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this is called Stokes-Raman scattering. In Stokes-Raman scattering, the scattered light always has a
frequency lower than that of the incident light. Finally, if a molecule that already exits at a raised
energy level of (hV ), is raised further to an excited state by the incident light and then falls back to the
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ground state, the frequency of the emitted light will now be (v + v ). This is called Anti-Stokes-Raman
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scattering, and the light scattered by this process always has a frequency greater than that of the
incident light.

Analysis of the Raman spectra from a wide range of compounds has shown that (hcDv) is almost
invariably equal to the change in rotational or vibrational energy of the molecule. Raman spectroscopy
discloses the presence of non-polar bonds and aromatic rings. It also discloses changes in polarization
and the 'shape' of the electron distribution in the molecules as it vibrates. In contrast, infrared
spectroscopy detects changes in the dipole moment of the molecule as it vibrates and is more sensitive
to polar functional groups. The information provided by the two techniques, Raman spectroscopy and
infrared spectroscopy, is often claimed to be complementary. When a particular bond between atoms
produces a strong infrared signal, it is less likely to produce a strong Raman signal, and vice versa. The
study of Raman spectra has a number of advantages as, by the appropriate choice of the incident
radiation, the scattered lines can be brought into a convenient region of the spectrum where they can be
easily observed. The energy of the incident radiation determines the spectroscopic region where the
Raman scattering is observed. Originally, both incident and scattered radiation were measured in the
visible region of the spectrum but, for various reasons, one of which is discussed below, the near-infra-
red radiation is now the most frequently employed.

In the early days, the practice of Raman spectroscopy was experimentally more difficult than today, as
the intensity of the scattered light is only 0.0001% of that of the incident light (one part in a million).
The difficulties were greatly reduced with the introduction of the laser light sources of high energy in
the 1960s and, in particular, the argon laser with its intense blue and green emission. Nevertheless,
although the high-intensity laser light sources has aided in Raman spectroscopic techniques, it has also
led to other problems, such as photochemical reaction in the