Raman Spectroscopy
1
Dr. Shivendra Singh
Assistant Professor, SLAS
shivendrasngh0@gmail.com
OUTLINES (UNIT-3)
I.
Raman spectroscopy: Basic
II.
Classical and quantum theories of Raman effect
III. Pure rotational, vibrational and vibrational-rotational Raman spectra
IV. Selection rules
V.
Mutual exclusion principle
VI. Resonance Raman spectroscopy
VII. Coherent anti stokes Raman spectroscopy (CARS)
…SPECTROSCOPY
Physical
stimulus
Molecule
response
Detecting
instrument
Visual (most common)
representation, or
Spectrum
i.e. Spectroscopy may be defined as the study of the interaction of
EMR with matter.
I. RAMAN SPECTROSCOPY: BASIC
Ø In previous lectures we have learnt that the Infrared spectroscopy is used to
record the absorption of vibrational and rotational states of molecules.
Ø The origin of this absorption is the change of the dipole moment due to the
vibrational and rotational motion.
Ø However, homonuclear diatomic molecules neither posses the permanent
dipole moment nor induce dipole moment due to the vibration. Thus these
levels are inactive or forbidden in case of Infra red absorption.
Ø Raman spectroscopy is a tool to overcome this problem. Here we will learn the
origin of the Raman spectrum and its applications.
…RAMAN SPECTROSCOPY: BASIC
q Raman scattering is the inelastic scattering of a photon – change in photon energy
(collision phenomenon, not absorption).
q By nature weak effect (approximately 1 in 107 photons)
q This inelastic scattering by molecules was first discovered by Sir C. V. Raman in 1928.
q He was awarded Nobel Prize for this fundamental work in 1930.
ν’
ν0
ν0 = ν’
Raman Effect
…RAMAN SPECTROSCOPY: BASIC
ν’
ν0
Raman Effect
ν0 = ν’
ν0
ν0
ν’
E1
Eo
ν’
ν0
ν’
E1
Eo
E1
Eo
Stokes Lines
Rayleigh scattering
Anti-Stokes Lines
Inelastic Collision
Elastic Collision
Inelastic Collision
…RAMAN SPECTROSCOPY: BASIC
q When strong light of 19436 cm-1 is scattered by a material consisting of
molecules containing carbonyl group, its spectrum exhibits two rather weak
lines at 17786 cm-1 and 21086 cm-1 apart from the line at 19436 cm-1.
Most intense
§ Carbonyl group = 1650
ü 19436 -1650 = 17786
ü 19436 +1650 = 21086
Least
intensity
Frequency on lower
side is known as
Stokes lines and on
higher side is known
as anti stocks lines.
The experimental Raman spectrum shows that the intensities of the Stokes lines
are higher than that of the anti-Stokes lines.
…RAMAN SPECTROSCOPY: BASIC
§ When a parallel beam of monochromatic light goes through a gas or liquid or transparent
solid, a part of light is scattered in all directions.
§ The intensity of scattered light is inversely proportional to the fourth power of wavelength.
i. e.
Scattering α 1/λ4
§ It is found that the scattered light contains exactly the same wavelength as the incident
light. This scattering is called as Rayleigh scattering.
§ Apart from this wavelength, the scattered light also contains some weak additional lines.
As said earlier that this phenomenon was first discovered by Raman and his
collaborators and is known as Raman Effect.
§ A comparison of the wave numbers of these additional lines shows that these Raman
lines are independent of the wavelength of the incident light but depends on the nature of
scattering substance.
II. CLASSICAL THEORY OF RAMAN EFFECT
(POLARIZABILITY THEORY)
We know that when an atom or molecule is brought into an electric field E, an
electric dipole moment µ is induced in the system. The positively charged nuclei
being attracted towards the negative pole of the field, the electrons to the positive
pole. The magnitude of this induced dipole moment (µ) is proportional to the
electric field,
IµI = α IEI ........ where α (alpha) is known as the polarizability.
Except for the case of spherical symmetry, the magnitude of the induced dipole
moment depends on the direction of the electric field.
E = E0Sin2πνt
α = α0 + β Sin2πνvibt
ν = Frequency of rotation
α0 = Equilibrium polarizability
β = Rate of change of polarizability with vibration
…CLASSICAL THEORY OF RAMAN EFFECT
(POLARIZABILITY THEORY)
Ø It must be noted, however, that if the vibration does not alter the
polarizability of the molecule, then β = 0 and the dipole oscillates only
at the frequency of the incident radiation; the same is true of a rotation.
Thus we have the general rule;
“In order to be Raman active, a molecular rotation or vibration must
cause some change in a component of the molecular polarizability.”
Ø Let us now consider briefly the shapes of the polarizability ellipsoids;
…CLASSICAL THEORY OF RAMAN EFFECT
(POLARIZABILITY THEORY)
For example, in case of a diatomic molecule, the induce dipole moment
will be higher in magnitude when the electric field direction is along the
inter-nuclear axis than that of the perpendicular to the inter-nuclear axis.
Electric field
H–H
H
I
H
Raman
Active
Raman
Inactive
…CLASSICAL THEORY OF RAMAN EFFECT
(POLARIZABILITY THEORY)
Major axis
Minor axis
H–H
Raman
Active
Polarizability
H
I
H
Raman
Inactive
Size of Ellipsoid α 1 / λα
Ellipsoid
…CLASSICAL THEORY OF RAMAN EFFECT
(POLARIZABILITY THEORY)
H
IIIIIIII
O
H
O
H
H
Less polarizable
Hence
Large ellipsoid
…CLASSICAL THEORY OF RAMAN EFFECT
According to classical electromagnetic theory, when a dipole oscillates, it radiates
with the same frequency of oscillation. This is nothing but the Rayleigh scattering.
Now, if the molecule vibrates i.e., the internuclear distance changes; changes the
polarizability. Also this polarizabitity depends on the orientation of the molecule.
So the rotation of the molecule also changes the polarizability.
ü The experimental Raman spectrum shows that the intensities of the Stokes
lines are higher than that of the anti-Stokes lines.
ü Classical explanation does not provide information about intensity for both the
lines and thus inadequate for explaining the experimental spectrum.
QUANTUM THEORY OF RAMAN EFFECT
q Let us consider for a system, E0 is the
ground electronic state and υ′′ =0, 1,......
are the vibrational states of the ground
electronic state.
q If the light frequency ν0 incident on this
system, three cases may arises;
…QUANTUM THEORY OF RAMAN EFFECT
Case-I:
q Molecules absorb the light of frequency hν0 and go to the vibrational
state as shown by dashed line in the figure.
q The vibrational state is created by the light and molecular interaction
and exists as long as the light exists. Thus by definition, the lifetime of
the virtual state is very very small.
q Now the molecules will back to the ground vibrational state (υ = 0) and
will emit the same frequency ν0 . This is the same Rayleigh scattering.
…QUANTUM THEORY OF RAMAN EFFECT
ν’
ν0
ν0 = ν’
ν0
ν’
E1
Eo
Rayleigh scattering
Elastic Collision
Raman Effect
…QUANTUM THEORY OF RAMAN EFFECT
Case II
q Molecules are transferred to the vibrational state by light ν0 and these excited
molecules may come back to the higher vibrational state (υ′′ = 1). In this case
the emitted frequency is ν0 −νυ.
q According to the energy conservation, the energy will be lost from the incident
photon energy hν0 to excite the vibrational frequency of the molecule and thus,
the emitted photon energy
hνstokes = hν0 −hνυ
ü Radiation scattered with a frequency lower than that of the incidents beam is
referred as Stokes radiation.
…QUANTUM THEORY OF RAMAN EFFECT
ν’
ν0
ν0 = ν’
ν0
ν0
ν’
E1
Eo
ν’
E1
Eo
Stokes Lines
Rayleigh scattering
Inelastic Collision
Elastic Collision
Raman Effect
…QUANTUM THEORY OF RAMAN EFFECT
Case III
q In thermal equilibrium, the excited vibrational levels are also populated and
molecules there can also absorb light and go to the virtual state. While coming
back to the same vibrational state, this will emit frequency ν0.
q But if molecules come back to ground vibrational state (ν=0) then the emitted
frequency will be (ν0 + νυ).
q Again, according to the energy conservation, the photon energy adds up the
vibrational energy and emitted energy will be hνantistokes = hν0 + hνυ
ü Radiation scattered with a frequency higher than that of the incidents beam is
referred as Anti-Stokes radiation.
…QUANTUM THEORY OF RAMAN EFFECT
ν’
ν0
Raman Effect
ν0 = ν’
ν0
ν0
ν’
E1
Eo
ν’
ν0
ν’
E1
Eo
E1
Eo
Stokes Lines
Rayleigh scattering
Anti-Stokes Lines
Inelastic Collision
Elastic Collision
Inelastic Collision
WHY THE STOKES LINES ARE HIGHER IN INTENSITY
THAN THE ANTI-STOKES LINES
The population to these vibrational states depends on the Maxwell Boltzmann
distribution.
The relative distribution of the no. of
molecules in a given vibrational state.
Ø The origin of anti-Stokes lines is from the
higher vibrational levels and the population
is lower in those states than the ground
vibrational level.
Ø As discussed that intensity of transitions
lines not only depend on the transition
probability, but also depend on the
population of the initial state.
Ø This is the reason for the lower intensity of
anti-Stokes lines than the Stokes lines.
EXPERIMENTAL SETUP
A typical Raman spectrum of CCl4
III. PURE ROTATIONAL, VIBRATIONAL AND
VIBRATIONAL-ROTATIONAL RAMAN SPECTRA
…PURE ROTATIONAL RAMAN SPECTROSCOPY
MW: RIGID DIATOMIC MOLECULE
In the rotational region spectra are usually discussed in terms of
wavenumber, so it is useful to consider energies expressed in these units.
Means,
εJ =
EJ
h
=
J (J + 1) cm-1 (J = 0, 1, 2…..)
hc
8π2Ic
Where,
c = velocity of light in cm s-1
This is usually abbreviated as;
εJ = BJ(J+1) cm-1
Where B is the rotational constant (B = h/8π2IBc cm-1)
Selection rule: ΔJ = ± 1
…PURE ROTATIONAL RAMAN SPECTROSCOPY
Which will show: H2, N2 (Homo-diatomic
molecules) AND HCl, CO
εJ = BJ (J + 1) – DJ 2 (J + 1)2 cm-1
Centrifugal distortion const
(Not relevent in Raman)
εJ = BJ (J + 1)
cm-1
Selection rule: ΔJ = ± 2, 0
The transition ΔJ = 0 is trivial,
since this represents no change
in molecular energy and hence
Rayleigh scattering only.
We can ignore the selection rule
ΔJ = - 2, since for pure rotational
change, the upper state quantum
number must necessarily be
greater than that in the lower
state.
…PURE ROTATIONAL RAMAN SPECTROSCOPY
Combining, them, ΔJ = +2
with the energy levels,
ΔE = EJ’ = J +2 – EJ’’ = J
= B(4J+6) cm-1
Since ΔJ = +2, we may label these
lines S-branch line to the low
wavenumber side of the exciting line
(Stokes lines). While, if the molecule
losses energy to the photon the S
branch lines appear on the high
wavenumber side (Anti-Stokes lines)
The wavenumbers of the corresponding
spectral lines are given by;
s
=
=
ex
± ΔE
s
ex ± B (4J + 6)
…PURE ROTATIONAL RAMAN SPECTROSCOPY
For J=0,
= BJ(J+1) cm-1
For J=2,
= B(J+2) + B(J+2+1)
Therefore,
Δ
= B(J+2) + B (J+3) - BJ(J+1)
= B (J2 + 2J + 3J + 6 – J2 – J)
= B (5J - J + 6)
Δ
= B (4J + 6)
For 0 to 2 transition
= - B (4J + 6)
For 2 to 0 transition
…PURE ROTATIONAL RAMAN SPECTROSCOPY
= B (4J + 6)
For Stokes
For J = 0,
= 6B
For J = 1,
= 10B
For J = 2,
For J = 3,
For Anti-Stokes
4B
= 6B
For J = - 1,
= - 10B
4B
For J = - 2,
= 14B
= 18B
For J = 0,
4B
For J = - 3,
4B
4B
= - 14B
= - 18B
4B
…PURE ROTATIONAL RAMAN SPECTROSCOPY
6B
18B 14B
10B 6B
Stokes
6B
6B 10B 14B 18B
Anti-Stokes
…PURE ROTATIONAL RAMAN SPECTROSCOPY
ΔJ = 0
Stokes
ΔJ = + 2
4B
4B
18B 14B
4B
10B 6B
6B
6B
Anti-Stokes
4B 4B 4B
6B 10B 14B 18B
ΔJ = - 2
…PURE ROTATIONAL RAMAN SPECTROSCOPY
Question: Rotational constant for 14N2 is 2 cm-1. The wavenumber of radiation in
Raman spectra is 20487 cm-1. What is wavenumber of scattered Stoke line.
Solution:
First Stoke line
= V – 6B
= 20487 – 6 x 2
= 20475 cm-1.
6B
18B 14B
10B 6B
Stokes
6B
6B 10B 14B 18B
Anti-Stokes
…PURE ROTATIONAL RAMAN SPECTROSCOPY
Question: 3rd & 4th line of rotational Raman spectra of CO are separated by 8 cm-1.
The CO bond length is given bySolution:
4B = 8 cm-1 (given)
So, B = 2 cm-1
h
B = ________
8π2Ic
r=
h
√ ________
r=
16π2µc
8π2µc2
4
2
3
h
√ ________
1
4B
6B
Ans
6B
I = µr2
h
B = ________
8π2µr2c
18B 14B
10B 6B
Stokes
6B 10B 14B 18B
Anti-Stokes
…PURE VIBRATIONAL RAMAN SPECTRA
Selection rule: Δv = 0, ± 1, ± 2, ± 3, ± 4 …...
For Anhormonic oscillators;
E = (v + ½ )ωe - (v + ½ )2 ωeXe
For ±1; ΔE = ωe (1- 2Xe)
For ±2; ΔE = 2ωe (1- 3Xe)
For ±3; ΔE = 3ωe (1- 4Xe)
Hot bands = ωe (1- 4Xe)
Xe : Anhamonicity const
…ROTATIONAL-VIBRATIONAL RAMAN SPECTRA
Selection rule: Δv = 0, ± 1 and ΔJ = 0, ± 2.
At room temp., v = 0 to v = 1
1. Δv = 0 and ΔJ = 0 >> Rayleigh line
2. Δv = ± 1 and ΔJ = 0 >> Q-Branch
(PURE VIBRATIONAL); In the
middle of the spectrum; Single line
3. Δv = ± 1 and ΔJ = - 2 >> O-Branch
4. Δv = ± 1 and ΔJ = + 2 >> S-Branch
…ROTATIONAL-VIBRATIONAL RAMAN SPECTRA
Selection rule: Δv = 0, ± 1 and ΔJ = 0, ± 2.
…ROTATIONAL-VIBRATIONAL RAMAN SPECTRA
IV. SELECTION RULES
…SELECTION RULES
Rotational
Selection rule:
ΔJ = ± 1
Vibrational
Selection rule:
Δv = ± 1
Vibrational-rotational
Selection rule:
Δv = ± 1, ΔJ = ± 1
Anharmonic:
Selection rule:
Δv = ±1, ±2, ±3..
Pure Rotational Raman
Pure vibrational Raman
ΔJ = ±2, 0.
Δv = 0, ±1, ±2, ±3….
Rotational-vibrational
Raman
Δv = 0, ± 1
ΔJ = ±2, 0.
V. MUTUAL EXCLUSION PRINCIPLE
q For carbon dioxide, the bending and antisymmetric modes
are infrared active, while the symmetric stretch mode is
Raman active. This behaviour is typical of all centrosymmetric molecules.
q Modes that are infrared active are Raman inactive and vice
versa. This is the Rule of Mutual Exclusion, which states that
“no normal mode can be both infrared and Raman active
in a molecule that possesses a centre of symmetry”.
…MUTUAL EXCLUSION PRINCIPLE
In fact, even for molecules which do not possess a centre of
symmetry, symmetric modes are weak in infrared and strong
in Raman, whereas bending and asymmetric modes are
weak in Raman.
…MUTUAL EXCLUSION PRINCIPLE
q If there is no centre of symmetry, then some vibrations will
be both Raman and IR active.
q Hence, if some vibrations are observed to give coincident
Raman and IR absorptions, it is certain that the species
has no centre of symmetry.
q Also, all symmetric modes of centrosymmetric molecules
are strong in Raman, and this helps in identifying
symmetric bands. The symmetric bands can also be
identified by studying their polarization.
RAMAN VS IR
RAMAN VS IR
1. It is due to the scattering of
light by the vibrating molecules.
1. It is the result of absorption of
light by vibrating molecules.
2. The vibration is Raman
active if it causes a change in
polarisability.
2. Vibration is IR active if there is
change in dipole moment.
3. The molecule need not
possess a permanent dipole
moment.
3. The vibration concerned
should have a change in dipole
moment due to that vibration.
RAMAN VS IR
4. Water can be used as a
solvent.
4. Water cannot be used due to
its intense absorption of IR.
5. Sample preparation is not
very elaborate, it can be in
any state.
5. Sample preparation is
elaborate (Gaseous samples
can rarely be used).
6. Gives an indication of
covalent character in the
molecule.
6. Gives an indication of ionic
character in the molecule.
7. Cost of instrumentation is
very high
7. Comparatively inexpensive.
ADVANTAGES OF RAMAN OVER IR
ü Water can be used as solvent.
ü Very suitable for biological samples in native state (because water can
be used as solvent).
ü Although Raman spectra result from molecular vibrations at IR
frequencies, spectrum is obtained using visible light or NIR radiation.
ü =>Glass and quartz lenses, cells, and optical fibers can be used.
Standard detectors can be used.
ü Few intense overtones and combination bands => few spectral
overlaps.
ü Totally symmetric vibrations are observable.
ü Raman intensities α to concentration and laser power.
ADVANTAGES OF IR OVER RAMAN
q Simpler and cheaper instrumentation.
q Less instrument dependent than Raman spectra because
IR spectra are based on measurement of intensity ratio.
q Lower detection limit than (normal) Raman.
q Background fluorescence can overwhelm Raman.
q More suitable for vibrations of bonds with very low
polarizability (e.g. C–F).
VI. RESONANCE RAMAN SPECTROSCOPY (RRS)
…RESONANCE RAMAN SPECTROSCOPY
q By employing RRS it is possible to:
ü study reactions taking place at the surface of electrodes,
ü obtained structural information from deep within complex
biological molecules
ü determine the shapes of potential surfaces and molecular
geometries in excited states,
ü record spectra of species with half-lives on the micro-second
level
ü obtain highly accurate values for physical parameters such as
anharmonicity coefficients, and
ü monitor reactions on catalytic surfaces
Journal of Chemical Education, 54, 8, 1977
…RESONANCE RAMAN SPECTROSCOPY
Ø In the past the situation for Raman spectroscopists was not this
advantageous. Early work, in what is referred to as normal Raman
Spectroscopy (NRS), used a low pressure mercury arc as the source
of electromagnetic radiation.
Ø Experiments involving NRS usually required neat liquids or solutions
with concentrations greater than 0.1 M and rather large volume
sample in order to obtain reasonable signal to noise ratios.
Ø These limitations were largely overcome with the advent of laser
sources whose powerful input could be focused into a very small
volume of sample. Using the laser, samples in the µl and mM ranges
can he readily studied.
Journal of Chemical Education, 54, 8, 1977
…RESONANCE RAMAN SPECTROSCOPY
Ø Early Raman measurements were made on compounds,
which were selected such that the Hg exciting lines fell
far short of the first excited electronic states.
Ø This was necessary since absorption would result in local
heating effects and consequently sample decomposition.
Journal of Chemical Education, 54, 8, 1977
…RESONANCE RAMAN SPECTROSCOPY
Ø Now this difficulty is circumvented by using the rotating
sample technique combined with laser sources as first
suggested by Kiefer and Bernstein.
Ø Since the laser beam can he focused on only a small
portion of the rotating sample, local heating is minimized.
When the exciting line is "tuned" into an electronic
absorption band, some of the Raman bands which are
related to the electronic transition that is responsible for
the absorption will he greatly enhanced.
Journal of Chemical Education, 54, 8, 1977
…RESONANCE RAMAN SPECTROSCOPY
Ø Since the intensity of these lines can be tremendously increased
under resonance conditions, samples as dilute as 10-7M can be
studied.
Ø The fact that only certain Raman bands are enhanced when the
exciting line is in resonance with an electronic band imparts a
selectivity to the effect. This selectivity has some rather important
ramifications.
Ø The recent development of CW tunable dye lasers gives an added
dimension to the field. Now the electronic absorption band may be
"scanned" with different exciting frequencies much as has been
done in fluorescence spectroscopy for years.
Journal of Chemical Education, 54, 8, 1977
…RESONANCE RAMAN SPECTROSCOPY
ü An example of RR enhancement is given
in Fig.1, where the Raman spectrum of
1.0X10-4 M tris(o-phenanthroline)iron(II)
sulfate in water is shown as obtained with
four different exciting lines.
ü An internal standard of 0.5M SO4-2 gives
rise to the band a t 983 cm-1.
ü Clearly, as the source approaches higher
frequencies the Raman bands are
strongly enhanced.
Journal of Chemical Education, 54, 8, 1977
…RESONANCE RAMAN SPECTROSCOPY
Ø Deciding at which frequency the bands maximize is less
straightforward since intensity corrections must be made for
self-absorption, that is to say the reabsorption of Raman
scattered radiation that still falls within the absorption band of
the sample: detector sensitivity, since the detector response is
not constant over the range of frequencies studied, and the
scattering dependence of the normal Raman effect.
Journal of Chemical Education, 54, 8, 1977
…RESONANCE RAMAN SPECTROSCOPY
q In order to understand the
origins of the RR effect, we will
consider a molecule with just
two low-lying excited electronic
states e and s; and the
changes in intensity that occur
when the exciting frequency is
moved into resonance with the
lower state e.
Journal of Chemical Education, 54, 8, 1977
VII. COHERENT ANTI STOKES RAMAN
SPECTROSCOPY (CARS)
q Coherent anti-Stokes Raman spectroscopy, also called Coherent antiStokes Raman scattering spectroscopy (CARS), is a form of
spectroscopy used primarily in chemistry, physics and related fields.
q It is sensitive to the same vibrational signatures of molecules as seen
in Raman spectroscopy, typically the nuclear vibrations of chemical
bonds.
https://en.wikipedia.org
…COHERENT ANTI STOKES RAMAN
SPECTROSCOPY (CARS)
Ø Unlike Raman spectroscopy, CARS employs multiple
photons to address the molecular vibrations, and produces
a coherent signal.
Ø As a result, CARS is orders of magnitude stronger than
spontaneous Raman emission. CARS is a third-order
nonlinear optical process involving three laser beams: a
pump beam of frequency ωp, a Stokes beam of frequency
ωS and a probe beam at frequency ωpr.
…COHERENT ANTI STOKES RAMAN
SPECTROSCOPY (CARS)
Ø These beams interact with the sample and generate a coherent
optical signal at the anti-Stokes frequency (ωpr+ ωp - ωS).
Ø The latter is resonantly enhanced when the frequency difference
between the pump and the Stokes beams (ωp-ωS) coincides
with the frequency of a Raman resonance, which is the basis of
the technique's intrinsic vibrational contrast mechanism.
…COHERENT ANTI STOKES RAMAN
SPECTROSCOPY (CARS): PRINCIPLE
The CARS process can be physically explained by using either a
classical oscillator model or by using a quantum mechanical model
that incorporates the energy levels of the molecule.
Classically, the Raman active vibrator is modeled as a (damped)
harmonic oscillator with a characteristic frequency of ωv. In CARS,
this oscillator is not driven by a single optical wave, but by the
difference frequency (ωp-ωS) between the pump and the Stokes
beams instead. This driving mechanism is similar to hearing the
low combination tone when striking two different high tone piano
keys: your ear is sensitive to the difference frequency of the high
tones.
https://en.wikipedia.org
…COHERENT ANTI STOKES RAMAN
SPECTROSCOPY (CARS): PRINCIPLE
Similarly, the Raman oscillator is susceptible to the difference frequency of two
optical waves. When the difference frequency ωp-ωS approaches ωv, the
oscillator is driven very efficiently. On a molecular level, this implies that the
electron cloud surrounding the chemical bond is vigorously oscillating with the
frequency ωp-ωS. These electron motions alter the optical properties of the
sample, i.e. there is a periodic modulation of the refractive index of the material.
This periodic modulation can be probed by a third laser beam, the probe beam.
When the probe beam is propagating through the periodically altered medium, it
acquires the same modulation. Part of the probe, originally at ωpr will now get
modified to ωpr+ωp-ωS, which is the observed anti-Stokes emission. Under
certain beam geometries, the anti-Stokes emission may diffract away from the
probe beam, and can be detected in a separate direction.
https://en.wikipedia.org
…COHERENT ANTI STOKES RAMAN
SPECTROSCOPY (CARS): PRINCIPLE
https://en.wikipedia.org
…COHERENT ANTI STOKES RAMAN
SPECTROSCOPY (CARS): PRINCIPLE
While intuitive, this classical picture does not take into account the quantum
mechanical energy levels of the molecule. Quantum mechanically, the CARS
process can be understood as follows. Our molecule is initially in the ground
state, the lowest energy state of the molecule. The pump beam excites the
molecule to a virtual state. A virtual state is not an eigenstate of the molecule and
it can not be occupied but it does allow for transitions between otherwise
unoccupied real states. If a Stokes beam is simultaneously present along with the
pump, the virtual state can be used as an instantaneous gateway to address a
vibrational eigenstate of the molecule. The joint action of the pump and the
Stokes has effectively established a coupling between the ground state and the
vibrationally excited state of the molecule. The molecule is now in two states at
the same time: it resides in a coherent superposition of states. This coherence
between the states can be probed by the probe beam, which promotes the
system to a virtual state.
https://en.wikipedia.org
…COHERENT ANTI STOKES RAMAN
SPECTROSCOPY (CARS): PRINCIPLE
Again, the molecule cannot stay in the virtual state and will fall back
instantaneously to the ground state under the emission of a photon at the antiStokes frequency. The molecule is no longer in a superposition, as it resides
again in one state, the ground state. In the quantum mechanical model, no energy
is deposited in the molecule during the CARS process. Instead, the molecule acts
like a medium for converting the frequencies of the three incoming waves into a
CARS signal (a parametric process). There are, however, related coherent
Raman processes that occur simultaneously which do deposit energy into the
molecule.
https://en.wikipedia.org
…COHERENT ANTI STOKES RAMAN
SPECTROSCOPY (CARS)
Additional
Coherent Stokes Raman spectroscopy (CSRS pronounced as
"scissors") is closely related to Raman spectroscopy and lasing
processes.
It is very similar to CARS except it uses an anti-Stokes frequency
stimulation beam and a Stokes frequency beam is observed (the
opposite of CARS).
https://en.wikipedia.org
…COHERENT ANTI STOKES RAMAN
SPECTROSCOPY (CARS)
OUTLINES (UNIT-3)
I.
Raman spectroscopy: Basic
II.
Classical and quantum theories of Raman effect
III. Pure rotational, vibrational and vibrational-rotational Raman spectra
IV. Selection rules
V.
Mutual exclusion principle
VI. Resonance Raman spectroscopy
VII. Coherent anti stokes Raman spectroscopy (CARS)