Raman spectroscopy is often considered to
be complementary to IR spectroscopy. For symmetrical molecules with
a center of inversion, Raman and IR are mutually exclusive. In other
words, bonds that are IR-active will not be Raman-active and vice versa.
Other molecules may have bonds that are either Raman-active, IR-active,
neither or both.
Raman spectroscopy measures the scattering of light by matter. The
light source used in Raman spectroscopy is a laser. The laser light
is used because it is a very intense beam of nearly monochromatic light
that can interact with sample molecules. When matter absorbs light,
the internal energy of the matter is changed in some way. Since this
site is focused on the complementary nature of IR and Raman, the infrared
region will be discussed. Infrared radiation causes molecules to undergo
changes in their vibrational and rotational motion. When the radiation
is absorbed, a molecule jumps to a higher vibrational or rotational energy
level. When the molecule relaxes back to a lower energy level, radiation
is emitted. Most often the emitted radiation is of the same frequency
as the incident light. Since the radiation was absorbed and then emitted,
it will likely travel in a different direction from which it came. This is
called Rayleigh scattering. Sometimes, however, the scattered (emitted)
light is of a slightly different frequency than the incident light. This
effect was first noted by Chandrasekhara Venkata Raman who won the Nobel
Prize for this discovery. (6) The effect, named for its discoverer,
is called the Raman effect, or Raman scattering.
Raman scattering occurs in two ways. If the emitted radiation is
of lower frequency than the incident radiation, then it is called Stokes
scattering. If it is of higher frequency, then it is called anti-Stokes
The green arrow in the picture to the left represents the incident radiation.
The Stokes scattered light has a frequency lower than that of the
original light because the molecule did not relax all the way back to the
original ground state. The anti-Stokes scattered light has a higher
frequency than the original because it started in an excited energy level
but relaxed back to the ground state.
Though any Raman scattering is very
low in intensity, the Stokes scattered
radiation is more intense than the anti-Stokes scattered radiation. The reason
for this is that very few molecules would exist in the excited level as compared to the ground
state before the absorption of radiation. The diagram shown represents
electronic energy levels as shown by the labels "n=". The same phenomenon,
however, applies to radiation in any of the regions.
In the infrared region, methods have been developed to enhance the intensity
of the scattered light in order to make Raman instruments more sensitive.
One of these methods involves placing the sample on roughened gold
or silver surfaces, then focusing the light on the sample. The laser
light is chosen such that it will interact with the surface particles in
order to resonate, therefore increasing the intensity of the scattered light.
This greatly enhances the ability to detect the scattered radiation. (7)
This is called Surface Enhanced Raman Scattering or SERS. SERS has made it much more practical to detect the scattered
light, making Raman a more available technique.