What is Raman spectroscopy?
When molecules interact with incident
radiation, most of the radiation is scattered elastically (Rayleigh
scattering). However, certain molecules channel some of the incident radiation
into vibrational modes, resulting in the scattering of a small part of the
radiation (on the order of 10-7) at different, longer wavelengths
(this is called Stokes Raman scattering). Since Raman spectra are unique, they
provide a sort of chemical fingerprint for unknown compounds.
1. Abstract
Previously a low-resolution, classical spectrograph
with a resolution of about 700 and a spectral range of about 200 nm was
constructed.
For vibrational spectroscopy, a Raman system was built
for the above spectrograph and also for use with a higher-resolution Littrow
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 which 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 flourescence in many samples.
2. Objectives
1. To construct a Raman spectrograph system using a
Raman head consisting of an excitation laser, a set of filters, appropriate
lenses, and a sample holder, to be attached to a spectrograph.
2. To test the Raman system with various Raman standards
and make modifications as required.
3.1.
The Raman effect
Raman spectroscopy,
like infrared spectroscopy, is a form of vibrational spectroscopy. In a Raman
spectrograph system, 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) (21).
Stokes scattering results when a molecule is in its ground state when it
interacts with the beam of light; some of the energy from the colliding photon
is channeled into the vibrational mode of the molecule. This causes the light to
be absorbed and then re-emitted at a lower frequency. Anti-Stokes scattering
occurs when a molecule is in a vibrationally excited state when it interacts
with the radiation; the interaction can cause it to drop to the ground state
and lose some of its vibrational energy to the re-emitted (higher 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) (21). 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 (2).
Fig. 1 - Jablonski energy diagram (17). The Raman
effect with Stokes scattering 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 (this is the quantum mechanical model
for the Raman effect). With Anti-Stokes scattering just the opposite happens.
(12) In Rayleigh scattering, the
molecule absorbs a photon and scatters it at the same energy.
Fig. 2 - Spectrum of CCl4 demonstrating
the symmetry of
Stokes and Anti-Stokes bands (12).
Not all vibrations will be observable with Raman spectroscopy, depending on the symmetry of the molecule, but there is usually enough information present to enable a precise characterization of molecular structure (17). Unlike infrared spectroscopy, which results from a change in the dipole moment, Raman scattering results from changes in the polarizability of a molecule; thus Raman complements infrared spectroscopy. The energy of a vibrational mode depends on molecular structure and environment, including atomic mass, bond order, molecular substituents, moelcular geometry and hydrogen bonding, all of which effect the vibrational force constant that in turn dictates the vibrational energy (19). Classically, a molecule can be thought of masses attached by strings governed by Hooke’s law, where k is the vibrational force constant. Especially with infrared spectral lines, which result from changes in the dipole moment that occurs as a result of molecular vibration, the frequency of a vibrational mode may be understood in light of the fact that an incident oscillating electric field is producing alternating electric forces on a molecule, causing the dipole spacing to alternately increase and decrease (8). The force constant in a diatomic molecule can actually be classically derived, if the masses of the atoms in question are known, as a function of the fundamental frequency of vibration: I have done this below using differential equations. For more complicated systems of atoms, knowing the relationship between the observed wavenumbers in a spectrum and the force constants between the atoms can be much more difficult and require computer modeling, even for very practised chemical spectroscopists (15).
3.2.
Derivation of the relationship between frequency and Force
Constant for a Diatomic Molecule
Not IR active; raman active IR
active; not raman active
Fig. 4 - Diagrams contrasting symmetric (left) and
anti-symmetric (right) stretches for CO2 (2).
Again, for a transition to be Raman
active, there must be a change in the polarizability of the molecule. But what is
polarizability? Polarizability is the change of the dipole moment over distance
(2), or the the ease of which an electron cloud can be distorted by an external
electric field. There must be a change in polarizability during the vibration
for that vibration to inelastically scatter radiation (19). In considering
polarizability, it is useful to consider whether there is a change in the
volume of the electron cloud during the vibration. For instance, the CO2
molecule (O=C=O) has a “symmetric stretch,” a vibrational mode in which both
oxygen atoms oscillate in opposite directions away from the carbon atom; since
the volume of the electron cloud around the molecule effectively grows smaller
and larger, the vibration is Raman active. On the other hand, there is another
vibration, an “antisymmetric stretch,” in which the oxygen atoms move in the
same direction; in this vibration, the increase in volume on one side is offset
by a decrease in volume on the other, so that the volume of the electron cloud
doesn’t effectively change and the vibration isn’t raman active. (3) Chemists
use polarizibility ellipsoids to describe polarizability; they compare
the ellipsoid at equilibrium bond length to the ellipsoid for the extended and
compressed symmetric motions. (2)
Fig. 5 -
Diagram of electron cloud around Carbon Tetrachloride (12).
1. Instrumentation
Raman systems are
generally composed of several main parts, as follows.
1. 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 green 532nm laser
I used in my project was a diode-pumped solid state laser. It uses an infrared
laser to pump a frequency-doubling crystal.
Fig. 5 - Internal construction of 532nm laser.
http://www.repairfaq.org/sam/dpmdpss1.gif
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 (12).
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.
Fig. 5 (below). A table
of wavelengths with associated wavenumbers for a laser of 532 nm is shown
below. The green and orange numbers represent wavelengths for a laser and for a
mercury lamp which was used for calibration.
Table 1. Frequency
vs. Wavelength Table
|
Wavelength (nm) |
n -1 (cm-1) |
Δ-1 |
|
Wavelength (nm) |
n-1 (cm-1) |
Δ-1 |
|
532 |
18796.99248 |
0.00 |
|
571 |
17513.13485 |
1,283.86 |
|
534 |
18726.59176 |
70.40 |
|
572 |
17482.51748 |
1,314.47 |
|
535 |
18691.58879 |
105.40 |
|
574 |
17421.6028 |
1,375.39 |
|
536 |
18656.71642 |
140.28 |
|
575 |
17391.3043 |
1,405.69 |
|
537 |
18621.97393 |
175.02 |
|
576 |
17361.1111 |
1,435.88 |
|
538 |
18587.36059 |
209.63 |
|
576.97 |
17331.9237 |
1,465.07 |
|
539 |
18552.8757 |
244.12 |
|
578 |
17301.0381 |
1,495.95 |
|
540 |
18518.51852 |
278.47 |
|
579.07 |
17269.0694 |
1,527.92 |
|
541 |
18484.28835 |
312.70 |
|
580 |
17241.3793 |
1,555.61 |
|
542 |
18450.1845 |
346.81 |
|
581 |
17211.70 |
1,585.29 |
|
543 |
18416.20626 |
380.79 |
|
582 |
17182.13 |
1,614.86 |
|
544 |
18382.35294 |
414.64 |
|
583 |
17152.66 |
1,644.33 |
|
545 |
18348.62385 |
448.37 |
|
584 |
17123.29 |
1,673.70 |
|
546.0731 |
18312.56658 |
484.43 |
|
585 |
17094.02 |
1,702.98 |
|
547 |
18281.53565 |
515.46 |
|
586 |
17064.85 |
1,732.15 |
|
548 |
18248.17518 |
548.82 |
|
587 |
17035.78 |
1,761.22 |
|
549 |
18214.93625 |
582.06 |
|
588 |
17006.80 |
1,790.19 |
|
550 |
18181.81818 |
615.17 |
|
589 |
16977.93 |
1,819.06 |
|
551 |
18148.82033 |
648.17 |
|
590 |
16949.15 |
1,847.84 |
|
552 |
18115.94203 |
681.05 |
|
591 |
16920.47 |
1,876.52 |
|
553 |
18083.18264 |
713.81 |
|
592 |
16891.89 |
1,905.10 |
|
554 |
18050.54152 |
746.45 |
|
593 |
16863.41 |
1,933.59 |
|
555 |
18018.01802 |
778.97 |
|
594 |
16835.02 |
1,961.98 |
|
556 |
17985.61151 |
811.38 |
|
595 |
16806.72 |
1,990.27 |
|
557 |
17953.32136 |
843.67 |
|
596 |
16778.52 |
2,018.47 |
|
558 |
17921.14695 |
875.85 |
|
597 |
16750.42 |
2,046.57 |
|
559 |
17889.08766 |
907.90 |
|
598 |
16722.41 |
2,074.58 |
|
560 |
17857.14286 |
939.85 |
|
599 |
16694.49 |
2,102.50 |
|
561 |
17825.31194 |
971.68 |
|
600 |
16666.67 |
2,130.33 |
|
562 |
17793.59431 |
1,003.40 |
|
601 |
16638.94 |
2,158.06 |
|
563 |
17761.98934 |
1,035.00 |
|
602 |
16611.30 |
2,185.70 |
|
564 |
17730.49645 |
1,066.50 |
|
603 |
16583.75 |
2,213.24 |
|
565 |
17699.11504 |
1,097.88 |
|
604 |
16556.29 |
2,240.70 |
|
566 |
17667.84452 |
1,129.15 |
|
605 |
16528.93 |
2,268.07 |
|
567 |
17636.6843 |
1,160.31 |
|
606 |
16501.65 |
2,295.34 |
|
568 |
17605.6338 |
1,191.36 |
|
607 |
16474.46 |
2,322.53 |
|
569 |
17574.69244 |
1,222.30 |
|
608 |
16447.37 |
2,349.62 |
|
570 |
17543.85965 |
1,253.13 |
|
609 |
16420.36 |
2,376.63 |
Intensity of the
Raman signal is inversely proportional to the 4th 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 flouresce 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. I chose a visible-band laser because I had one for pointing out
stars. The optics involved with infrared and ultraviolet lasers and their
associated filtering mechanisms seemed out of my price range. In retrospect, the inexpensive 532nm
lasers (I tried several) oscillated at 3 frequencies, making Raman analysis
difficult.
2. 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) (13). In my Raman system, the laser-line filter’s cutoff was too
far from the 532nm laser line to clean up the harmonics.
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)
(12). 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 radiation may pass through) (13).
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 was used in my Raman
system 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; (12) they can also be arranged so
that the scattered light is collected at an oblique angle, as on the previous
page.
|
Fig. 6 - Graph of notch
excitation filter (13). |
Fig. 7 - Graph of barrier
long-wave-pass filter (13). |
3. Lens to collect scattered light.
A lens collects the Raman
scattered light coming through the dichroic mirror and focuses it to the slit
of a spectrograph. Basically the job of the spectrograph is to transform the Raman
light into a spectrum, which, upon being collected by the CCD camera can be
analyzed by a spectral processing program. In older systems before CCD camera
were available a grating was mechanically scanned and a photomultiplier tube
was used as a single pixel detector. A device with an input slit, lenses or
mirrors, a grating, and an output
slit is called a monochromator.
Fig. 8 - Jobin Yvon T64000 Raman system using triple monochromator, similar to that used at OU by Dr. R. Freck (15). Such systems cost over $100,000. The CCD Camera, which is what is sticking up in the photo, is liquid nitrogen cooled. (17)
Littrow spectrograph design and CCD camera considerations
Since
I wanted my Raman spectrograph to be medium- to high-resolution, I chose to
upgrade the low-resolution spectrograph I built in my project two-years ago to
a higher resolution spectrograph with an entirely different design, called a
Littrow spectrograph. Unlike the classical spectrograph, the littrow
spectrograph only uses one lens, which decreases its size and cost and
increases its stability (theoretically). This lens collimates light entering
the spectrograph from a slit and sends it to the grating; after the light is
dispersed, the lens focuses the spectrum onto a camera chip.
Spectrographs output graphs of
intensity versus wavelength, meaning that
they show the different wavelengths of a particular light source. The spectrograph generally incorporates a spectral dispersion device (a diffraction grating), the collimation lens, and the collection device (for me, a CCD camera).
My final drawn design for my Raman system. You can
click it to see an enlarged version.
A photo of my Raman system. In this photo the Raman
head is not attached to the Littrow
spectrograph (diagrammed above); it is attached to the Classical spectrograph.
You can click on the photo to see it enlarged.
6. Data Analysis
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. 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 higher-groves 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 * 105, 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.
7. Conclusions
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. 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
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.
8. Acknowledgements
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 have been virtually impossible. I would also like to thank Alan Holmes and the Santa Barbara Instrument Corporation (SBIG) for letting me borrow an ST-10 CCD camera for future use with my project. I would like to thank the IAPPP (International Amateur Photoelectric Photometry) and the AAS (American Astronomical Society) for granting me and my previous partner Sarah Howell the Richard D. Lines award last year, which helped to finance my project this year. I give thanks also Dr. Roger Freck of the chemistry department at the University of Oklahoma, for taking the time to graciously explain the fundamentals of the Raman effect to me, especially the classical and quantum explanations of how an incoming laser beam affects molecules, and letting me see his two Raman spectrographs. 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.
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