The document discusses the principles and mechanisms of Raman spectroscopy, including Rayleigh and Raman scattering, the significance of polarizability changes during molecular vibrations, and various spectrometer setups. It highlights the advantages of Raman spectroscopy over IR spectroscopy for inorganic and organic compounds, emphasizing the utility in analyzing molecular structures and properties. Additionally, the document outlines the instrumentation involved, sample preparation methods, and applications of Raman spectroscopy across different chemical systems.

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