Mary Masterman - Oklahoma Biological Survey - University of ...

Discussion of Results
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Eventually, both the Raman head and the Littrow spectrograph were successfully constructed and attached. Raman spectra of
various substances, including standard Raman chemicals like acetone and toluene and also other various materials, including ethyl
alcohol, glass, diamond, acetaminophen (Tylenol), and Teflon, were successfully observed. With many materials, fluorescence was
a problem, probably due to the relatively high frequency (as compared to infrared) of the laser. Spectra were calibrated with a
mercury lamp, and the observed wavenumbers for various lines in the spectra correlated very well with published wavenumbers.
The resolution of the Littrow spectrograph with the 1200 lines/mm grating was about 7000, and the range was about 300
Angstroms, which turned out to be enough to cover most of the desired spectral range with Raman. It did prove to be a challenge to
maximize the intensity of the Raman signal, especially with the higher resolution spectrograph and the very thin slit required of the
higher resolution setup; but when exposure lengths were adjusted, spectra could still be successfully obtained of molecules with
pronounced Raman resonances.
After the Raman spectrograph system was completed, the main obstacle to good spectra proved to be an unexpected one: all of
the Raman lines, as it became apparent as I narrowed my slit and got higher resolution spectra, did not have sharp, well-defined
profiles as described in the literature but instead were complex blends of three or four peaks. At first I attributed this to some flaw of
the Littrow spectrograph, which was actually producing lines with a slight ghost on the red side (perhaps because of the highergroves
per mm diffraction grating). I noticed, however, that the laser line in particular was characterized by the complex profile, and
the mercury lamp calibration lines were not. When I took spectra of the laser line by itself, without any filters, I saw that it was not
outputting light at one specific frequency, but rather at several, and that—since Raman frequencies are independent of the laser
frequency—the Raman lines were mimicking the same spectral profile. To produce better quality Raman spectra, I would need a
more expensive single-mode laser.
Since a laser line was visible in most of the spectra obtained, I wondered what the attenuation of the laser line by the barrier filter
was. It was found the ratio of the laser line intensity to the intensity of the blocked laser was 1.25 * 10 5 , with a 3.5% error (over two
trials). When the intensity of the Raman signal for toluene, an enthusiastic Raman scatterer, was compared to the computed,
normalized full-intensity laser beam, it was found that the ratio between the intensities of the Raman Stokes scattering and the laser
line was 3.7 * 10 -6 . This was comparable to the 10 -6 or 10 -7 value mentioned in the literature.