1. Introduction & Its instrumentation
RAMAN SPECTROSCOPY
Introduction
&
Its instrumentation

2. INTRODUCTON
The Raman Effect was named after one of its discoverers, the Indian scientist Sir C. V. Raman who observed the Raman effect.
The pioneering theory of Raman effect was develop by George Placzek between 1930 & 1934.The mercury arc became the principal light source.

3. Originally, heroic measures were required to obtain Raman spectra due to the low sensitivity of the techniques.
To maximize the sensitivity, the sample was highly concentrated and relatively large volumes were used.

4. In 1960s the advent of the laser resulted in simplified Raman spectroscopy instruments & also boosted the sensitivity of the techniques.
In 1923, A Smekal predicted that when light is scattered by transparent medium, the scattered radiation might contain frequencies other than those present in the original incident radiations.

5. In 1928 Sir C.V Raman discovered that on passing monochromatic light through some organic substance such as benzene, Light contain some additional frequencies over & above that of the smekal’s prediction.
The lines are therefore identified as stokes &
antistokes lines.

6. THEORY OF RAMAN EFFECT
Monochromatic radiation on incident upon a sample will interact & light will scattered.
If the frequency of the scattered light is analyzed, a small amount of radiation is scattered at some different wavelength.
It is the change in wavelength of the scattered photon that provides the chemical & structural information.

7. CHARACTERISTICS OF RAMAN LINES
The intensity of the Stokes lines is always greater than corresponding antistoke’s lines.
Raman shift generally lies within the range of 100 to 3000 cm-1 which lies in far infra & near infra red regions of the spectrum.
3) The Raman lines are symmetrically displaced about the parent line

8. MECHANISM OF RAMAN EFFECT
QUANTUM THEORY
This treats radiation of frequency v as consisting of a stream of particles i.e. Photon having energy hv where h is Planck constant.
During perfectly elastic collision, photon are deflected unchanged.
But during inelastic collision, energy may be
Exchange between molecule & photon.

9. If molecule gain the energy ∆E, the photon will be scattered with energy hv-∆E & the equivalent radiation will have a radiation will have a frequency v-∆E/h .
Conversely, if the molecule loses energy ∆E, the scattered radiation will have a frequency v+∆E/h.

10. CLASSICAL THEORY
When a molecule is put into a static electric field it suffer distortion, the positively charged nuclei are attracted towards the negative pole of the field, the electrons to the positive pole.
This separation of charge centre causes an induced electric dipole moment to be set up in the molecule & the molecule is said to be polarized.

11. The size of the induced dipole M, depends both on the magnitude of the applied
field E & on the ease which the
molecule can be distorted. Thus we may write
M=α E (1)
Where α is the polarizability of the molecule.

12. When a sample of molecule is subjected to a beam of radiation of frequency v the electric field experience by each molecule varies according to the equation
E=E0sin2пvt (2)
Where E 0 = equilibrium value of the field strength.

13. Thus induced dipole also under goes oscillation of frequency v
Thus induced dipole also under goes oscillation of frequency v .On combining eq. (1) & (2) we get
M=αE0 sin2пvt (3)
Thus from eq. (3) if follows that the electromagnetic radiation of frequency v induces in the atom or molecule a dipole which oscillate with the same frequency.

14. INSTRUMENTATION COMPONENTS OF RAMAN SPECTROSCOPY
Excitation source (laser)
Sampling device
Device to filter/reject light scattered at the laser wavelength
Wavelength processing unit
Detector and electronics

15. RAMAN SPECTROSCOPY

16. A laser source is needed to excite the target species.
A filter collects the Raman scattered light (Stokes) and filters out the Raleigh and Anti Stokes light.
. A diffraction grating bends the Raman shifted light according to wavelength.
A detector records the signal and passes the signal to a computer

17. MECHANISM OF RAMAN SPECTROSCOPY
A sample is normally illuminated with a laser beam. Scattered light is collected with a lens and sent to filter or spectrophotometer to separate desired Raman modes & to obtain the Raman spectrum.
Researchers typically use commercially available filters to eliminate the stray light.

18. Researchers traditionally used single-point detectors such as photon-counting Photomultiplier Tube(PMT) .
Nowadays, researchers use multi-channel detectors like Photo diode or, Charge-Coupled Devices (CCD) for detecting Raman scattered light. Sensitivity and performance of modern CCD detectors are rapidly improving.

19. THANK YOU