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

Construction and Operation of a Raman Spectrograph
Mary Masterman
Westmoore High School
March, 2006

Abstract
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Previously a low-resolution, classical spectrograph with a resolution of about 700 and a spectral range of about 200 nm was
constructed.
A Raman system was built for the above spectrograph. It used a green 532 nm laser to stimulate Stokes scattering in samples.
The laser beam was reflected off a dichroic mirror at an angle of 90°. A microscope objective then focused the laser beam onto the
sample and collimated the returning light. Light of the laser wavelength was filtered out by an interference filter that reduced the
laser signal by approximately 10 5 . A 50 mm f.l. lens then focused the waveshifted light on the entrance slit of the spectrograph.
Measured wavenumbers correlated well to those in published spectra. Difficulties included false spectral lines created by the
multimode laser and fluorescence in many samples.
A Littrow spectrograph was then constructed. A medium-format 200 mm lens was used as a combined collimation and camera
lens. A body, utilizing a diagonal mirror, connected the lens with the camera. The modular design facilitated interchanging of the
lens, camera, and grating holder with different models. An adjustable slit was created, with light entering through a standard SCTthread
adapter. The spectrograph could be mounted directly to the Raman head or used in other applications, including astronomy.
The F/4.8 spectrograph had a resolution of 6500 and a spectral range of 80 nm with a 1200 l/mm grating. The spectrograph was
tested with a consumer DSLR camera as well as a cooled CCD camera.

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Introduction
Objectives
• To construct a Raman spectrograph system using a Raman head consisting of a set of filters and a sample holder,
appropriate lenses, and an appropriate excitation laser, to be attached to a spectrograph.
• To build a medium- to high-resolution inexpensive amateur Littrow spectrograph to be attached to the Raman head.
• To test the Raman system with various Raman standards and make any modifications as necessary.
Theory
Raman spectroscopy, like infrared spectroscopy, is a form of vibrational spectroscopy. The Raman effect occurs when a photon
striking a molecule excites an electron into a higher “virtual” energy level, and the electron decays back to a lower level, emitting a
scattered photon (6). Unlike infrared spectroscopy, which results from a change in the dipole moment, Raman scattering results
from changes in the polarizability of a molecule.
In Raman, a high-intensity beam of light is directed to a sample. The vast majority of the light is scattered elastically (Rayleigh
scattering) and the reflected light has the same wavelength as the incident light. A very small portion of the scattered radiation,
however (on the order of 10^-7) is shifted to a different wavelength. The scattered photons are known as Raman scattering. A few
are shifted to higher energy wavelengths (anti-Stokes radiation) but most are shifted to lower frequencies (Stokes radiation) (6). If a
molecule is in its ground state when it interacts with the beam of light, some energy from the colliding photon can be channeled into
the vibrational mode of the molecule, causing the light to be absorbed and then re-emitted at a lower frequency (Stokes scattering).
If a molecule is in a vibrationally excited state when it interacts with the beam of light, the interaction can cause it to drop to the
ground state and lose some of its vibrational energy to the re-emitted (high frequency) light. (3) Stokes scattering is much more
common than anti-Stokes, because at normal temperatures, electrons are most likely to be in their lowest energy state (a result of the
Boltzmann energy distribution) (6). A spectrum of anti-Stokes scattering is identical in pattern, but much less intense, than a
spectrum of Stokes scatterings;for this reason, only Stokes scattering is typically used in Raman spectroscopy. (Fig. 1)
Fig. 1 - Spectrum of CCl 4 demonstrating the symmetry of Stokes and Anti-Stokes bands (4).

Raman Instrumentation
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Raman systems are generally composed of several main parts, as follows.
A. The excitation source (laser).
Lasers make modern Raman feasible because they give a coherent beam of monochromatic light and can have very high power.
They must be of sufficient intensity to produce the desired and detectable amount of Raman scatter, and they should be free of
extraneous bands. They should thus exhibit good wavelength stability and low background emission.
The frequency of a Raman shift is independent of the laser wavelength used for excitation. This is an important concept in
Raman. In Raman, unlike electronic spectroscopy, wavelengths are not normally measured in Angstroms or nanometers. Since the
frequency shift, not the actual frequency, is what is important, Raman spectra are generally plots of intensity versus wavenumber:
, where c is the speed of light in cm/s and v is the frequency in s^-1. (4) Since it is independent
of the laser wavelength, the wavenumber will be the same for a 532 nm or a 785 nm laser. The wavelength difference for a
particular band, however, is not necessarily the same. For instance, a wavenumber of 1000 cm^-1 for a 532 nm laser would result in
a Stokes band ~30 nm away from the laser line. But the same wavenumber for a1064 nm laser would result in a Stokes band ~130
nm away from the laser line.
Intensity of the Raman signal is inversely proportional to the 4 th power of the laser wavelength (3):
. It would seem,
then, that laser radiation of a high frequency would be ideal. However, since samples tend to fluoresce at higher-frequency
wavelengths, and since fluorescence is a much more efficient process than Raman, this is not necessarily true. Thus, infrared lasers
have become increasingly popular in Raman, despite their low frequencies. On the other hand, it has also been demonstrated that at
wavelengths under 260 nm, there is essentially no fluorescence interference (3); so ultraviolet lasers have the benefits of stimulating
strong Raman scattering and not causing samples to fluoresce.
B. System to irradiate sample and collect scattered light (three types of filters).
Light from the laser is generally passed through an excitation filter in Raman systems. The excitation filter purifies the laser
beam by permitting light only of a very small selected wavelength range to pass through, eliminating background emission (1).
Excitation filters may be Short-Wave-Pass (only extraneous light of longer Stokes wavelengths than the laser is blocked) or Laser-
Line (extraneous light of wavelengths shorter and longer than the laser is blocked) (5).
After passing through an excitation filter, the laser beam is usually focused on the sample in order to increase the intensity of the
electric field on the sample surface (the magnetic component of the electromagnetic radiation is generally not relevant to Raman)
(4). Raman scattered light is then collected and passed through a barrier filter. The barrier filter prevents Rayleigh-scattered light of
the same wavelength as the laser form passing on to the slit of the spectrograph (the Rayleigh scattering is so much more intense
than the Raman that it would saturate the CCD chip).

Barrier filters may be Long-Wave-Pass (only Stokes radiation may pass through) or Notch (both Stokes and anti-Stokes
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radiation may pass through) (5).
A third type of filter, known as a dichroic mirror, reflects light of the wavelength of the laser and allows Raman scattering to pass
through it. It is (sometimes) used between the excitation and barrier filters at an angle of 45 degrees, to direct the laser beam to the
sample.
There are a variety of optical configurations for the Raman probe head. The filters can be arranged so that the scattered Raman is
collected at an angle of 90 degrees or 180 degrees from the incident laser beam; (4) they can also be arranged so that the scattered
light is collected at an oblique angle, as on the previous page.
Fig. 2 - Graph of notch excitation filter (5).
Fig. 3 - Graph of barrier
long-wave-pass filter (5).
C. Lens to collect scattered light, spectrograph, and collecting device.
A lens collects the Raman scattered light coming through the dichroic mirror and focuses it to the slit of a spectrograph. The
spectrograph built in this project is known as a Littrow spectrograph, meaning that it uses just one lens as a collimation lens for the
Raman light and as a lens to focus the dispersed light onto the collecting device (a camera). (2) A device must be built to hold the
grating in place / allow it to be tilted, preferably with a stop so that the grating can be flipped between first and zeroth order.
Basically the job of the spectrograph is to transform the Raman light into a spectrum, which, upon being collected by the camera /
processor, can be analyzed by a spectral processing program.

Materials and Methods
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Primary Materials
Drill press with various drill bits, C-clamps, taps, and screws; Lathe with 3-jaw and 4-jaw chucks; Various scrap aluminum
tubing, L-brackets, and plates; Black foamie w/ double-stick tape; Canon EOS 300D digital camera; 300 mm medium-format
camera lens; Mounting board with aluminum rails; 1200 lines / mm 5 cm^2 diffraction grating; Vice and files; Saw; Hand-drill;
Canon 50 mm camera lens; Chroma Corporation filter set (including barrier filter, excitation filter, and dichroic mirror); Thin brass
sheet to hold dichroic mirror in place; Canon 67 mm filter to hold barrier filter; 532 nm 5 mW green laser; Red laser, level, and ruler
(for calibration); Flat diagonal mirror with adjustment screws; Micrometer head (to adjust angle of grating).
Methods
Build Raman filter cube using square aluminum tubing. Carefully contruct brass sheet to hold dichroic mirror and drill holes in
filter cube for other filters. Very carefully, and with gloves, slide dichroic mirror into brass sheet so that it is at a 45° angle to filter
cube. Construct mounting board for Raman system using large wood board and aluminum rails. Attach lenses to rails using L-
brackets and and attach to mounting board. Construct holder for laser using aluminum tubing and screws. Attach barrier filter to lens
using 67 mm filter holder. Attach excitation filter to laser holder. Align optical system.
Construct backing-ring to attach Pentax 6x7 ring to Canon EOS 300D Camera. Construct holder for diagonal on lathe using
round aluminum tubing. Attach diagonal mirror to holder using double-stick tape. Construct grating holder using micrometer head,
brass rod for grating to turn on, and part of an aluminum L-bracket. Attach grating with double-stick tape. Align optical system.
Attach spectrograph to Raman head. Collect spectra. In the Raman system, the dichroic mirror was not initially at the correct 45º
angle, so that the filter cube had to be made tiltable and separate holders for the barrier and excitation filters had to be constructed.
Due to the necessity of placing the invisible Raman signal on the slit of the spectrograph, alignment of the Raman system’s
optical path was very challenging. The Raman system didn’t work at all until the sample focusing lens was replaced with a
microscope objective, which presumably dramatically intensed the intensity of the laser beam on the sample. It was very difficult to
design the Littrow spectrograph such that the diagonal and SCT ring would fit in the flange-to-film distance of the lens, partly
because the distance was not as published.
Alignment of the diagonal mirror so that the entrance image of the spectrograph resulted in a centered and focused image on the
camera chip proved to be tedious.

Fig. 4 – My diagrams of spectrograph system. At the top, top and side diagrams of the Littrow spectrograph are shown. A photo of
that system is on the bottom right. A photo of the Raman system is on the bottom left.
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Results
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Aromatic ring breathing at
1004 cm^-1
Wavenumber (cm^-1)
Fig. 5 - 10 second spectrum of toluene gathered with spectrograph. The mercury calibration spectrum is superimposed.
Fig. 6 - Actual raw image acquired by spectrograph.
Fig. 7 - Molecular structure of toluene. The aromatic
benzene ring associated with the band at 1004 cm^-1 is
visible. the lines in the toluene spectrum above are due to
the vibrational stretching of some of the bonds in toluene
upon interaction with the laser beam.

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.

Conclusion and Recommendations
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In the end, a Raman spectrograph system was successfully constructed. The process of building the Raman head was difficult,
especially the optical alignment, and only through trial and error were Raman spectra finally achieved. The Littrow spectrograph
(attached to the Raman head) still proved to be more difficult to construct than the Raman head due to the even more complicated
process of optical alignment associated with the diagonal mirror between the collimation lens and the camera chip. Optical
alignment of the Raman system was difficult due to the faintness of the Raman signal (it is invisible to the naked eye, so it is
difficult to get the signal aligned on the slit of the spectrograph). The difficulty associated with accounting for the small backfocus
of my digital camera proved to be a severe problem. The Littrow spectrograph produced spectra that were higher resolution than the
classical spectrograph constructed in my previous project; however, all in all, I would say that the difference in resolution and
spectral dispersion were not worth the added trouble of building and collimating the new spectrograph. Still, the idea that Raman
spectrographs—high or low resolution ones—might be someday constructed for very wide-ranging industrial applications and for a
relatively small cost is very exciting.
In the future I would like to create a program to quicken data analysis of spectra produced by the Raman system, and even write
a program to use least-squares analysis to automatically determine the composition of compounds using a library of Raman spectra
of common compounds. I would also like to examine the relationship between compound concentration and intensity of the Raman
signal, which theoretically are directly proportional. This will require a carefully alignable sample holder, with a constant thickness,
since the Raman signal varies depending on how much of the sample the laser light passes through. I would like to experiment with
the sample holder and the laser alignment to see if the Raman signal can be maximized.
Raman spectroscopy has an amazingly diverse array of applications in society, including analysis of synthetic polymers like
rubber, identification of legal and illegal drugs (forensics) and identification of explosives, determination of unsaturation in food
oils and fats, determination of defects in semiconductors; in-situ measurements of rocks and minerals during planetary missions,
real-time detection / diagnosis of cancer, and analysis of artwork. These applications pose a seemingly unlimited number of
potential project ideas, and emphasize the significance that amateur Raman spectroscopy could have on the entire field.

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Acknowledgement
I would like to thank Chroma Corporation for courteously donating me a set of Raman filters, including a dichroic mirror, barrier
filter, and excitation filter, for use in my research; without these filters, my project probably would not even have been possible. I
would also like to thank Alan Holmes and the Santa Barbara Instrument Corporation for letting me borrow an ST-10 CCD camera
for future use with my project. I wish to thank Dr. Roger Freck of the Chemistry Department at the University of Oklahoma for
taking the time to explain the fundamentals of vibrational spectroscopy to me and clearing up all of my misconceptions about
Raman spectroscopy. I would like to thank the IAPPP (International Amateur Photoelectric Photometry) and the AAS (American
Astronomical Society) for granting me and my partner Sarah Howell the Richard D. Lines award last year, which helped to fund my
project this year. I would also like to thank Mr. Jeffrey Baughman for helping me with all of the advice he offered me concerning
my project and his immense help with regards to the forms and other technicalities required. I would most like to thank my parents
for their support, specifically my dad, who taught me how to use AutoCAD to design my spectrograph and instructed me in the
difficult process of machining the parts for my spectrograph.

References
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1. Abramowitz, M., & Davidson, M. (2004). Molecular Expressions Microscopy Primer: Specialized Microscopy Techniques:
Fluorescence Filters. Retrieved November 10, 2005, from
http://microscope.fsu.edu/primer/techniques/fluorescence/filters.html.
2. Buil, Christian. (n.d.) The Littrow Spectrograph. Retrieved Novemeber 4, 2005, from
http://www.astrosurf.com/buil/us/spectro8/spaude_us.htm.
3. Brief description of Raman scattering. (n.d.) Retrieved September 28, 2005, from http//www.omegafilters.com/index.php?page =
omegatext/prod_raman_scatter.
4. Ferraro, J. & Nakamoto, K. (1994). Introductory Raman Spectroscopy. San Diego, C.A.: Academic Press, Inc.
5. Filter Types for Raman Spectroscopy Applications (2005). Retrieved November 23, 2005, from
http://www.semrock.com/Catalog/Raman_filtertypes.htm.
6. Raman Spectroscopy - An Overview (2001). Retrieved November 23, 2005, from
www.kosi.com/raman/resources/technotes/1101.pdf.