raman effect Abhishek meena

3.  एक सतह पर गिरे हुए द्रव्य से जब प्रकाश िुजरता है। तब उस
प्रकाश में विद्यमान फोटॉन कण द्रव्य के इलेक्ट्रॉन से टकराते
हैं। इस टकराहट के दो प्रकार होते हैैः लोचदार टक्ट्कर ( elastic
collision ) और बेलोच टक्ट्कर ( inelastic collision ) । लोचदार
टक्ट्कर में इलेक्ट्रॉन अपनी energy level से higher energy तक
उछलते हैं और जब िह इलेक्ट्रॉन अपनी original level में
िावपस लौटकर आता है, तो िो इलेक्ट्रान एक फोटॉन कण
उत्सर्जित करता है। बने हुए इस नए फोटॉन की ऊजाि पहले
टकराए फोटॉन की ऊजाि के समान ही होती है और इसे
rayleigh scattering कहते हैं। बेलोच टक्ट्कर में इलेक्ट्टॉन अपनी
higher energy से लौटकर िावपस आता है, तो उससे उत्सर्जित
फोटॉन का energy level पहले िाले फोटॉन की तुलना में
अपेक्षाकृ त कम होता है और यही अद्भुत घटना रामन ् प्रभाि को
जन्म देती है।

4.  the scattering of light by matter, accompanied by a noticeable change
in the frequency of the scattered light. If a source emits a line
spectrum, the Raman effect produces additional lines, whose number
and location are closely related to the molecular structure of the
substance, in the spectrum of the scattered light.
 The Raman effect was discovered in 1928 by the Soviet physicists G.
S. Landsberg and L. I. Mandel’shtam during studies of the scattering of
light in crystals and simultaneously by the Indian physicists C. V.
Raman and K. S. Krishnan during studies of the scattering of light in
liquids. In the Raman effect the transformation of the primary light flux
usually is accompanied by transition of the scattering molecules to
other vibrational and rotational energy levels in such a way that the
frequencies of the new lines in the scattering spectrum are
combinations of the frequency of the incident light and the frequencies
of the vibrational and rotational transitions of the scattering molecules
(hence the Russian name, kombinatsionnoe rasseianie sveta, or
“combination light scattering”).

5.  To observe Raman-effect spectra, an intense light beam must be concentrated on
the object under study. A mercury lamp— or, since the 1960’s, a laser beam—is
most often used as the source of the exciting light. The scattered light is focused
and strikes a spectograph, where the Raman-effect spectrum is recorded by
photographic or photoelectric methods.
 The Raman effect is most often associated with a change in the oscillatory states of
molecules. Such a Raman-effect spectrum consists of a system of companions that
lie symmetrically about an exciting line with frequency v (Figure 1). A companion
with frequency v+ v, (a violet, or anti-Stokes, companion) corresponds to every
companion with frequency v — vi (a red, or Stokes, companion). Here vi is one of
the natural oscillation frequencies of the molecule. Thus the frequencies of the
natural (or normal) oscillations of a molecule, which are manifested in the Raman-
effect spectrum, may be determined by measuring the frequency of Raman-effect
lines. Similar mechanisms also exist for a rotational Raman-effect spectrum. In this
case the frequencies of the lines are determined by the rotational transitions of the
molecules. In the simplest case a rotational Raman-effect spectrum is a sequence of
nearly equidistant, symmetrically situated lines whose frequencies are combinations
of the rotational frequencies of the molecules and the frequency of the exciting light.

6. Figure 1. Diagram of Stokes lines (with
frequencies v — v1v — v2 and v — v3) and
anti-Stokes lines (v + v1,v + v2, and v + v3)
during Raman scattering of light with
frequency v

7.  According to quantum theory, the process of the
Raman effect consists of two interconnected
events, the absorption of a primary photon with
energy hv (where h is Planck’s constant) and the
emission of a photon with energy hv' (where v’ = v ±
vi), which take place as a result of the interaction of
the molecule’s electrons with the field of the
incident light wave. Under the action of a quantum
with energy hv through the compound state, a
molecule in an unexcited state passes into a state
with oscillatory energy hvi, emitting a quantum h(v
— vi). This process leads to the appearance

8.  of a Stokes line with frequency v — vi in the scattered light (Figure
2,a). If the photon is absorbed by a system in which oscillations
already have been excited, then after scattering it may pass into a zero
state; here the energy of the scattered photon exceeds the energy of
the absorbed photon. This process leads to the appearance of an anti-
Stokes line with frequency v + vi (Figure 2,b).
 The probability w of the Raman effect (and consequently the intensity
of thn Raman-effect lines) depends on the intensity of the exciting
radiation I0 and the scattered radiation I:w =aI0(b + I), where a and b
are some constants; when the Raman effect is excited by ordinary light
sources (such as a mercury lamp), the second term is small and may
be disregarded. The intensity of the Raman-effect lines is extremely
low in most cases, and at ordinary temperatures the intensity of anti-
Stokes lines Ia generally is much less than the intensity of the Stokes
lines Is. Since the probability of scattering is proportional to the number
of scattering molecules, the ratio Ia/Is is defined by the

9.  Figure 2. (a) Stokes transitions, (b) anti-Stokes transitions during Raman scattering: (G) ground
level, (hvi) vibrational level, (hvi) intermediate electron level of the molecule
 ratio of the populations of the ground and excited levels. At ordinary temperatures the population
of the excited levels is not great, and consequently the intensity of the anti-Stokes component is
low. The population rises with increasing temperature, leading to an increase in the intensity of
the anti-Stokes lines. The intensity of Raman-effect lines / depends on the frequency v of the
exciting light: at great distances (on the frequency scale) from the molecules’ region of electron
absorption, I ~ v4; as the electron absorption band is approached, a more rapid increase in their
intensity is observed. In some cases, when the concentration of matter is low, it is possible to
observe a resonance Raman effect, in which the frequency of the exciting light enters the region
of the substance’s absorption band. When the Raman effect is excited by high-powered lasers,
its probability increases and a stimulated Raman effect, whose intensity is of the same order as
that of the exciting light, arises.
 Raman-effect lines are polarized to a greater or lesser extent. Here various companions of a
given exciting line have different degrees of polarization, but the nature of the polarization of
Stokes and anti-Stokes companions is always identical.
ratio of the populations of the ground and excited levels. At ordinary temperatures the population of the excited levels is not great, and consequently the intensity of
Raman-effect lines are polarized to a greater or lesser extent. Here various companions of a given exciting line have different degrees of polarization, but the natur