Geoscience Reference
In-Depth Information
where
λ
i
is the wavelength of the incident radiation and Δ
ν
is the frequency of the mode,
which corresponds to the energy difference between the initial and final vibrational levels
of the scattering process.
The intensity of a mode depends on the symmetry of the corresponding vibrational
motion and the wavelength of the incident radiation. For most vibrational modes, the inten-
sity of Raman scattering varies as the inverse fourth power of the Raman wavelength (
α
λ
r
-4
). For some modes the wavelength dependence becomes much stronger when the exci-
tation approaches an absorption band of the molecule (resonance excitation). Raman scat-
tering is an inelastic (involving energy transfer) interaction process, having efficiencies of
up to three to four orders of magnitude weaker than Rayleigh scattering. This is an impor-
tant consideration because to retrieve a Raman signal from a species at a concentration in
the region of parts per million of a host of species, an optical filter having a Rayleigh line
rejection factor of 10
-9
-10
-10
would be required.
In water, Raman scattering is less probable (approximately one tenth) than elastic scat-
tering (Rayleigh-Mie). Raman scattering occurs when molecules are irradiated with inci-
dent photons. The scattering molecule immediately scatters photons that are different in
energy to the incident photon. Because this energy difference requires energy transfer,
Raman scattering is said to be inelastic in nature as opposed to Rayleigh scattering, which
is
elastic
. The difference in energy between the incident and scattered photons (either pos-
itive or negative) corresponds to the energy difference between two energy levels of the
molecule. For a given pair of energy levels, the energy difference can be observed as a
constant frequency difference, for example, 3400 cm
-1
for the OH stretch vibrational mode
in water, or a variable wavelength difference between the incident and scattered photons.
Because Raman scattering follows a
λ
-4
relation law, there is a greater production of water
Raman photons at shorter wavelengths.
Understanding the Raman scattering phenomena with respect to water molecules
is important as water Raman scattered signals have been used extensively to normalize
fluorescence signals. Although inherently a weak interaction, water Raman signals are
commonly observed in the fluorescence spectra of aquatic samples given the high number
of water molecules present per fluorophore. In addition, because irradiated water molecules
will scatter photons probabilistically then water Raman signals observed in fluorescence
spectra of aquatic samples can be used as an internal standard that allows the correction for
differential absorption (inner filter) effects caused by the sample.
1.3.4.11 Normalization of Fluorescence Intensities
When a volume of water is probed with an incident beam of light the fluorescence emission
is related to the number of the fluorophores present within the beam. As such the observed
fluorescence emission is subject to large changes due to variations in the penetration of
this incident light. These changes are produced by the optical attenuation properties of the
sample caused by variations in the concentration of either the fluorophore(s) under inves-
tigation or other substances such as suspended particles and dissolved organics. The net
