Dr. Yogita Sahebrao Thakare provides an introduction to photochemistry and molecular spectroscopy in her document. She discusses key topics like electromagnetic radiation, parameters of electromagnetic radiation like wavelength and frequency, and principles of photochemistry such as the Grotthuss-Draper and Stark-Einstein laws. She also covers characteristics of electromagnetic radiations and important formulae related to wavelength, frequency, velocity, and energy of electromagnetic waves.

1. Dr. Y. S. THAKARE
M.Sc. (CHE) Ph D, NET, SET
Assistant Professor in Chemistry,
Shri Shivaji Science College, Amravati
Email: yogitathakare_2007@rediffmail.com
B Sc- III Year
SEM-V
PAPER-III
PHYSICAL CHEMISTRY
UNIT- V and VI
Introduction to syllabus
Photochemistry
Electromagnetic radiation
Parameters of electromagnetic radiation
29-August -20 1

2. Unit V
Photochemistry
Introduction, Electromagnetic radiation, parameters of
electromagnetic radiation, spectrum of electromagnetic
radiation, problems, Interaction of radiation with matter, types
of chemical reaction- thermal reaction and photochemical
reaction, laws of absorption of light- Lambert law and beers
law, problems, laws of photochemistry- Grotthus Draper law
and stark Einstein's law of photochemical equivalence,
Quantum yield, experimental arrangement, reasons for high
and low Quantum yield, problems, photochemical reactions,
kinetics of photochemical decomposition of HI,
photosensitized reaction, mechanism of photosensitization,
Joblonski diagram, chemiluminescence, bioluminescence,
problems.
Electrode – The metal rod dipped in its salt solution29 -August -20 Dr. Yogita Sahebrao Thakare 2

3. Unit VI
Molecular spectroscopy
Molecular spectroscopy, electromagnetic radiation, parameters of
electromagnetic radiation, spectrum of electromagnetic radiation,
problems, types of spectra, degree of freedom, energy level
diagram, rotational spectroscopy, expression for moment of inertia
for diatomic rigid rotator, rotational quantum number, origin of line
in rotational spectrum, isotopic effect, applications of rotational
spectra for microwave spectroscopy, problems, Vibrational
spectroscopy, condition for IR active diatomic molecule,
expression for vibrational energy of diatomic molecule considering
it as a simple harmonic oscillator, zero point energy, relationship
between vibrational frequency and force constant, problems
Raman spectroscopy, characteristics of Raman spectrum, origin of
stokes and antistokes line in Raman spectrum, rotational Raman
spectrum vibrational Raman spectrum, problems
29 -August -20 Dr. Yogita Sahebrao Thakare

4. Unit V
Photochemistry
Photochemistry : Branch of chemistry which deals with the study of chemical
reactions that proceed with the absorption of light by atoms or molecules.
Example: photosynthesis, the degradation of plastics and the formation of vitamin D
with sunlight.
Principles
Light is a type of electromagnetic radiation, a source of energy. The
Grotthuss- Draper law states that light must be absorbed by a chemical substance in
order for a photochemical reaction to take place. According Stark-Einstein law in
primary photochemical reaction each molecule of reacting substance absorbs one
quantum of radiation which leads to photochemical change. For each photon of light
absorbed by a chemical system, no more than one molecule is activated for a
photochemical reaction.
29-August -20 Dr. Yogita Sahebrao Thakare

5. OR
Photochemistry deals with the study of photochemical reactions.
The photochemical reactions are caused by the absorption of light
radiations (photons) from visible and ultra violet regions (200-800nm).
The photons supply the necessary energy to the reactants so then the
excited reacting molecule reacts to produce product.
29-August -20 Dr. Yogita Sahebrao Thakare

6. 29-August -20 Dr. Yogita Sahebrao Thakare

7. Electromagnetic radiations
Electromagnetic Radiation
Electromagnetic radiation is a form of energy that is transmitted through space with
velocity of light (C = 3 × 108 ms-1). Electromagnetic radiation is the combination of
electric & magnetic fields which are perpendicular to each other and also
perpendicular to direction of propagation ( Fig. 1).
Thus electromagnetic radiation can be considered as a simple harmonic wave
prorogated in space, in a straight line. Electromagnetic radiations are spread over a
range of wavelengths. Visible light, UV light, IR radiations, X-ray,
29-August -20 Dr. Yogita Sahebrao Thakare

8. 29-August -20 Dr. Yogita Sahebrao Thakare

9. 29-August -20 Dr. Yogita Sahebrao Thakare

10. Electromagnetic radiations exhibit dual nature (Wave as well as particle)
some phenomena like reflection of light, refraction of light, interference
of light etc. could be explain only in terms of wave characters where as
other phenomena like blackbody radiations spectrum, photoelectric
effect, Compton effect etc. could not be explain in terms of wave nature.
However, the later phenomena could be explained in terms of particle
nature.
According Planck’s Quantum Theory of Radiation radiant energy emits
or absorbed not continuously (not as continuous wave) but
discontinuously or discretely in the form of small packets or bundles or
discrete unit of waves. Each of this unit is called a quantum (plural
quanta) which can exist independently.
The energy associated with each quantum or photon (in case of light) is
proportional to the frequency (ν) of emitted radiation,
i.e. E α ν or E = h ν29-August -20 Dr. Yogita Sahebrao Thakare

11. General characteristics of electromagnetic radiations:
1. All the electromagnetic radiations are associated with oscillating
electrical and magnetic vectors at right angle to each other and
also right angle to the direction of propagation.
2. All the electromagnetic radiations have the same speed in vacuum.
They travels in vacuum with speed of light.
3. Energy can be transmitted through space by electromagnetic
radiations.
4. Different types of electromagnetic radiations differ from one
another in their wavelength.
5. Since velocity of all types of electromagnetic radiations is constant,
the grater the frequency of radiations smaller is its wave length.
6. A electromagnetic radiation is characterized by the nature of its
wave which are characterized by wave length, wave number,
frequency, time period, amplitude wave front and velocity
29-August -20 Dr. Yogita Sahebrao Thakare

12. Parameters of EMR : Electromagnetic radiation can be characterized in terms
of following parameters-
I) Wavelength : Distance between two successive crests (maxima) or
troughs (minima) in an electromagnetic wave is defined as wavelength.
It is denoted by the Greek letter Lambda (λ).
Units of Wavelength
nm (nanometer), µm (micrometer), mm (millimeter), cm(centimeter) or m
(meter), Ao (Angstron)
1 nm = 10-9m, 1 µm = 10-6m, 1 mm = 10-3m,
1 cm = 10-2m, 1 Ao = 10 -8 cm 1=10-10 m
II) Frequency : The number of waves passing through a given point in unit
time is called as the frequency of radiation.
It is denoted by the Greek letter ν (nu) and has a unit of reciprocal time.
It is generally expressed in cycles per second or in Hertz or kilocycles per
second or megacycles per second.
1 Hz = cycle / second or S-1
1 MHz = 103 KHz = 106 Hz
29-August -20 Dr. Yogita Sahebrao Thakare

13. III) Wave number :- It is defined as the number of waves per unit length and it is
denoted by ν (nu bar). In spectroscopy, wave number is usually expressed in terms of
cm-1 or m-1
ν =
1

=
1
𝑤𝑎𝑣𝑒𝑙𝑒𝑛𝑔𝑡ℎ
But ν =
𝐶

OR  =
𝐶
ν
OR
1

=
ν
C
OR ν =
ν
C
IV) Velocity: - The distance travelled by the wave in one second is called the velocity
of the wave. It is denoted by C and expressed as centimeter per second (cm/s) or
meter per second (m/s).
The product of wavelength and frequency
gives the velocity of the wave
i.e. C = ν. 
Where C= Velocity of light in vacuum
But for velocity in air C=V
Where V= velocity of light in air
OR V= ν. 
29-August -20 Dr. Yogita Sahebrao Thakare

14. V) Energy (E) :- The energy (E) associate with quantum of electromagnetic
radiation is given by
E = h ν = h C / = h Cν (It is called as energy per
Where ,
h = planck`s constant (h= 6.62 × 10-34 JS),
ν = frequency (Hz)
 = Wavelength of EMR (m)
C = Velocity of light (C = 3 × 108 m/s)
The energy of radiation can be calculated in earg mol-1 or J mol-1 or Kcal mol-1
Energy per mole or energy in Einstein is given by
E = NA h ν = NA h C / = NA h C ν
Where, 𝑁𝐴 = 𝐴𝑣𝑜𝑔𝑎𝑑𝑟𝑜𝑠 𝑛𝑢𝑚𝑏𝑒𝑟 = 6.023 × 1023
𝑚𝑜𝑙−129-August -20 Dr. Yogita Sahebrao Thakare

15. Impotant Formulae:
1. 𝜈 =
1

=
1
𝑤𝑎𝑣𝑒𝑙𝑒𝑛𝑔𝑡ℎ
2. 𝜈 =
𝐶

3. E = h 𝜈
4. E = h 𝜈 = h c / = hc 𝜈
5. h = plank constant= 6.626 x 10−34 𝐽𝑆
6. 1nm = 10−9m
7. 1𝐴0
= 10−10
𝑚
8. 1 𝐽 = 107
𝑒𝑎𝑟𝑔 = 0.239 𝑐𝑎𝑙
9. 1 𝑐𝑎𝑙 = 4.184 𝐽
10. 1 𝑒𝑉 = 1.6021 × 10−19 𝐽
11. 𝑁𝐴 = 𝐴𝑣𝑜𝑔𝑎𝑑𝑟𝑜𝑠 𝑛𝑢𝑚𝑏𝑒𝑟 = 6.023 × 1023
𝑚𝑜𝑙−1
29-August -20 Dr. Yogita Sahebrao Thakare