This very important Concept in Electronic Spectroscopy / U.V spectroscopy.Useful for UG ,PG Chemistry ,pharmacy and B.Tech students.

1. Achyuth’s
Chemistry Bees
The Online
Chemistry
Classes
By
G . Achyuth kumar M.Sc., B.Ed.(M.Phil.)

2. Frank condon principle
U. V-VISIBLE SPECTROSCOPY

3. contents
History
Statement of Frank Condon Principle
Explanation using Morse Diagram
Application of Franck Condon Principle

4. History
 Frank-Condon principle was proposed by
German Physicist James Frank and U.S
Physicist Edward U. Condon in 1926

5. What is Frank Condon Principle?
Or
Statement of Frank- Condon principle
An electronic transition takes place so rapidly that
vibrating molecule does not change its
internuclear distance appreciably.

6. Explanation
 When Electronic transitions are takes place , the time scale
of transition is so fast( 10-16s) compared to nucleus motion
(10-13s) that we can consider nucleus to be static(no change
in the position) and vibrational transitions from one
vibrational state to another state is more likely to be happen.
If these sates have a large overlap.
 The nuclear motions are negligible during the time required
for an electronic Excitation.

7. Explanation using Morse Diagrams
 The principle is clearly related to Born-Oppenheimer approximation
.
 Therefore during absorption of radiation the molecule is get excited
to different energy state is represented by different potential energy
curve .
 Thus every excited molecule is considered as a molecule which has
a slightly different Morse Potential energy curve using Morse
Diagram.
 There is no selection rule for ∆v and transition will occur at the
position where wave function is maximum.
 From Quantum mechanics we know that the molecule is in centre of
ground state( VII = 0).If the internuclear distance in Ground state
and excited state is equal then the most probable transition is VII = 0
to VI = 0 .

8. Explanation using Morse Diagrams
 Case:1

11. Applications
 Frank-Condon Principle can successfully explains the
intensities of UV absorption peaks or bands.
 It is also Explains the Intensities of spectral lines in
Fluorescence