Geoscience Reference
In-Depth Information
Table 5.2.
Classes of scattering phenomena
Class
Scatter Method
Kayleigh
Elastic scattering from small particles such as atoms or molecules, resulting in
scattered radiation that occurs in all directions uniformly
Debye or Mie
Elastic scattering from large particle with dimensions comparable to the
incident radiation wavelength (Debye) or much larger than the incident
wavelength, where the result from scattering is nonuniform
Brilliouin
Inelastic scattering where the frequency of the reflected radiation is changed by
thermal sound waves
Raman
Inelastic scattering where the frequency of the reflected radiation is changed by
the gain or loss of vibrational/rotational quantum of energy by the analyte
molecule
through the Boltzmann distribution. Thus by measuring the emission spectrum from a sam-
ple, one can apply known spectra from a library to identify and quantify the concentration
of species in the sample. It is important to note here that many systems do not exhibit ther-
mal equilibrium and therefore the measured radiant power does not always directly relate
to the population densities of the excited states and or to the analyte concentration.
5.2.3 Scattering
Radiation incident on an analyte may be scattered as well as absorbed by the sample. The
intensity, angular distribution, and radiation frequency of the scattered light can be used
as a means of analysis. Several classes of scattering are possible and are simplistically
described in
Table 5.2
.
The most widely used scattering phenomenon in spectroscopy is Raman scattering.
Incident photons scattered inelastically from an analyte may either gain or lose energy
(Raman and Krishnan,
1929
; Long,
2002
). Observed energy differences typically corre-
spond to one quantum of vibrational energy of the species and the wavelength of the scat-
tered light shifts accordingly. In simple terms, the most common form of Raman, called
Stokes scattering, involves a loss of energy causing a red shift in the scattered photon
while a gain or increase in energy is called anti-Stokes scattering. In certain circumstances
(anharmonicity) overtone and combination bands can be observed in a Raman spectrum.
For example, in the gas phase there are rovibrational structures whereas in the liquid phase
these signals are far too weak and instead the full width at half maximum (FWHM) is
related to the molecular reorientation time.
Raman spectroscopy, much like IR absorption spectroscopy, provides a method of
determining the unique “fingerprint” of a species. Unlike in IR absorption spectroscopy,
however, there is no need to detect the incident light. In most cases, light scatters in all
directions, making Raman spectroscopy especially useful for the analysis of opaque solids;
