Raman Scattering
Introduction
The Raman effect is the appearance of weak lines in
the spectrum of light scattered by a substance which has been illuminated by a
monochromatic light (with angular frequency w). The lines occur close
to, and on each side of, the incident light frequency, and hence are optical
sidebands. The sidebands arise from the nonlinear interaction of the light with
atomic or molecular quantum states in the scattering material. In a classical picture, the light induces a
dynamic (time dependent) response in the polarizability of the substance, and
then the product of the polarizability with the original light field results in
the optical sidebands. In a quantum mechanical picture, the nonlinearity is
equivalent to second order time-dependent perturbation theory. In this case,
one encounters a product involving a quantum state a with time dependence exp(-iwat) , the complex conjugate of a quantum state b with time dependence exp(iwbt) , and the electromagnetic field with time
dependence cos(wt). Using simple trig identities, one obtains
a resultant time dependence cos[(w - (wb - wa)) t]
and cos[(w + (wb - wa)) t]
. By analogy with the terminology used in fluorescence, the lines corresponding
to a lower frequency are called Stokes lines and those corresponding to a
higher frequency are called Anti-Stokes lines. By measuring the frequency
shifts and wb - wa, the structure of the system can be
determined. [1-8]
Recalling
that second order perturbation theory involves a sum over virtual states, a
pictorial mnemonic for Raman scattering may be viewed as in Fig. 1.
Figure 1.
Illustration of the quantum state transitions for the Stokes (left) and
Anti-Stokes (right) processes.
Carbon
tetrachloride provides a good example of a Raman sample; the low-lying levels
are different vibrational states of the molecule and the virtual state lies
near an excited electronic state of the molecule. By examining the Raman
spectrum, the frequency of the vibrational modes of the molecule can be
deduced. Additional information can be obtained from the strength of the
various lines and the polarization dependence of the spectra, which may be found
from the details of the time dependent perturbation theory.
Since the
Raman effect is second order (nonlinear), the effect is weak, and a strong
source of incident light is required. The Raman experiment uses a powerful
argon (Ar) ion laser as the incident source, and the weak Raman sidebands are
detected with a double monochromator scanning spectrometer and a sensitive
photomultiplier tube (PMT) which is cooled to reduce its intrinsic thermal
noise.
Equipment
Hg lamp, Oriel Calibration Lamp Manual, and He-Ne Laser
Spectrometer for analyzing light spectra, with scanning power supplies
Detector (photomultiplier and housing attached to spectrometer and amplifier) with Peltier cooler and power supplies
Computer with data acquisition program
Lenses and samples (including carbon tetrachloride)
CAUTION: The Ar ion Laser can burn eyes, fingers,
clothing, etc. Laser safety goggles must be worn at all times. Do not put shiny
objects into the beam. Use care when aligning or realigning mirrors. Do all
alignments at minimum power (see laser manual for details of power adjustment).
Use bent pipes as optical “dumps” to stop stray laser light.
In this
experiment, the Raman spectra will be measured for several liquids, starting
with carbon tetrachloride (CCl4). Other liquids include chloroform
(CHCl3) and dichloromethane (CH2Cl2). CAUTION: The Raman samples are contained
in sealed glass vials, because they are hazardous and create noxious odors. Be
careful when handling the samples. It may be necessary to prepare fresh
samples.
An
illustration of the Raman scattering experiment is shown in Fig. 2. A moderate
power He-Ne laser is used to align the optical system. The beam from the
powerful Ar ion laser is deflected upward by a mirror so as to pass through the
Raman sample (not shown in this figure; the sample would be at the height of
the He-Ne laser). Two lenses are used to focus an image of the scattered light
inside the illuminated sample onto a narrow opening (slit) on the spectrometer.
As discussed in the spectrometer manual, the spectrometer has three adjustable
slits: an entrance slit, an internal slit, and an exit slit. The
photomultiplier unit, which is optically sealed to the exit slit, consists of
the PMT, a housing, a socket base with coaxial connections for the PMT, and a
thermoelectric cooler with cables passing from the housing.
CAUTION: At times it may be necessary to open the cover of the spectrometer.
Before opening the spectrometer, make sure the shutter in front of the PMT is
closed, otherwise the PMT may be ruined. The control knob for the shutter is
located below the exit slit. Turn the knob so that the black lines are aligned
(you will also feel it click into place); this is the closed position. The open
position is the other position where the knob clicks.
CAUTION: Use only the side cover on the spectrometer (as shown in Fig. 2). Keep
the cover closed except when opening is absolutely necessary; dust is the
biggest problem with the spectrometer. Also, never touch the surface of any
optical component; these surfaces cannot be cleaned.
Other
equipment not shown in Fig. 2 includes a NIM crate with a power supply and a
preamplifier connected to the PMT anode (signal output) with a BNC cable, a
high voltage power supply connected the PMT cathode with a coaxial cable having
special high-voltage BNC connectors, a power supply connected to the the PMT
thermoelectric cooler, a scan control unit for the spectrometer, and a computer
for data acquisition. The PMT preamplifier and an output from the spectrometer
are connected to the computer. Two small digital multimeters (DMM's) are
connected to the Ar laser and the PMT cooler power supply.
Figure 2. A schematic of the Raman
scattering experiment.
Procedure
1. The first
step is to check the status of the PMT thermoelectric cooler. The cooler is
based on the thermoelectric effect, in which a DC electric potential difference
generates a temperature gradient; by anchoring the high end of the gradient
near room temperature with a flowing water heat exchanger, the low temperature
end of the gradient can be used to cool the PMT. The thermoelectric effect is
reversible, so that a temperature gradient can generate a potential difference.
There are some potential diffculties with this cooler, so that it cannot be
turned on until two checks are performed. The reason is that the cooler may
have been used earlier, and the heat exchange water may have been stopped when
the cooler was turned off. This may result in two problems. The first problem
is that parts of the cooler which remain cold after shut-down may cause the
water (no longer flowing) to freeze and plug the heat exchanger. The second
problem is that the residual temperature gradient may establish an electric
potential across the leads connected to the cooler power supply, with the
consequence that when the power supply is turned on, the extra potential causes
too much current to flow, and a fuse is blown. The steps to check the status of
the cooler are as follows:
(a) Turn on
the DMM connected to the PMT cooler power supply (which should be off), and set
it to measure DC voltage. The absolute value of the voltage should be less than
0.1V; if not, there may be a residual temperature gradient on the thermoelectric
cooler, and you must wait until it reduces.
(b) Follow
the rubber tubing from the PMT housing to the source of the heat exchanger
water at a green-handled ball valve on a back wall near the floor. Turn on the
water by rotating the green handle until it is parallel to the outlet pipe. Do not
alter the round-handled throttling valve which is upstream from the
green-handled ball valve. Water should exit the other rubber tube into a drain
pipe, and the lights on a safety flow switch mounted on the wall near the PMT
housing should change from red to green. If water does not flow, the PMT cooler
may be plugged with ice, and you will have to wait until it melts. If the water
does flow, turn it off by rotating the green handle until it is perpendicular
to the outlet pipe. Note that the water is filtered, and that the filter should
be replaced if it appears dirty.
If the checks
indicate that the PMT cooler is operational, do not turn on the power supply at
this point. Also do not turn on the water at this point.
2. Check that
the spectrometer exit slit shutter (at the PMT unit) is closed. Turn on the PMT
high voltage power supply and set it at 900 V (the actual output voltage is negative).
Turn on the PMT preamp and set its gain to 64. The PMT preamp output should be
kept connected to the computer. You may check the PMT preamp output by
inserting a BNC tee and viewing the output with an oscilloscope [How should the
connection be terminated?]; negative pulses should be observed. The pulses have
about the same amplitude (a few hundred mV); the magnitude of the signal from
the PMT is taken as the rate that the pulses occur in time, expressed as
counts-per-second (cps). When taking data with the computer, disconnect the PMT
preamp output from the oscilloscope.
3. Turn on
the computer and make sure no anti-virus software is running; such software may
disable the computer's data acquisition hardware driver. Start the raman computer
program by double-clicking its icon on the desktop, or by running the program
C:\Program Files\DevStudio\MyProjects\raman\Release\raman.exe.
Check the Settings menu, and make sure the Timeinterval is set at 0.8 s. Use
File -> Open to open a new file and at the prompt “How long do you want to
measure?,” enter the value 2000s; proceed to step 2, and at the prompt “Enter
counter value,” accept the default (0); finally, Start the data acquisition.
The data presentation will default to a graph display of PMT pulse rate (in
cps, determined from the number of counts occurring over the time interval
given by the “Timeinterval” setting) versus time. Using the View menu, turn on
the Listview display; you should see a list of numbers scrolling up. The pulse count
rate from the PMT will show up as numbers in a column labeled as “Frequency
(Hz);” the numbers should have values of approximately 200 to 300 cps. This is
the room temperature PMT dark current count rate. The data acquisition mode may
be exited at the end of the run (or at any earlier time) by clicking the Stop
button. Leave it running for now.
4. Turn on
the water to the PMT cooler, and turn on the PMT cooler power supply. As the
PMT tube cools down, you should see its dark current count rate decrease, reaching
values of only 2 or 3 cps in 30 minutes. You can monitor the PMT cool-down with
either the Graphview or Listview display on the computer.
5. Near the
end of the PMT cool-down, you can check for light leaks. Close the cover on the
scanning spectrometer if it is open (as shown in Fig. 2), close the entrance
slit, open the internal and exit slits, and finally open the shutter on the
exit slit. Turn the room lights on and off; the PMT signal should remain close
to 2 or 3 cps. When done with this check, close the shutter on the exit slit.
Alignment of the optical system
1. While the
PMT is cooling down, the optical system may be aligned. Remove the two lenses
from their bases. After making sure the shutter on the exit slit is closed,
open the cover to the spectrometer (as shown in Fig. 2) and place a cardboard
disk (with marks indicating its center) over the first mirror (opposite the
entrance slit). Open the spectrometer entrance slit to almost fully open.
CAUTION: Laser safety goggles must be worn from this point on.
Turn on the He-Ne
laser, and adjust its position so that its beam passes through the center of
the entrance slit and falls on the center of the disk at the first mirror. This
defines the optical axis for the spectrometer.
2. Turn on
the DMM connected to the Ar ion laser, so it can be used to monitor the laser's
anode current; use the laser's manual to determine how the DMM reading relates
to the anode current. Be careful that the current never exceeds 95% of the
maximum rating found in the laser's manual. You may find that the anode current
creeps slowly with time, so monitor it and adjust as necessary. Using the
manual instructions, turn on the Ar ion laser, and adjust the anode current to
its minimum (the Ar laser beam should still be visible). The Ar laser beam
should be deflected upward by the Ar laser mirror, shown in Fig. 2. Adjust the
position of the cardboard pointer (illustrated in Fig. 2) so that both the
He-Ne laser beam and the Ar laser beam are visible near the tip of the pointer.
Turn the adjustment screws on the Ar laser mirror so that the deflected Ar
laser beam is vertical and intersects the He-Ne laser; it may be helpful to
adjust the cardboard pointer so that its tip is just illuminated by both laser
beams. A plumb line may be used to see if the deflected Ar laser beam is
vertical. After aligning the Ar laser beam, the cardboard pointer may be removed.
3. Install
the 8.1 cm focal length lens into the lens-holder base nearest the Ar laser
beam, so that the He-Ne laser beam passes through near the center of the lens.
Adjust the base so that the lens is about 10 cm from the Ar laser beam. Use the
fine positioning adjustments on the base so that the He-Ne laser beam again
falls on the center of the first spectrometer mirror.
4. Install
the 44.7 cm focal length lens into the second lens-holder base so that the
He-Ne laser beam passes through near the center of this lens. Adjust the base
so that this lens is about 10 cm from the first lens. Use the fine positioning
adjustments on the base so the the He-Ne laser beam again falls on the center
of the first spectrometer mirror. Slide the bases of both mirrors back and
forth about 1 cm and see that the He-Ne laser beam remains near the center of
the first spectrometer mirror; if not, the beam supporting the lens bases must
be re-aligned.
5. The Raman
sample holder is a metal cylinder (with an inside diameter of about 1 cm, and
an opening on one side) with a stand positioning it at about the same height as
the cardboard pointer tip. Place a strip of cardboard inside the sample holder
and position it so that the deflected Ar laser beam produces a ~2 cm streak
along the cardboard strip which can be seen when viewed through the opening in
the holder; the opening should be facing the lenses. Adjust the positions of
the lens bases so that this streak is imaged at the spectrometer entrance slit.
This should maximize the amount of light which falls on the first spectrometer
mirror. The optical system is now aligned.
Calibration of the scanning spectrometer
Before doing
any Raman scattering experiment, it is necessary to become familiar with the scanning
spectrometer, and to calibrate the scanning motor counter against the
wavelength. The calibration can be accomplished using the He-Ne laser, the Ar
laser, and the known lines from a mercury (Hg) lamp. A calibration graph should
be made for future reference. The slope of the calibration line should be a
ratio of two small integers. The reason is that at some point a grating inside
the spectrometer was replaced; the old and new gratings had standard line densities,
but they were two different standards, which differed by the ratio of two small
integers.
1. Read the
manual for the scanning spectrometer. The scanning is accomplished by a
precision motor drive, which is controlled by a unit external to the
spectrometer. Always make measurements in the same scan direction due to screw
lag. Note that turning on or stopping the motor drive at high speed can ruin
your calibration; always turn on or stop the drive at low speeds (approximately
10 on the control unit dial read-out), slowly accelerating or decelerating to
the desired speed. Note that the scanning spectrometer needs electrical power,
and it is turned on by plugging its power cord into a wall socket; lights near
the scanning motor counter should turn on. When shutting down the experiment,
remember to unplug this power cord. If the counter lights are off when the
spectrometer is plugged in, then use a DMM to check connections, bulbs, etc. in
the vicinity of the counter.
2. Using a
strip of paper, trace the light from the Ar laser (which should be present as a
result of the optics alignment procedure) through the first monochromator; it
may be necessary to turn off the room lights. Run the spectrometer's scanning
motor (rotating the gratings) so that the scattered light from the Ar laser
falls on the internal slit. Note the scanning motor counter reading; this gives
the “ballpark” position for the subsequent calibration at this wavelength. Note
that the Ar ion laser may be tuned to produce different wavelengths of light;
the possible discrete wavelengths may be found in the raman lab manual
notebook. Use the table of colors and wavelengths in the lab notebook and the
observed color of the Ar laser beam to determine the actual wavelength produced
by the Ar laser. Do not use color charts in textbooks, because they are very
inaccurate.
3. Close the
cover of the scanning spectrometer, and set the internal and exit slit openings
to 0.3 mm. Close the entrance slit opening to zero. The PMT cool-down data acquisition
on the computer should have completed. [If you wish, you may save the cool-down
data for your lab report.] Restart the computer data acquisition as for the PMT
cool-down, and recheck that the PMT has cooled down so that its dark current is
2 or 3 cps. Open the shutter on the exit slit, and slowly open the entrance
slit; the count rate from the PMT should increase. Use the scanning
spectrometer motor drive to position the monochromator for a maximum count
rate. Increase the entrance slit opening until 0.3 mm is reached, or the count
rate reaches 105 cps, whichever comes
first.
CAUTION: Never let the PMT count rate exceed 106 cps.
4. Rewind the
spectrometer scanning motor away from the “ballpark” position for the Ar laser
wavelength so that the count rate drops to the background rate (near or at the
dark current rate). For the calibration you will want to scan through the Ar
laser wavelength to acquire a fully resolved peak. You will want to use a
position where the PMT is at the background rate as your desired starting
position for the scanning motor counter when initiating the calibration scan;
make a note of this desired starting position. NOTE: The resolution of the
spectrometer depends critically on the width of the various slits in the
spectrometer and, in general, reducing the slit width will both increase the
resolution and decrease the signal to the PMT. Also, weak peaks, possibly next
to a strong peak, will require longer scan times. These aspects should be kept
in mind when the scans for Raman lines are performed.
To obtain a
good calibration peak, you should try different values for the spectrometer scanning
motor speed, the slit openings, and, within the raman data acquisition computer
program, the length of time for data acquisition (the default value is 1000 s
and the maximum value is 50,000 s) and the Timeinterval setting. You might want
to note how the height, width, etc. of the recorded calibration peak varies
with the different parameters.
To take
calibration data, start the raman program (if not already started), change the Timeinterval
if desired, open a new file, enter how long you want to measure, and go to Step
2. At the “Enter counter value” prompt, enter the desired starting position for
the scanning motor counter; do not click Start. If you have not already done so,
rewind the scanning motor to a position preceding the desired starting
position, decelerate, and stop the motor. Then start the motor forward, towards
the desired starting position, and slowly change to the desired constant
scanning speed. When the desired starting position is reached on the counter,
click the Start button, and begin the data acquisition. As the counter changes,
electrical pulses will be sent from the spectrometer to an input at the back of
the computer (different from the PMT preamp input), and these pulses will
increment the numbers in the “Countervalue” column in the data acquisition
listing. Note that the counter readings and the changes in the “Countervalue”
column numbers will not be exactly synchronized; you can use the first and last
Countervalue entries from the complete run and the list line number to obtain
accurate values for the counter. During a data acquisition run, it is essential
that you keep the scanning motor at constant speed; after a run, slowly
decelerate the scanning motor before stopping it. Using File -> Save, store
the calibration data using a suitable filename.
Repeat the
calibration procedure above replacing the light from the Ar ion laser with
light from the He-Ne laser and several strong lines (two yellows and a green)
of known wavelength from a Hg lamp. Set up the Hg lamp so that it shines
strongly into the spectrometer entrance slit. CAUTION: Ultraviolet light from the Hg lamp is harmful; do not look
at it when on, and keep it covered to avoid accidents. The calibration will be
given as a straight line fit of the actual wavelength of the incident light as
a function of the counter value at the peaks of the calibration data sets. After
an initial calibration with strong, obvious lines, you may want to look for
some weaker lines from the Hg lamp. For some Hg lines and for the Raman
measurement, it may be necessary to increase the spectrometer slit openings.
Note that
there are several types of Hg lamps, with different spectra. Also, if the
counting rate is too high (too much light is entering the spectrometer) then
you may get too many unidentifiable lines. If necessary, close down the
entrance slit; recall that the maximum count rate should be below 106 cps.
Finally note that some strong lines from the Hg lamp may place second order
diffraction peaks in your spectra. An initial calibration with strong, obvious
lines will alleviate line identification problems.
Raman data acquisition
The liquid
samples for Raman scattering measurements are placed in transparent vials which
fit into the sample holder. The side of a vial may be partially covered with
aluminum foil to increase the amount of scattered light sent to the lenses.
When placing the vials into the sample holder, some wedging material may be
used so that the vials do not shift during data acquisition. Recall that the
light scattered from the samples will have an intense Rayleigh line in addition
to the much weaker Raman sidebands. Adjust the position of the sample holder
(and the position of the sample vial within the holder as well) in order to
obtain the maximum signal from the Rayleigh line.
For the Raman
data acquisition scans, the Ar ion laser anode current should be increased to 95%
of its maximum rating. Then perform a slow scan of an appropriate range of
wavelengths to look for the Raman lines. If you have trouble finding the Raman
lines, you might want to look up the known values of the wavelength shifts to
make sure you are scanning over the correct range. If you have carefully
adjusted the optics, maximized the Ar ion laser output, and varied the sample
vial position and you cannot see Raman lines or they are very faint and hard to
discern, you may want to try adjusting the alignment of the mirrors internal to
the laser head, but only as a last resort. Consult the lab instructor before
attempting this, as it is easy to throw the laser off its tuning.
INSTRUCTOR ONLY: The B screw in the back of the laser,
if tweaked, will change the frequency of the laser output light. If only
adjustments of the mirror alignments are desired, the A screws and the B screw
in the front of the laser can be VERY SLIGHTLY tweaked while monitoring
the current draw to minimize the current (a very slight adjustment makes
a huge impact on current draw). To avoid accidentally blowing the fuse, this
adjustment should be done only at low currents (e.g. 7 A). Once the current
draw has been minimized, it can be set back to 10 - 11 A
References
[1] J. Loader, Basic
Laser Raman Spectroscopy, Heyden & Son,
[2] D. Long, Raman
Spectroscopy,
[3] T. R. Gilson
and P. J. Hendra, Laser Raman Spectroscopy, Wiley,
[4] A. Anderson,
ed., The Raman Effect, Marcel
[5] R. Chang, Basic
Principles of Spectroscopy,
[6] W. Demtroder, Laser
Spectroscopy Basic Concepts and Instrumentation, Springer Verlag,
[7] S.
[8] B. P.
Straughan and
[9]
C.V. Raman and K.S. Krishnan, Indian J. Phys. 2, 387
(1928).
[10] G. Herzberg, Infrared and Raman Spectra of
Polyatomic Molecules,