2. Raman Effect
• A change of wavelength
exhibited by sum of the
radiation scattered in a
medium .
• The effect is specific to the
molecule which cause it.
3. Raman Scattering
• Raman scattering involves two photons one in , one out.
• it is more complex then the one photon processes .
• In this a photon is scattered by a crystal with the creation of a phonon.
4. Raman Scattering
• The process is identical to the inelastic scattering of x rays.
• It is similar to the inelastic scattering of neutrons by a crystal.
5. SELECTION RULES
• The selection rules for the first order raman effect are
• Where refer to the incident photon .
• refer to the scattering photon .
• Ω, K refer to the phonon created or destroyed in the scattering event.
6. 2nd ORDER RAMAN EFFECT
• Two phonon are involved in the inelastic scattering of the photon .
• The raman effect is made possible by the strain dependence of the electronic
polarizability .
• Polarizability α associated with the phonon may be written as a power series
in the phonon amplitude u.
7. STOKES
• The photon at is called the stokes line .
• The intensity of the stoke line involves the matrix
elements for phonon creation .
8. ANTI STOKES
• The photon at is called the anti
stokes line.
• The anti stokes lines involves phonon
annihilation with a photon intensity
proportional to
9. RESULTS
• Relative intensity of the anti stoke lines vanishes at temperature T = 0,
because here there are no thermal phonons available to be annihilated.
10. INTENSITY RATIO
• If the phonon population is initially in thermal equilibrium at temperature T ,
the intensity ratio of the two lines is
11. GRAPHICAL REPRESENTATION
• There is a graph between intensity and raman
shift, cm-1
• First order raman spectra of the K=0 , optical
mode of a silicon crystal observed at three
temperature
• The optical phonon frequency equal to
frequency shift depend upon T.
12. GRAPHICAL REPRESENTATION
• Intensity ratio of anti stoke to stoke lines is
as a function of temperature.
• It is for the observation on the optical
mode of silicon .
13. GRAPHICAL REPRESENTATION
• Raman spectrum of GaP at 20K .
• The two highest peaks are the first order raman
line associated with a excitation of an LO phonon
at 404cm-1 and a TO phonon at
366cm-1
all the other peaks involve two phonon
