatomic or polyatomic molecule, one or several series of also quantized vibrational and rotational states are superimposed on each electronic state. If the molecule is complex, the various vibrational and rotational states lie very close together.


In the Born-Oppenheimer approximation the probability of a radiative franck condon principle is written as see eq. The integral over represents overlap of the vibrational wave functions.

Regardless of origin the model above was created using a Gaussian broadening The nuclear displacement between the ground and excited state determines the shape of the absorption spectrum.

Franck–Condon principle

Let us examine both a smaller and a large excited state displacement. Note that the zero-zero or S0,0 vibrational transition is much large in the case where the displacement is small. Franck condon principle maximum intensity will be achieved in higher vibrational levels as shown in the stick spectrum.

The absorption spectra plotted below all have the same integrated intensity, however their shapes are altered because of the differing extent of displacement of the excited state potential energy surface. Stick spectra, dressed with Gaussians, for the small to large displacements in harmonic oscillator system described above.

So the nature of the relative vibronic band intensities can tell us whether there is a displacement of the equilibrium nuclear coordinate that accompanied a transition.

When will there be an increase in bond length i.