178. ABSORPTION OF RADIATION(9) – FRANCK -CODON PRINCIPLE(3).

In the previous post, we studied the Franck-Codon principle, which states that an electronic transition takes place so rapidly, that a vibrating molecule does not change its internuclear distance appreciably in that time frame. In this post, we will discuss how these transitions take place.


RADIATIVE TRANSITIONS ARE VERTICAL AND PERPENDICULAR. When an electron absorbs energy in the UV – visible spectrum range, it gets excited from the lower energy level to a higher energy level. There is NO horizontal movement of the nuclei during such transitions. The electron goes up in a straight line at an angle of 90° only as shown below-

Thus, the Franck-codon principle is based on the fact that electronic transitions occur vertically in potential energy diagrams. The electronic transitions happen most favorably when the nuclear structures of the initial and final states are most similar. Also, the electron goes to a region where the probability of finding an electron is more. Let us try to understand this by looking at the figure below –

i) (0,0) transitions

The transition of electrons from v’=0 → v”=0 are termed as (0,0) transitions.These are the most favorable transitions. As seen in the figure above, in such transitions, the electron goes up straight from a high probability region(shown by a black peak) in the ground state (E0) to another high probability region in the excited state(E1).

ii) (1,0) transitions

The transitions of an electron from the v’ = 0 level of the ground state to the v”=1 vibrational level of the excited state are denoted as (1,0) transitions.

These transitions are not very favorable, as the electron moves from a high probability region in the ground state to a zero probability region in the excited state.

ii)v’=0 → v”=2 (2,0) transitions

Here, the electron travels from a high probability region(big peak) to a low probability region(small peak). Thus, the occurrence of these transitions is less too.

A graph of these observations would look like this –

The probability of the (0,0) transitions is the highest and thus we get a taller line for such transitions.

We end our discussion on the Franck-codon principle with this post. In the next post, we shall start discussing a new topic in photochemistry. Till then,

Be a perpetual student of life and keep learning…

Good Day!

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