Shortly Description About Of Light Scattering

1. Guidance by:-
Dr. Goverdhan
Yadu
Submitted by:-
Rahul Sahu
M.Sc.- Physics 4th
Sem

2.  Scattering
 Rayleigh and Raman Scattering
 Stimulated Raman Scattering
 Hyper Raman Scattering

3.  Scattering occurs when light or other energy waves pass
through an imperfect medium (such as air filled with
particles of some sort) and are deflected from a straight
path. A great example is when the sun's rays pass
through clouds. The light is deflected off of its straight
path and scatters in many directions.

4.  Atoms or molecules which are exposed to light absorb light energy and
re-emit light in different directions with different intensity. This
phenomenon known as Scattering.
 Scattering may also refer to particle-particle collision between
molecules, atoms, electrons, photons and other particles.
 When radiation is only scattered by one localized scattering
center, this is called single scattering. It is very common that
scattering centers are grouped together; in such cases, radiation
may scatter many times, in what is known as multiple scattering.
The main difference between the effects of single and multiple
scattering is that single scattering can usually be treated as a
random phenomenon, whereas multiple scattering can be
modeled as a more deterministic process because the combined
results of a large number of scattering events tend to average
out.

5.  Raman scattering is conceptualized as involving a virtual electronic
energy level which corresponds to the energy of the exciting laser
photons. Absorption of a photon excites the molecule to the imaginary
state and re-emission leads to Raman or Rayleigh scattering.
 In all three cases the final state has the same electronic energy as the
starting state but is higher in virtual energy in the case of Stokes
Raman scattering, lower in the case of Anti-Stokes Raman scattering
or the same in the case of Rayleigh scattering.
 Normally this is thought of in terms of wavenumbers, where ῦ0 is the
wavenumber of the laser and ῦM is the wavenumber of the vibrational
transition. Thus Stokes scattering gives a wavenumber of ῦ0 - ῦM
while ῦ0 + ῦM is given for anti-Stokes. When the exciting laser energy
corresponds to an actual electronic excitation of the molecule then
the resonance Raman effect occurs.

7.  The frequency shifts of the Raman lines, their intensity and polarization
are characteristic of the scattering substance. According to the classical
theory, Stokes and Anti-Stokes Lines should appear with equal intensity.
Experiments, however, show that Stokes lines are far more intense. The
upper level shown in figure does not, in general, correspond to a stationary
state. It is a virtual level. Since energy is not preserved in a transition to a
virtual level, the molecule will immediately return to a stationary state
under emission of a photon. The principle difference between fluorescence
and Raman effect is that in fluorescence there is always a transition to a
stationary upper level, whereas in Raman effect there is no such state.
 The high intensity and narrow line width of the laser radiation have made
it possible to measure the Raman scattering properties of materials under
high resolution and have, thus propelled Raman spectroscopy into its
present popularity. The use of laser for exciting Raman scattering has
made it possible to observe some new Raman scattering phenomena, of
considerable fundamental interest.

8.  The stimulated Raman scattering (SRS) process occurs when the
light intensity inside the Raman non-linear medium reaches a certain
(threshold) level. The incoming pump light induces intense molecular
or lattice vibrations and these modulate the incoming light beam,
generating frequency-shifted radiation. The interaction of the intense
electrical field with Raman material causes the output radiation shift
toward a longer wavelength (first Stokes shift). For sufficiently high
applied pump intensities, other additional lines at longer, as well as
shorter, wavelengths with respect to the pump wavelength will be
generated (anti-Stokes and higher Stokes lines). The spectral areas
reachable with SRS extend from ultraviolet to mid-infrared,
depending on the pump laser and Raman material used.

9.  Stimulated Raman scattering occurs when an excess of Stokes photons that
were previously generated by normal Raman scattering are added to the
excitation beam. The mode that is the strongest in the regular Raman
spectrum is then greatly amplified.
 Stimulated Raman is an example of non-linear Raman spectroscopy – with a
4-5 order of magnitude enhancement in Raman signal. As the Stokes beam is
unidirectional with the incident laser beam, only the strongest Raman signal
is amplified, all other weaker signals are not present within the spectrum.

11.  Hyper Raman scattering is a modified version of Raman scattering, where
the scattered light occurs at frequencies somewhat lower than twice the
frequency of the pump light. This means that two pump photons are
converted into one photon of Raman scattered light and one phonon. This
effect is usually fairly weak, but it has aspects which make it interesting
for Raman Spectroscopy. In particular, hyper-Raman spectra can provide
vibrational information on molecules where ordinary Raman scattering is
suppressed due to symmetry issues . The scattering rate can be substantially
enhanced near optical surfaces.
 The scattered radiation was found to vary as the square of the laser intensity
and occurred only in the focal region. These elastic and inelastic scattering
are now referred to as hyper-Rayleigh and hyper-Raman scatterings
respectively.