CHEM01200604009 Sreejith Kaniyankandy - Homi Bhabha ...

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sample. Raman spectroscopy is based on inelastic scattering of light. On excitation of a
sample, the electrons are pushed to a virtual level and from the virtual levels electrons returns
to ground state (Rayleigh scattering) or return to higher vibrational state (Stokes line).
Additionally the sample may be excited from higher vibrational state to virtual state and
returns to ground vibrational state also called anti stokes line. Anti stokes light has a greater
energy than the incident light.
_
1 1
0
s
(2.7)
s
and 0
is wavelength of scattered and incident (excitation) light in the above equation.
The Stokes line is more intense compared to Antistokes line as at room temperature ground
vibrational state is most populated state of a molecule. Visible wavelengths are used in
Raman spectroscopy, as scattering of visible light is more efficient compared to IR due to
inverse scaling of scattering cross section with wavelength. Very often a tunable excitation
source is used to achieve a resonance in scattering when incident light is close to excited
molecular level. Such a process is called resonance Raman scattering. In case of resonance
Raman scattering, cross section for scattering increases by ~10 4 . Scattered light is collected
at either in 90˚ or 180˚ with respect to excitation light. CCD detectors are used for detection
in most of current spectrometers. In present thesis we have used a LABRAM1, ISA make
spectrometer in the back scattering geometry with 488 or 514 nm line of Ar ion laser.