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Topic: Raman Spectroscopy
Have you ever wondered
what life may have been like if it ever existed on Mars? On the 30th
of July 2020, NASA undertook that ambitious goal by launching a rover called “Perseverance”
equipped with the most advanced analytical technologies of the 21st
century. (NASA, n.d.) On board the rover was an instrument called a Raman
Spectrometer. Its purpose is to detect and identify organic compounds. (NASA,
n.d.) By detecting for life characteristic compounds such as amino acids or
complex organic substances, one may be able to deduce whether life was present.
In addition, Raman Spectroscopy is used frequently in Chemistry and Forensics to
ascertain the identity of unknown compounds. (AZO Materials 2018) This technology
relies on many facets of optics, such as lenses, reflection, diffraction, and
the inelastic scattering of photons. (P. J. Larkin 2011, 28-31)
Figure 1: Raman Spectrometer diagram
(simplified)
Molecules can exist at
different vibrational and rotational energy levels caused by the various
vibrational and rotational modes due to their structure. (P. J. Larkin 2011,
17-21) When the monochromatic light from the laser source impinges on a
chemical substance, it causes the molecule to adopt an energy equivalent to the
energy of the laser source. This is called a virtual state since it is not an
actual energy state of the molecule and is only transient. If the molecule in
question goes back into its ground state (lowest energy at a particular
temperature), then it would release a photon that has the same energy as the
photon from the source. When this happens, the scattering is referred to as
Rayleigh Scattering. It turns out this happens much of the time, however, in
some cases the molecule may return to an energy level that is higher than the
ground state i.e. (v = 1, 2, 3, etc.). When this occurs, the photons emitted
are lower in energy and hence have a longer wavelength. The resultant spectral
lines are called stokes lines. A third, even less likely case can occur where
the molecules are already in an excited state. In that scenario the light from
the source caused the molecules to be excited to a virtual state higher than previously.
As the molecule goes to a lower energy level, for example the ground state, the
energy of the emitted photon has a higher energy, thereby having a shorter
wavelength. The lines caused by this secondary process is known as anti-stokes
lines. (Mettler Toledo, n.d.) The resulting spectrum would appear as shown in
figure 2 below.
Figure 2: Raman Spectrum (Illustrative)
The wavelengths of light produced by
this process are characteristic of the molecule and the chemical bonds in such
a molecule. As a result, it can be used as a unique signature to identify
unknown substances and elucidate their structures. The laser produces a
monochromatic (one wavelength) beam. The light from the laser is directed into
a beam splitter. Half of the beam exits the beam splitter and is lost. The
other half of the beam is directed into a convex lens which focuses the beam
down to a very small high-intensity point via refraction. The light then hits
the sample and is scattered. This light then goes through the lens and hits the
beam splitter where half is reflected into the laser and the other half is
directed through a filter. This filter, sometimes referred to as a notch
filter, removes most of the Rayleigh Scatter light which has the same
wavelength as the laser light. Therefore, the filter isolates the useful
signals for analysis i.e., the Stokes and Anti-stokes wavelengths. The light is
then focused to a point by another convex lens and then passes through a narrow
slit. The beam of light expands from the slit due to diffraction where it is
reflected off a plane mirror onto a reflective grating. The grating functions
to split the light into its individual wavelengths via interference and
diffraction. These wavelengths are
however uncollimated, so a concave mirror aids to focus this beam to form a
spectrum on a detector. The detector is typically highly sensitive, such as a charged
couple detector or a photomultiplier. The spectrum is then amplified and the
intensities of each of the wavelengths are recorded.
Figure 3: Real Raman Spectrum of polystyrene
(Renishaw n.d.)
It
is very important that the notch filter removes most of the Rayleigh light
otherwise the signal would be overwhelmed with this wavelength intensity,
thereby sequestering the other wavelengths. Raman spectrometers use detectors
such as CCDs (Charged Coupled Devices). A CCD is a silicon (Si) multichannel array
detector capable of detecting in UV, visible and near-infrared light. Various
types of CCDs can be used that are optimized for specific wavelength ranges. The
intensity of each wavelength is determined by these detectors, creating a spectrum
of intensities as seen in figure (3). (Horiba, n.d.) These peaks are highly
specific to the compound. The data from the CCDs are first amplified and
filtered to remove noise in the signal. A computer then uses this information
to plot a graph of intensity versus wavenumber (inverse of wavelength expressed
in cm-1). These peaks are then compared to a database that has the
wavenumbers for every organic functional group known. Alternatively, the entire
spectrum can be compared with a known database of compounds to distinguish what
compound it is.
Raman spectroscopy has
had limited application in its early developments due to the low efficiency of
normal Raman scattering, however, new developments of optical instruments,
lasers, and nanotechnology have allowed greater sensitivities and resolutions. (S.
Weng et al. 2019, 2) One such advancement to Raman spectroscopy is coherent
anti-stokes Raman spectroscopy. One of the major drawbacks of Raman
spectroscopy is that fluorescence can occur in molecules and often have a
greater intensity than the stokes and anti-stokes lines, hence increasing the
noise in the spectrum which makes identification difficult or impossible.
Coherent anti-stokes Raman spectroscopy addresses this issue by using two
pulsed lasers. The frequency is kept constant with one laser and the second
laser tuned so that the frequency difference between the two lasers equals the
frequency of a Raman-active mode of interest. (Teledyne, n.d.) In addition, it enhances
the weak Raman signal by using a four-wave mixing process. (Semrock, n.d.) Another
advancement in Raman spectrometry is Fourier-transform Raman spectroscopy which
uses a neodymium-doped yttrium aluminum garnet (Nd: YAG) laser which has a
wavelength of 1064 nm as the light source. This eliminates the fluorescence
interference problem and reduces the laser damage to samples caused by higher frequency
light. (S. Weng et al. 2019, 2)
The Raman spectrometer
is an important analytical instrument, which for the first time is being used
on another planet (Mars). (NASA, n.d.) Only speculation remains at what ‘Perseverance’
would find on Mars when it touches down, however, it would surely alter humanity’s
understanding of life itself, our origins and help scope for the possibility of
becoming a multi-planetary species. Raman spectrometry is a vital tool in
chemistry and forensics, used to determine the identities of compounds and the
composition of mixtures and works by using light to excite the vibrational or
rotational modes of molecules from virtual states. This requires innovative
uses of optics which focuses the light, filters it, reflects it, diffracts it,
and collects it onto sensitive detectors to be measured. The Raman spectrometer
still has a lot of areas for development. Within these areas of improvement are
the filters and lasers i.e., to create much more efficient narrower filters and
to have lasers of higher intensities.
References
AZO Materials. 2018. “Raman
Spectroscopy and its Most Common Applications.” Accessed 10, December 2020. https://www.azom.com/article.aspx?ArticleID=16150.
Horiba. n.d. “Instrument
Presentation.” Accessed 8, December 2020. https://www.horiba.com/en_en/ramanspectrometerpresentation/.
Mettler Toledo. n.d.
“Raman Spectroscopy.” Accessed 9, December 2020. https://www.mt.com/us/en/home/applications/L1_AutoChem_Applications/Raman-Spectroscopy.html.
NASA. n.d.
“Instruments.” Accessed 9, December 2020. https://mars.nasa.gov/mars2020/spacecraft/instruments/.
NASA. n.d. “Mars 2020.”
Accessed 1, December 2020. https://mars.nasa.gov/mars2020/.
P. J. Larkin. 2011. Infrared
and Raman Spectroscopy: Principles and Spectral Interpretation. USA:
Elsevier.
Princeton Instruments.
n.d. “Raman Spectroscopy.” Accessed 5, December 2020. https://www.princetoninstruments.com/learn/raman.
Renishaw. n.d. “Raman
Spectra Explained.” Accessed December 5, 2020https://www.renishaw.com/en/raman-spectra-explained—25807.
S. Weng, et al. “Recent
advances in Raman technology with applications in agriculture, food and
biosystems: A review.” Artificial Intelligence in Agriculture 3, no. 1-10
(October) https://doi.org/10.1016/j.aiia.2019.11.001.
Semrock. n.d. “Coherent
anti-stokes Raman Scattering.” https://www.semrock.com/coherent-anti-stokes-raman-scattering-cars.aspx.
