1. Electronic spectroscopy relies on quantized energy states of electrons. Absorption of photons promotes electrons to excited states, and fluorescence occurs when electrons return to lower states. 2. Molecular electronic spectra involve changes in electronic, vibrational, and rotational energies of molecules. They appear in the visible and ultraviolet regions. 3. Potential energy curves describe different electronic states of diatomic molecules. Transitions between states emit or absorb radiation and give rise to band systems consisting of vibrational and rotational transitions within those bands.

1. Molecular Electronic Spectra Formation and
Electronic Band System
Presented By
Gaurav Yadav
M.Sc. Applied Physics
Amity University Rajasthan

2. Contents
1. Introduction
2. Silent feature
3. Formation of Spectra
4. Vibrational structure
5. Energy level diagram
6. References

3. Electronic Spectroscopy relies on the quantized nature of energy
states. Given enough energy, an electron can be excited from its
initial ground state or initial excited state (hot band) and briefly
exist in a higher energy excited state. Electronic transitions involve
exciting an electron from one principle quantum state to another.
Often, during electronic spectroscopy, the electron is excited first
from an initial low energy state to a higher state by absorbing
photon energy from the spectrophotometer. If the wavelength of
the incident beam has enough energy to promote an electron to a
higher level, then we can detect this in the absorbance spectrum.
Once in the excited state, the electron has higher potential energy
and will relax back to a lower state by emitting photon energy. This
is called fluorescence and can be detected in the spectrum as well.
Introduction:

4. Molecular Electronic Spectra
1. Electronic spectra are the most complex.
2. Appear in visible and ultravoilet region.
3. Involve change in electronic, vibrational and rotational
energy of the molecule.
4. All diatomic molecules exhibit electronic spectra.
5. Homonuclear molecules gives only electronic spectra.
Silent features

5. Formation of electronic spectra
When one of the atoms forming a diatomic molecule is
electronically excited, that is, its valance electron is pushed into an
orbital farther from the nucleolus, the molecule is said to be in an
electronically excited state. The various possible arrangements for
a given molecule form a pattern of allowed electronic states for the
molecule. For each of the electronic states there is a different
dependence of the molecule’s energy on its internuclear separation.
That is, each electronic state of the molecule has a different
potential energy curve characterized by a minimum at an
equilibrium internuclear distance re , dissociation limit De , a set of
discrete vibrational levels and sets of discrete rotational levels.

6. De''
υ00
Te' - Te'' = υe
re'
0
1
2
3
De'
3
2
1
0
Aexc
Inter Nuclear Distance
MoleculeEnergy
re is the equilibrium internuclear
distance
De is the dissociation limit
Te‘ is the electronic energy in upper state
Te'' is the electronic energy in lower state
Te' - Te'' = νe

7. 1. The two typical potential curves for a diatomic molecule are
shown in figure 1, the lower one representing the ground state
and the upper one representing the electronically excited state
of the molecule.
2. The curves differ in shape and size as well as in position of
minimum because the atoms are more loosely bound in the
excited states.
3. The curves showing the molecules potential energy as a
function of internuclear distance becomes shallower and
broader, and the equilibrium internuclear distance increases
with increasing electronic excitation.
4. The difference between two minima is Te' - Te'' = νe
5. When a transition of molecules from one electronic state to
other electronic state take place the emitted or absorbed
radiations fall in the visible or ultraviolet region.

8. The electronic transitions are accompanied by a no. of
transitions involving a vibrational level of upper and a
vibrational level of lower electronic state. Each such vibrational
transition is, in turn accompained by a no of transitions
involving a rotational level of the lower vibrational state (one
such emission transition has been shown in fig 1 ) the
rotational transition gives rise to a group of fine lines which
constitutes a “band” thus a band arises from a particular
vibrational transition, and the bands arising from all the
vibrational transitions constitutes a “band system”. Thus a
single electronic transition in a molecule gives rise to a band
system.

9. Vibrational Structure of Electronic
Band –System in Emission
Electronic transitions
Involve
changes
Electronic
energy
Vibrational
energy
Rotational
energy
Therefore wave numbers arising from an electronic transition are given by
ν = (Te' - Te'') + (G' - G'') + (F' - F'')
νe = (Te' - Te'') , a constant for a given electronic transition. G' and G'' corresponds
to vibrational levels associated with different electronic states, and so have
different .

10. Let us ignore the rotational structures of individual bands
(F’ - F'‘ = 0). Then the wave number corresponding to the
band origins of the systems are given by
ν0 = νe + G'(ν') - G''(ν'')
= νe + {ωe'(ν' +1/2) - ωe'χe'(ν' + 1/2 )2} – { ωe''(ν'' +1/2) -
ωe''χe''(ν'' + 1/2 )2
For an electronic transition, there is no restriction on the
change in the vibrational quantum no. i.e.
∆ν = unrestricted

11. ∆ν = +1
∆ν = 0 ∆ν = -1 ∆ν = +1
ν
Lower
electronic
state
Upper
electronic
state
4
3
2
1
0
ν'
ν''
∆G‘1/2
∆G'3/2
∆G''3/2
∆G'‘1/2
(Fig. 2)
Energy Level Diagram

12. 1. Transitions from each vibrational level of the upper
electronic state can take place to each of the vibrational
levels of the lower electronic state.
2. Large no. of bands from a single transition.
3. If the temp. of the emission source is higher than the room
temp. a good no of the vibrational levels of the upper
electronic state are appreciably populated and hence a
good no of emission band are obtained as shown in fig.
4. It is seen from the energy level diagram that the entire
band system can be divided into a no. of easily recognized
groups, each group consisting of a few close bands. These
are called the ‘sequences’ and the band of the sequence
line

13. Separation between the corresponding vibrational levels of upper electronic state
The second difference of successive vibrational quanta

14. Applications:
The two principal applications are structure determinations
and quantitative analysis.
The position and intensity of an electronic absorption band
provides information as to chemical structure. Such
absorptions normally are not as useful as infrared absorptions
because they do not give as detailed information.

15. References:
1. http://talkphysic.blogspot.in/2015/02/electronic-spectra-of-
diatomic-molecules
2. http://www.organicchemistry.com/electronic-spectra-in-
raman-spectroscopy
3. Atomic and molecular spectra: laser by Raj Kumar.
4. http://www.chemteam.info/Electrons/LightEquations1.html