Raman Spectroscopy is a non destructive chemical analysis technique which provides detailed information about chemical structure, crystallinity and molecular interactions. The raman effect involves scattering of light by molecules of gases, liquids, or solids. Raman Spectroscopy is sensitive to homo-nuclear molecular bonds. It is able to distinguish between single, double, and triple bonds between carbon atoms.Raman spectroscopy is the study of matter by the inelastic scattering of monochromatic light. It has become a ubiquitous tool in modern spectroscopy, biophysics, microscopy, geochemistry, and analytical chemistry. In contrast to typical absorption or emission spectroscopy experiments, transitions among quantum levels of atoms or molecules are induced by the absorption or emission of photons (IR, visible, UV). In a typical Raman experiment, a polarized monochromatic light source (usually a laser) is focused into a sample, and the scattered light at 90 degree to the laser beam is collected and dispersed by a high-resolution monochromator. The incident laser wavelength (chosen such that the sample does not absorb, in ordinary Raman Spectroscopy) is fixed, and the scattered light is dispersed and detected to obtain the frequency spectrum of the scattered light. The scattered light is very weak (<10-7 of the incident power), so that monochromators with excellent straylight rejection and sensitive detectors are required. In a much rarer event (approximately 1 in 10million photons)Raman scattering occurs, which is an inelastic scattering process with a transfer of energy between the molecule and scattered photon. If the molecule gains energy from the photon during the scattering (excited to a higher vibrational level) then the scattered photon loses energy and its wavelength increases which is called Stokes Raman scattering . Inversely, if the molecule loses energy by relaxing to alower vibrational level the scattered photon gains thecorresponding energy and its wavelength decreases; which is called Anti-Stokes Raman scattering. • Quantum mechanically Stokes and Anti-Stokes areequally likely processes. However, with an ensemble of molecules, the majority of molecules will be in the ground vibrational level (Boltzmann distribution) and Stokes scatter is the statistically more probable process. As a result, the Stokes Raman scatter is always more intense than the anti-Stokes and for this reason, it is nearly always the Stokes Raman scatter that is measured in Raman spectroscopy. Raman spectroscopy is used in chemistry to identify molecules and study chemical bonding and intramolecular bonds.In solid-state physics, Raman spectroscopy is used to characterize materials, measure temperature, and find the crystallographic orientation of a sample . In nanotechnology, a Raman microscope can be used to analyze nanowires to better understand their structures, and the radial breathing mode of carbon nanotubes is commonly used to evaluate their diameter.

1. Advanced Analytical Chemistry
SURIYA MARIMUTHU
Ph.D. Scholar,
C/O Prof. Chia-Hsiang Chen,
KMU-Taiwan
RAMAN SPECTROSCOPY

2.  When a beam of light falls on a substance, it may be transmitted,
absorbed on scattered. If the substance is transparent and does not
absorb in the visible region of the electromagnetic spectrum, nearly
all of this light is transmitted.
 A small fraction of the incident light, however emerges in all
directions as a result of scattering. If the light is monochromatic,
nearly all of the scattered light is observed to be of the frequency as
the incident light.
 This is called Rayleigh scattering.
 This phenomenon is referred to as Raman scattering.
Introduction

3. Theory and Excitation of Raman Spectroscopy
(a) radiation from a source that is
incident on the sample produces
scattering at all angles.
Lower frequency emissions called Stokes Scattering.
Higher-frequency emissions termed anti-Stokes scattering.
Elastically scattered radiation is of the same frequency as
the excitation beam and is called Rayleigh scattering.

4. Mechanism of Raman and Rayleigh Scattering
 The heavy arrow on the far left depicts the
energy change in the molecule when it
interacts with a photon from the source.
 The increase in energy is equal to the
energy of the photon hѴex.
 It is important to appreciate that the
process shown is not quantized.
 The middle set of arrows depicts the
changes that produce Rayleigh scattering.
 Finally, the energy changes that produce
Stokes and antiStokes emission are
depicted on the right.

5. Raman Spectrum of Carbon Tetrachloride
 Raman spectrum of CCl4, excited by laser
radiation of 𝜆ex=488 nm (𝜈ex=20,492 cm-1 ).
 The number above the Raman lines is the
Raman shift, Δ𝜈 = 𝜈𝑒𝑥 ± 𝜈𝜈, in cm-1 .
 Stokes-shifted lines are often given positive
values rather than negative values as shown.
 It is important to appreciate that the
magnitude of Raman shifts is independent of
the wavelength of excitation.

6. Selection Rule For Raman Scattering
Change in polarizability during the vibration must be greater than zero.
Δ𝜈 = ±1
Raman Depolarization Ratios
The depolarization ratio p is defined as
The depolarization ratio depends on the symmetry
of the vibrations responsible for the scattering.

7. Raman Spectrum Vs IR Spectrum
Water is quite useful as a solvent in
Raman spectroscopy.
Water cannot be used as a Solvent due to
intense absorption
The vibration is Raman active if it causes
a change in polarisability.
Vibration is IR active if there is change in
dipole moment.
Light Scattering Light absorption
Sample preparation is not very elaborate,
it can be in any state.
Sample preparation is elaborate
Gaseous samples can rarely be used.
Gives an indication of covalent character
in the molecule
Gives an indication of ionic character in the
molecule.
Cost of instrumentation is very high. Comparatively inexpensive.

8. Instrumentation
The laser radiation is directed into a sample cell.
The Raman scattering is usually measured at
right angles to avoid viewing the source
radiation. A wavelength selector isolates the
desired spectral region. The transducer converts
the Raman signal into a proportional electrical
signal that is processed by the computer data
system.
Instrumentation for modern
Raman spectroscopy consists of a
1) Laser source
2) Sample illumination system
3) Spectrometer

9. Laser Source
Laser Type Wavelength, nm
Argon ion 488.0 or 514.5
Krypton ion 413.1, 530.9, 647.1
Helium neon 632.8
Diode 660-880
Nd-YAG 1064
Some Common Laser Sources for Raman
Spectroscopy
Spectra of anthracene taken with a
conventional Raman instrument with an
argon-ion laser source at 514.5 nm (A)
and with an FT-Raman instrument with
a Nd-YAG source at 1064 nm (B).
The sources used in modern Raman
spectrometry are nearly always lasers because
their high intensity is necessary to produce
Raman scattering of sufficient intensity to be
measured with a reasonable signal-to-noise
ratio.

10. Sample-Illumination System
In (a), a gas cell is
shown with external
mirrors for passing the
laser beam through the
sample multiple times.
Liquid cells can be
capillaries (b) or
cylindrical cells (c).
Solids can be
determined as powders
packed in capillaries or
as KBr pellets (d).

11. Fourier Transform Raman Spectrometers
The laser radiation passes
through the sample and then into
the interferometer, consisting of
the beamsplitter and the fixed
and movable mirrors. The output
of the interferometer is then
extensively filtered to remove
stray laser radiation and Rayleigh
scattering. After passing through
the filters, the radiation is
focused onto a cooled Ge
detector.

12. Nitrogen, Hydrogen , Chlorine has Raman
active but IR inactive – Why?
A homonuclear diatomic molecule such as nitrogen, chlorine, or hydrogen has no
dipole moment- IR inactive, but they are Raman active. Because of the stretching
and contraction of the bond changes the interactions between the electrons and nuclei,
this causes a change of polarizability.
vibrational
modes of CO2

13. Applications of Raman Spectroscopy
Raman Spectra of Inorganic Species
Raman technique is often superior to IR spectroscopy for investigating
inorganic systems – Why?
Raman studies are potentially useful sources of information concerning the
composition, structure and stability of coordination compounds.
Metal-oxygen bonds are also Raman Active.
Dissociation constants for strong acids such as H2SO4, HNO3, H2SeO4, and
H5IO6 have been calculated.

14. Raman Spectra of Organic Species
Raman studies are likely to yield useful information about the olefinic functional
group that may not be revealed by IR spectra – Why?
Raman band region for cycloparaffin is 700 to 1200 cm-1.
Raman spectroscopy thus appears to an excellent diagnostic tool for the estimation
of ring size in paraffins.
Quantitative Applications
Raman spectra tend to be cluttered with bands than IR spectra - Why?

15. Drawbacks
Matrix effect can occur.
Instrument instability.
Other Types of Raman Spectroscopy
I. Resonance Raman Spectroscopy.
II. Surface-Enhanced Raman Spectroscopy.
III.Nonlinear Raman Spectroscopy

16. Thank You !
Suriya Marimuthu
2023.12.18