TOPIC – COHERENT ANTI-STOKES
RAMAN SPECTROSCOPY (CARS)
S.P.C. GOVT. COLLEGE , AJMER
SEMINAR SESSION : 2021-22
SUBMITTED TO : DEPARTMENT OF CHEMISTRY
SUBMITTED BY : ANKIT JOSHI
M.Sc. PREVIOUS CHEMISTRY SEMESTER – 2nd

CONTENTS :-
Introduction
Theoretical Background
Some Basics Of Rayleigh And
Raman Scattering
Coherent Anti-Stokes Raman Spectroscopy
(CARS) :-
CARS Process
Advantages of CARS
Limitations of CARS
Applications of CARS
Summary

Introduction
 A spectroscopic technique based on coherent anti-Stokes
Raman scattering (CARS) was first demonstrated by Maker
and Terhune in 1965.
 CARS spectroscopy is a powerful technique which has been
widely applied essentially in interdisciplinary research fields on
the borders of biology, chemistry, physics, healthcare, defense,
remote sensing, forensics, material science and so on.
 The recent breakthrough achievements such as detection
bacterial spores, implementation of coherent Raman
microscopy, gas-phase thermometry of reacting and non-
reacting flows and many others have been its state-of-art
successes.

Theoretical Background
 We have read earlier that the probability or relative intensity of Rayleigh
lines, stokes and anti-stoke lines occurs inside solutions are in 1:10−3
:10−6
ratio.
 Anti-stock lines have the least probability of occurring in the solution
because when the ground state falls bellow the original level in the anti-stokes
lines, its probability decreases. And when we take its signal, it fails.
 Thus anti-stoke lines are so weak that’s why we cannot study it with the help
of normal Raman spectroscopy, So we need coherent anti-stokes Raman
spectroscopy (CARS).
 A simple comprehensive theory for CARS is introduced in this PPT. It
begins with CARS formulation and, based on obtained solutions, a newly
recognized (somewhat neglected in the past) effect such as enhancement in
coherent Raman spectroscopy at positive probe delay is introduced in detail.
At the end of this section, the experimental observations are elucidated with
the CARS formulation.

SOME BASICS OF RAYLEIGH AND
RAMAN SCATTERING
 The source of energy in Raman spectroscopy is laser, so
laser is used in Raman spectroscopy.
 Scattering of light – When sunlight
enters the atmosphere of the earth.
The atoms and molecules of different
gasses present in the air absorb the
light. Then these atoms re-emit light
in all directions. This process is known
as scattering of light.
 The atoms or particles that scatter light are called scatters.
 Radiation – Radiation is energy or particles that comes
from a source and travel through space at the speed of light.

TYPES OF SCATTERING
𝟏. Elastic scattering 2. Inelastic scattering
If the energy of the
incident beam of
light and the
scattered beam of
light are same, then
it is called as ‘elastic
scattering’.
If the energy of the
incident beam of light
and the scattered
beam of light are not
same, then it is called
as ‘inelastic
scattering’.

𝑹𝒂𝒚𝒍𝒆𝒊𝒈𝒉 scattering Raman scattering
Rayleigh scattering is the elastic
scattering process in which the
electromagnetic radiation is
elastically deflected by particles
of matter, without a change of
frequency but with a phase
change.
Raman scattering is the
inelastic scattering of photons
by matter, meaning that there is
both an exchange of energy and
a change in the light’s direction.
Raman Stokes scattering
When the energy of the
scattered photons is less than
the incident photons, the
scattering is known as Raman
Stokes scattering.
When the energy of the
scattered photons is more than
the incident photons, the
scattering is known as Raman
anti-Stokes scattering.
Raman anti-Stokes scattering

hυ1
hυ1
hυ1
hυ2
hυ2-Δυ
hυ2+Δυ
Δυ
Incident
radiation
Transmitted
radiation

Coherent Anti-Stokes Raman Spectroscopy
(CARS) :-
 In this technique of coherent anti-Stokes
Raman spectroscopy (CARS), two sufficient
laser beams of high intensity with frequency
υ1 and υ2 are focussed on a sample. Mixing
in the sample generates a new coherent beam
of low intensity at a frequency 2υ1- υ2= υ3.
υ1 + υ1 − υ2 = 2υ1 − υ2
If frequency υ1 is fixed and the value of
frequency υ2 is varies such that :
υ1-υ2=υR
Then the scattered radiations are occurs
i.e. υ1+υR=υ3
Where υ1 = Incident radiations frequency
υ2 = Transmitted radiations frequency
υR = Raman active vibrational of molecular
system
υ3 = Relative frequency in υ1 Or Anti-stoke
Raman frequency

 The relative radiation frequency υ3 of υ1 is the anti-stokes
Raman radiation. Which is very intense and paired. This is
called coherent anti-stokes Raman spectroscopy (CARS).
 The intensity of a Raman spectrum can be increased by CARS.
 This technique is based on the fact that if two laser radiation
whose frequency are υ1 and υ2, is passed through a sample. So
they are paired in such a way that the coherent frequencies of
the different values can be obtained.
 One of these frequencies can be written as :-
υ’ = 2υ1 - (υ1 - Δυ)
υ’ = υ1 + Δυ
 This frequencies is the corresponding frequency of the anti-
stokes line. In this way, coherent radiation of high intensity
will be obtained which are called coherent anti-stokes
Raman spectroscopy (CARS).

CARS Process
 The CARS process consists of
two coherent laser radiation beams
υ1 and υ2 , which are almost collinear
in the molecular medium.
 If the frequency υ2 of the other
beam is varied by keeping the
frequency υ1 constant of one beam.
Then the frequency difference between the two beams come.
Which is equal to Raman shift υR.
Due to which a third beam υ3 is produced which is called
coherent anti-stokes Raman spectroscopy (CARS).

 This is almost parallel to the incident beam(υ1) and
It is also paired with them.
υ3 = υ1 + (υ1-υ2)
= 2υ1 - υ2
or υ3 = υ1 + υR
 The υ3 beam can be separated from the incident
beam (υ1) by filtering.
 The CARS process depends on the square of the
normal Raman scattering and the square of the
number of molecules.

Advantages of CARS
I. CARS signals are stronger by 8-10 orders of magnitude than
normal Raman experiment.
II. CARS signal can be easily visualized compared to
fluorescence.
III. Scattering intensity is increase enormously. ( It is non linear
technique. It reveals high resolution which is determined by
line width of lasers.
IV. Even microquantities (10−5- 10−7) can be detected by
resonance CARS.
V. High Raman conversion efficiencies are obtained.
VI. Excellence collection efficiency is possible since CARS is
generated as a beam.
VII. Narrow spectra are obtained without the need for a
monochromator.

Limitations of CARS
I. Requires complicated set up, difficult adjustment
costly equipment.
II. Spectra evaluation is non-trivial.
III. Band shapes are often distorted.
IV. Samples may decompose by high power laser beam
focusing.
V. Signal fluctuation are caused by frequent
instabilities of the lasers.
VI. No quantitative conclusion can be drawn from
signal intensity.

Applications of CARS
I. Molecular structure determination is important tool.
II. To study the rotational spectrum of gases. Even difference between υrot.
& υvib. and derivation from Boltzmann distribution can be analysed.
III. Gas phase combustion process are mostly studied by CARS. (CO2, CO,
O2, N2, CH4, H2O, H2 etc. in flames can be studied).
IV. Excellence technique for studying biological samples in aqueous
solution which often produce strong fluorescence in normal Raman
scattering.
V. Enhancement of CARS signals occurs of radiation approach the
electronic absorption frequency of the system.
VI. Medicinal samples like ferrocytochrome, cyanocobolamin have been
studied in dilute (10−5M) aqueous solution.
VII. Numerous application of CARS in molecular beam studies energy
transfer and plasma diagnostics are excepted.

Summary
 This ppt presented a brief overview to the basics of coherent anti-
Stokes Raman spectroscopy. First we introduced the CARS technique
and its strengths and barriers. Namely, we presented the integral
formulae for coherent anti-Stokes and Stokes Raman scattering, and
discussed the closed-form solutions, its complex error function, and
the formula for maximum enhancement of the inferred pure coherent
Raman spectra. The time-resolved coherent Stokes Raman scattering
experimental observations were also quantitatively elucidated as an
example. Moreover, various experimental realizations of narrowband
probe pulses were illustratively explained. Finally, several experimental
data were presented and discussed based on all Gaussian approach
presented in this review. Understanding the essentials of coherent
Raman spectroscopy promotes importance of a number of experiments
including the ones utilizing a broadband excitation with a narrowband
delayed probing for successful background suppression emphasized in
this work.

References :
FUNDAMENTALS
OF MOLECULAR
SPECTROSCOPY
BY P. S. SINDHU