The Raman effect was discovered in 1928 by Indian scientist C.V. Raman. When light interacts with molecules, most light is scattered at the same frequency as the incident light (Rayleigh scattering) but a small amount is scattered at shifted frequencies. This shifted light is called the Raman effect. There are three types of scattered light - Rayleigh lines have the same frequency, Stokes lines have lower frequencies, and anti-Stokes lines have higher frequencies. Raman spectroscopy analyzes these scattered light frequencies to identify molecules based on their vibrational and rotational states.
2. Raman Effect
• Given by Great Indian Scientist Sir C.V. Raman 1928.
• When visible or UV light fall on gas, liquid or crystal the
scattered light contains two components, one with same
frequency as that of incident light and other with changed
frequency (frequency less or more than the incident light).
Incident Frequency
Same Frequency
(Rayleigh Lines)
Decreased Frequency
(Stoke’s Lines)
Decreased Frequency
(Anti-Stoke’s Lines)
Unchanged Frequency Changed Frequency (Weak lines)
Scattering
3. Lines in Raman Effect
• Let
– Incident Frequency = ν0
– Scattered Frequency = ν’
– Change in frequency, (Raman Shift) Δν = ν0 -ν’
• Then if
ν’ = ν0 then Rayleigh Lines [Raman Shift, Δν = 0]
ν’ < ν0 then Stoke’s Lines [Raman Shift, Δν = +ve]
ν’ > ν0 then Anti- Stoke’s Lines [Raman Shift, Δν = -ve]
4. Raman Spectra
• Pure Vibration Raman Spectra (Low Resolution Spectrometer)
• Pure Rotational Raman Spectra (High Resolution Spectrometer)
5. Experimental Set for Raman Effect
Experiment by Raman consisted of following parts:
• Raman Tube
– Cylindrical tube made of glass or quartz. Its purpose is to
place Gas, liquid, transparent crystal used for scattering of
light. It is covered with water tube to cool down from heat
developed in the tube.
– For liquid its diameter is with approximate length 15 cm
and diameter 2 cm and for air diameter are approximate
length 15 cm and diameter 2 cm
• Proper Source of Light
– Source of high intensity monochromatic light – mercury
lamp with monochromatic filter or laser light (preferred)
• Spectrograph or Spectrometer
– High resolution and light gathering power
6. Experimental Set for Raman Effect
• Raman Spectra does not depend upon the frequency of incident or scattered light
rather it depend upon the nature of material placed in Raman Tube.
7. Classical Theory of Raman Effect
The equation of Electric Field in incident light of frequency ν0 can
be written as
The molecules will vibrate with the frequency of incident light,
hence Rayleigh Lines will appear in the spectra.
8. Classical Theory of Raman Effect
Due to vibrational and rotational motion of molecules a phase shift of
() will appear causing appearance of an additional factor in the
equation of Electric Field
The molecules will vibrate with the frequency different from the
frequency of incident light. Hence Stoke’s and Anti-Stoke’s lines will be
formed
9. Classical Theory of Raman Effect
• Limitations of Classical Theory
– As per the equation of polarizibility the intensity of
Stoke’s and Anti-Stoke’s lines must be equal but in
spectra the intensity of Stoke’s lines is higher then the
Anti-Stoke’s lines. Classical Theory fails to explain this
difference in intensity.
– At ordinary temperature the vibrational motion is very
less and only rotational motion dominates but
classical theory does not explain any difference
between contribution of vibrational and rotation
motion.
10. Quantum Theory of Raman Effect
• When incident light fall over the scatterer then a
collision between incident photon and molecule
will happen and photon get scattered. This
collision may be:
– Elastic collision (photon scatter without change in
energy. Then Rayleigh lines will be observed.)
– Inelastic Collision
• Photon scatter with loss of energy i.e. Incident Frequency
(ν0) > Scattered Frequency (ν’) (Stoke’s lines)
• Photon scatter with gain of energy i.e. Incident Frequency
(ν0) < Scattered Frequency (ν’) (Anti-Stoke’s lines)
