Raman Scattering
As stated earlier, light of a specific energy can be absorbed
by or emitted from a substance, depending on its allowable levels of vibrational,
rotational, and electronic energy. Another absorption and re-emission phenomenon
of light scattering occurs in a very small percentage of interactions. In this process,
called Raman scattering, light in the visible and ultraviolet range is absorbed by
molecules of a substance, thereby producing unstable vibrational or rotational energy
states. Because these excited states are unstable, some of the absorbed energy is
immediately re-emitted, thereby allowing the molecule to relax into a stable state.
This Raman scattering signal occurs very infrequently; therefore, most photons pass
through the gas sample without taking part in this particular absorption-re-emission
phenomenon. If the intensity of the transmitted light is sufficiently great, the
Raman-scattered signal can be measured and used to identify the molecules within
the gas sample. Because the signal is scattered light, it is emitted in all directions
relative to the incident beam.[18]
[26]
The Raman light is of low intensity, so it is best to measure it at right angles
to the high-intensity exciting beam.
An advantage of Raman scattering over gas analysis based on absorption
of infrared light is that a spectrum of Raman scattering lines can be used to identify
all types of molecules in the gas phase (e.g., O2
and N2
).
Because it involves only vibrational and rotational energy states, Raman scattering
cannot be used to identify single atoms. As discussed earlier, infrared absorption
can be used to analyze molecules with a dipole moment and therefore cannot be used
to identify oxygen or nitrogen. A disadvantage of Raman scattering as a clinical
tool is the requirement for a very high-intensity laser light source to produce the
small Raman-scattered signal.[18]
[26]
