The document summarizes the Franck-Condon principle, which states that during an electronic transition between two states of a molecule, the transition occurs so rapidly that the positions of the nuclei remain almost unchanged. It describes the different types of molecular energy levels and vibrational transitions. It also provides three cases that illustrate how the Franck-Condon principle determines the likelihood of vibrational transitions between different electronic states based on the equilibrium internuclear distances in each state.

1. Samrat Prithviraj Chauhan Government College
Ajmer
2020-2021
FRANCK CONDON
PRINCIPLE
Submitted By
Bhuvneshwari
Saini
M.Sc. Chemistry
Semester 2nd
Department of chemistry

2. Table of Content
 Introduction
 Energy Levels
 Vibrational Transition
 Frank Condon Principle
 Explanation of FCP
 Reference

3. Introduction
 Franck Condon Principle was proposed by
German physicists James Franck and U.S.
Physicists Edward U. Condon in 1926.
 Before the development of the Schrödinger
equation Franck put forward qualitative
arrangements to explain the various type of
intensity distribution found in vibronic transition in
a molecule take place much more rapidly than a
vibrational transition.
James Franck Edward U. Condon

4. Energy levels
 A quantum mechanical system or particle that is bond- that
is confined spatially can only take on certain discrete
values of energy called energy levels.
 Types of Energy levels :
1. Translation Energy: It is generated by the translation
motion of the molecule.
2. Rotational Energy: It is generated by the rotational
motion of the molecules. It means it is generated due to
the rotational form of centre of gravity of the molecule.
Minimum rotational energy-0.005eV. Selection Rule
∆J=+1,-1. Example – Rigid Rotor

5. 3. Vibrational Energy: It is generated due to the
vibration of the atom of the molecule with respected to
each other.
Vibrational Energy= 0.1eV. Selection Rule ∆J=+1,-1
and ∆V=+1,-1
Example- Harmonic Oscillator

6. 4. Electronic Energy :The electronic energy levels are
quantized and discrete and the energy gap between
the highest occupied and lowest unoccupied orbital
determines the large degree of their stability,
reactivity, and electronic properties.

7. Molecular Spectra

8. Vibrational Transition
 Vibrational Transition of a molecule refers to the
moment of the molecule from one vibrational level to
another.
 We can also name it as vibronic transition
 This type of transition occurs in between different
vibrational levels of the same electronic state.

9. Franck Condon Principle
 It states that an electronic
transition take place so
rapidly that a vibrating
molecule does not change
its internuclear distance
appreciably during the
transition.
 This principle is, to a first
approximation, true since
the electrons move so
much fast than the nuclei
that during the electronic
transition than nuclei do
not change their position. Internuclear Distance
Energy

10. The most likely place
to find and
oscillating object is
at its turning point.
The nuclei do not
have a chance to
move. The transition
represented by
“VERTICAL ARROW”
on the diagram.
In the ground state
the molecule is most
likely in v=0

11. Explanation of FCP
 CASE 1: Here the excited
electronic state has a slightly
smaller internuclear separation than
the ground state. A vertical
transition form the v"=0 level will be
more likely to occur into the upper
vibrational state v'=2, transition to
lower and higher state being less
likely; in general the upper state
most probably reached will depend
on the difference between the
equilibrium separations in the lower
and upper states. In case of CO
molecule this type of band system
is observed.
Internuclear distance
Energy
Intensity

12.  CASE 2:When upper
electronic state having the
same equilibrium
internuclear distance as the
lower. Now the FCP
suggests that a transition
occurs vertically on this
diagram since the
internuclear distance does
not change. Thus the
strongest spectral line of the
v" =0 progression will be the
(0,0) . This observed in O₂
molecule.
Internuclear distance
Energy
Intensity

13.  CASE 3: When the upper
state separation is drawn as
considerably greater than that
of the lower state. We see
that. The vibrational level to
which a vertical transition take
place has a high v' value,
here transitions can occur to a
state where the excited
molecule has energy in
excess of its own dissociation
energy. From such states the
molecule will dissociate
without any vibrations, since
the atoms which are formed
may take any value of kinetic
energy the transition are not
quantised and a continuum is
observed. This type of band
system is observed in I₂
Internuclear distance
Energy
Intensity

14. Reference
 Atomic and Molecular Spectra: Laser
by Raj Kumar
 Fundamentals of Molecular Spectroscopy
by C.N. Banwell
 Modern spectroscopy
by J.M. Hollas (Wiley)

15. Thank you