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Raman Spectroscopy
Joe A. Granados
Summer Bridge Program
July 17, 2003
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Raman Spectroscopy
Introduction:
Rubies! Emeralds! Diamonds! Tables upon tables of these precious stones are collected
from all over the world at the Gem and Mineral Show. Crowds of people gather to buy,
sell, and trade their little rarities. What if amidst all of these items were a few eye
catching gems that could only be found on another planet? How much would people pay
to have a ring with a Martian mineral imbedded on its surface? In the year 2008 another
rover is going to make a six to eight month trip to Mars. Onboard the rover, will be a
probe attached to a device that can identify minerals with more efficiency than any other
Mars bound instrument. This device is called a Raman Spectrometer. It is governed by
the theory of Raman scattering which was discovered in 1928 (Skoog). Now, with the
advent of modern technologies, Raman spectroscopy is a valuable tool for analyzing
molecules.
Objective:
Before this upcoming rover mission can begin, a Raman instrument specific to the Mars
bound machine needs to be constructed as well as a database and a program for operating
the instrument and interfacing with the database. The purpose of this project was to assist
in the creation of the mineral portion of the database. For this, a basic understanding of
Raman spectroscopy was required in order to use a Raman spectrometer in a lab setting
for the purpose of collecting data for the database. Knowledge of the instrument’s devices
and the program that links the operator to the instrument is also vital in creating a library
of Raman spectra.
Theory:
Raman spectroscopy is the creation and investigation of Raman scattering for the purpose
of identifying molecules (C. & G.). Raman scattering is created by irradiating a sample
with a light source at one specific wavelength or color. Today, this monochromatic light
source is usually in the form of a laser. In order for this to work, the laser has to be at a
wavelength so as not to be absorbed by the molecule (Skoog). Absorption occurs when
the source has enough energy to bring the molecule into an exited electronic state. The
resulting observed light radiating from the sample will be in the form of luminescence.
This can be florescence which is radiation emitted as the molecule transitions from an
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excited state to a lower one of the
phosphorescence which is
radiation emitted as the molecule
transitions from an excited state
to a lower one of a different
multiplicity (Karplus). To prevent
this type of absorption the light
source needs to have energy
lower than the energy required to
bring a molecule from the ground
state to the lowest electronic
state. E = h * ν = h * (c / λ) where
the energy is equal to plank’s
constant multiplied by the
frequency of the light which is
nothing but the velocity of light
divided by its wavelength. This
suggests that a light source with
the longest possible wavelength
would be ideal. However, there is
another form of absorption that is
used for infrared spectroscopy.
IR absorption spectra of
substances can be acquired by
irradiating molecules with
infrared radiation of varying
frequencies to achieve a
resonance that matches the
vibration of the molecule. IR is light at wavelengths greater than 700 nm. Also, the
detector that picks up the light spectra has a detection limit that begins to drop off at 1050
nm and terminates at 1100 nm. Since the Raman Shift is measured in wave numbers
away from the light source, not all of the Raman scattering may be observed by the
detector. For example a light source at 900 nm would be restricted to Raman scattering
less than 1587 cm-1 or wave numbers from converting the laser wavelength and the
detector drop off wavelength to wave numbers by: cm-1 = (107 nm) / (λ nm cm). The
company that is constructing the instrument for the a Mars rover would like to view
Raman shifts up to 4000 cm-1. Furthermore, the intensity of the scattering looks like the
following: Φ ~ σ(υex)υex4Eonie-Ei/kT where υex is the frequency of the excitation energy and
Eo is the power of the laser (Ingle). With frequency inversely proportioned to the
wavelength υ = c / λ, a longer wavelength will yield a smaller frequency even though
more power can be used without compromising the sample. This is because of the
excitation energy raised to a power of four. The term σ(υex) is Raman cross section which
is dependent on the sample, ni is the number density of state i, and e-Ei/kT is the Boltzmann
factor for state i (Ingle).
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Raman and Rayleigh scattering can now occur with the right light source generally closer
to a wavelength near IR allowing for a smaller excitation energy. This can be an energy
that brings the molecule to
some virtual state between
the ground state and the
lowest electronic state. Most
of the time the molecule
will jump back down to
where it originated from
emitting Rayleigh
scattering. This scattering is
the same wavelength as the
light source because there is
no change in excitation
energy which is a function
of hυ. where υ is the
frequency of the scattered
light and h is Planck’s
constant. When the
molecule leaves the virtual
state and finishes on the first
vibrational level of the
ground state, then ΔE is
subtracted from the initial
Energy of the light source
equaling E – ΔE. Since the
energy is smaller, the
wavelength is longer. This
is know as Stokes
emissions. if the molecule
starts off in the first
vibrational level in the
ground state when it is
irradiated, and the molecule
travels back down to the
lowest ground state a
change in energy is also
observed. This is an
increase in energy equaling
E + ΔE which indicates an
emission of a shorter
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wavelength. In general
Rayleigh scattering is 103
times more intense than the
stokes and anti-stokes lines.
Stokes lines are usually
more intense than antiStokes lines, but the ration
of anti-stokes to stokes
increases with temperature
as molecules appear in the
first vibrational level of the
ground state more often.
Though anti-Stokes lines are
not affected by florescence
as Stokes lines are. Raman
scattering is also about 10-5
of the power of the initial
light source.
The laser has an electric
field that is described by:
E = Eocos(2πυext) where E0
is the amplitude of the wave (Skoog). There an induced dipole moment (m) when the
electric field of the laser interacts with the molecule. m = αE = α Eocos(2πυext) and α is
defined as a constant called the polarizability of the molecular bond. This constant
measure the molecules ability to move in response to a passing electric field. If the
polarizability varies as a function of the distance between nuclei of the atoms in the bond,
then the molecule is said to be Raman active. This function of the polarizability will look
like the following: α = αo + (r - req)(бα/бr) where αo is the polarizability of the bond at the
equilibrium internuclear distance req and r is the internuclear separation at any instant.
The change in internuclear separation varies with the frequency of the vibration υv as
given by: r – req = rmcos(2πυvt) where rm is the maximum internuclear separation relative
to the equilibrium position. I then have the equation: α = αo + rmcos(2πυvt)(бα/бr). The
expression for the induced dipole moment is no m = α Eocos(2πυext) + Eo *
rmcos(2πυvt)(бα/бr) * cos(2πυext). Then using the trigonometric identity of cos x cos y =
(cos (x+y) + cos (x-y)) / 2, I get m = α Eocos(2πυext) + (Eo/2)rm(бα/бr)cos(2π(υex - υv)t) +
(Eo/2)rm(бα/бr)cos(2π(υex + υv)t). The first term in the equation represents Rayleigh
scattering which is at the same frequency as the light source. The second term in the
equation represents the stokes as well as the third term represents the anti-stokes
emissions. This shows that the magnitude of the Raman Shift is independent of the
frequency of the laser (Skoog). With this theory in place, All that is needed is an
instrument.
Procedure:
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On the first day of working in the Laboratory, I was to report to a scientist by the
name of John Algeo. I was also partnered with another REU student by the name of Niki
Farnsworth. John gave Niki and I a thorough tour of the lab that we were to be working
in. The tour covered the operations and functions of the Raman instrument. He also told
me what I would be doing in the lab. I would be first identifying the chemical formula of
new minerals delivered to the lab. Then, I was to use a digital camera and take pictures of
the individual minerals like the picture of gearksutite that is below.
I then needed to transfer the picture files from the camera to the computer’s hard drive. I
then was to save the pictures under there specific name and UAMM number which was
the University of Arizona Mineral Museum’s method of classification. After preparing
the minerals, I was then ready to run the Raman spectrometer and obtain spectra or
separated light scattered from the mineral.
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Gearksutite spectra with intensity as a function of Raman shift:
After optimizing the Raman spectra by obtaining as high of an intensity as possible, I
then had to format the spectra under two different file types. The first was a datalogue file
that contained the parameters of the instrument, spectra, and mineral information. The
second file was a file that could be read by a program called Omnic, which allows for the
comparison against other spectrum. With these instructions John was ready to set us off
on the tasks.
Raman spectroscopy is the creation and investigation of Raman scattering for the
purpose of identifying molecules. This is done with an instrument called a Raman
spectrometer. This works by having a monochromatic light source which is light at one
specific wavelength. The light then travels through some focusing optics before hitting
the sample. The sample then radiates scattered light. Collecting optics grab some of the
scattered light and direct it into some filtering optics that take any unwanted light. This
light is then separated out by its wavelengths before entering the detector.
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The Raman spectrometer that I have been working with uses a diode laser that operates at
785 nm. This is a measurement of nana-meters which is a measurement of the
wavelength of the light. Therefore the light has a long wavelength with a small amount of
energy. The laser then travels through a transmission grating which spreads out the
outlying light of slightly differing wavelengths while reflecting the light ninety degrees.
Next the laser light passes through the collimating lens which straightens the light and
concentrates the light into a point. From there, the light passed through an iris that only
allows the central light that is at 785 nm through. The outlying light is absorbed by the
iris. The laser then irradiates the sample which is placed on a movable stage. Below is a
cross section of the laser as it passes through each device before hitting the sample.
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The stage is adjustable so that maximum scattering can be collected by the off axis
paraboloid. The scattered light is then reflected ninety degrees and directed through the
holographic notch filters. These filters are designed to take out a small range of light. In
this case the range is at wavelengths of 785 nm give or take a few nana-meter’s. The
notch filters are used to block the Rayleigh scattering which is of the same wavelengths
as the diode laser. After the filters comes the focusing lens that focuses the light into an
entrance slit. This slit is the point were the scattered light begins its journey through the
spectrograph. This device bounce the light around reflecting out any remaining Rayleigh
scattering and converts the Raman scattering into a detectable spectra of light
wavelengths. A charged couple device then converts the light signal into electric signal,
amplifies the signal and sends the signal to a card on the computer for processing.
A computer program is used to interpret the signal from the card in the computer and
create a set of data points to graph. The computer language used to create the program is
called Labview. This programming language is a good platform for running and
collecting data from scientific instruments. For the Raman spectrometer, the main
important experimental data is saved as a datalogue file. Below is an image of the
program that operates and accepts data from the Raman instrument.
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The only problem with this format is that when the programming language that reads
these kinds of files is updated, the files become difficult to deal with. In this case the
program that runs the Raman spectrometer in the Denton group uses labview version five
programming. All the other computers in the laboratories contain labview version six. In
the past the only way to view the datalogue files was in the Raman lab. Therefore in
addition to building a mineral database, John Algeo gave Niki and I the task of creating a
program using labview version six.
Before I began working with the Denton group, I interviewed with Dr. Denton. I told him
that I would be interested in learning the labview programming language. I believe that is
why he had John Algeo start me off by going through a tutorial of the program. I did this
for one day and realized that I wasn’t going to gain very much by continuing. I had
learned about the basic functions of the program, but I was having a hard time looking at
example programs and learning from them. Luckily the next day John revealed the
conversion problem. I received a formally typed description of a program that he wanted
to create. This program was called Mineral Library Format Conversion. What it needed
to do was be able to read old datalogue files in labview version six without reformatting
the files. John then wanted to be able to save more experimental information to the files
and be able to view them again with the same MinLibFmtCon program. As a plus, the
files would still hopefully be viewable on the program that runs the Raman instrument.
On the next page is image of the Mineral Library Format Conversion program. This
shows the added features such as a link to a picture and many other text boxes and drop
down menu options.
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For the first two to three weeks of working in the Gould Simpson Building at the U of A,
I was building a mineral database using Raman spectroscopy for mineral identification
and creating a program that reads the mineral database files and allow for files
modification. After the general completion of the program, a post doctorate by the first
name of Kevin Toerne asked me if I would help him out with the programming that
operates his mass spectrometer. He wanted to be able to zoom in on his graphs of the data
points while the program was operational. He also wanted to be able to create graphs that
contained fifteen to thirty-two thousand data points with out having to reformat the graph
with each file, because the program that he has creates fifty files with fifteen to thirty-two
thousand data points roughly every half minute that the instrument is operational. During
an experiment with several solutions, that have several minutes in which ionization
occurs, many data files can be created. In affect, I spent another week or two learning
about his program that runs the mass spectrometer and the functions of the instrument
itself, while getting his labview program to run better, and creating a program that uses
an entirely different programming language called Visual Basics to operate Microsoft
Excel and create standardized graphs for his large data files.
The rest of my time in the program was spent in meetings on average of twice a week and
with discussions with Dr. Denton, John Algeo, and the other scientists in the Denton
group. There was also much collaboration with Niki Farnsworth in regards to running the
Raman instrument, creating the mineral database, and working on the Mineral Library
Format Conversion program. I have conducted much research through books from the
library, books from the Denton group, and articles published from the Denton group in
Raman spectroscopy. This was for the purpose of creating an educational power point
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presentation for my peers as well as for a formalized report containing the details of my
experience in the Summer Bridge Program.
Acknoledgements:
I would like to give thanks to Dr. Bonner Denton for taking me in and spending the time
to teach me of the many aspects of analytical chemistry. I also give thanks to John Algeo,
John Yamasaki, and everyone else in the Denton group who has guided me. I want to also
give thanks to Niki Farnsworth who I worked on the project in the program with. The
mentors James Little, John Lawver, Erwin Sucipto also played a huge roll in keeping
everything on track.
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Bibliography
Bilhorn, R. B., J. V. Sweedler, P.M. Epperson, and M. B. Denton. “Charge Transfer
Device Detectors for Analytical Optical Spectroscopy—Operation and
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C. & G. Merriam Co. “Webster’s Revised Unabridged Dictionary.” Dictionary.com
(1988): 18 July 2003 <http://dictionary.reference.com/>.
Ferraro, John R., and Kazuo Nakamoto. Introductory Raman Spectroscopy. San Diego:
Academic, 1994.
Ingle, James D. Jr., and Stanley R. Crouch. Petrochemical Analysis. Upper Saddle River:
Prentice-Hall, 1988.
Karplus, Martin, and Richard N. Porter. Atoms & Molecules. Menlo Park: Benjamin,
1970.
Miessler, Gary L., and Donald A. Tarr. Inorganic Chemistry. Englewood Cliffs: PrenticeHall, 1991.
Skoog, Douglas A., F. James Holler, and Timothy A. Nieman. Principles of Instrumental
Analysis. Philadelphia: Harcourt, 1998.
Sweedler, Jonathan V., Robert B. Bilhorn, Patrik M. Epperson, Gary R. Sims, and M.
Bonner Denton. “High-Performance Charge Transfer Device Detectors.”
Analytical Chemistry. 60(5), 282A (1988).
Venkataraman, G. Journey Into Light. Sadashivanagar Bangalor: Indian Academy, 1988.
Weissbluth, Mitchel. Atoms and Molecules. New York: Academic, 1978.