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. The lines occur close to, and on each side of, the main spectral lines, and arise from the inelastic scattering of the photons with atomic or molecular vibrations or rotations in the scattering material. By analogy with the terminology used in fluorescence, the lines corresponding to a loss of energy are called Stokes lines and those corresponding to a gain of energy are called Anti-Stokes lines.
An example of this process is shown below. The system shown has two low-lying levels (a and b) and the interaction takes place through a virtual level, and results in a transfer of population between the levels a and b. The frequency difference between the incident photon and the scattered photon gives the energy separation between a and b. By measuring the energy shift of the scattered photon, the structure of the system can be determined. For example, with carbon tetrachloride, 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.
truecm Figure 1. Illustration of the energy level transitions for the Stokes and Anti-Stokes processes.
CAUTION: The Argon 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 whem aligning or realigning mirrors. Do all alignments at low power (laser beam barely visible). Use bent pipes as optical ``dumps'' to stop stray laser light.
CAUTION: The Raman samples are contained in sealed glass vials, because they are hazardous and create noxious ordors. Be careful when handling the samples.
CAUTION: Before opening the spectrometer, make sure the shutter in front of the PMT is closed.
The output of the spectrometer is a photomultiplier connected to a photon counting system controlled by a microcomputer. The PMT should be operated at a voltage of 900 V. Count rates should be limited to less than 1 samples/s. The scanning is accomplished by a precision motor drive. Always make the measurement in the same scan direction due to screw lag. Note that turning on the motor drive at high speed can ruin your calibration! Always turn it on at low speeds ( 10).
In this experiment, the Raman spectra will be measured for several liquids, starting with carbon tetrachloride. Raman scattering is by its nature very weak. You must devise a method for getting the Raman scattered light into the spectrometer, while keeping out the direct laser light and any light which is scattered from other objects. The alignment is important here to make sure that the scattered light is going through the spectrometer. It is useful to use a vial of grape juice (which scatters strongly, although not by the Raman effect) as a scatterer for these alignment procedures. Note that 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.
Note that there will always be a strong unshifted line at the frequency of the laser due to Rayleigh scattering. Weaker lines require increased scan times.
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. The control 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.
Before doing any Raman scattering experiment, it is necessary to become familiar with the spectrometer, and to calibrate the motor settings against the wavelength. This can be accomplished using the mercury (Hg) lamp by observing the known lines from Hg. A calibration graph should be made for future reference.
Step-by-step Procedure for Calibration (and Operation)
1. Turn on cooling water for photomultiplier, as follows:
a. Refer to the diagram on the wall for the location of the main water valve and photomultiplier cooling valve.
b. Turn on the water for the main line (small blue knob).
c. Turn on the water for the photomultiplier (small red knob).
d. Turn the brass lever of the cross valve to REG for regular water flow.
2. Turn on the power supply for the photomultipliers Peltier cooler.
3. Let the cooler operate for approximately 30 minutes before continuing on to Step 4.
4. Turn on the high voltage supply for the photomultiplier. (if perfonning the mercury lamp calibration, steps 5 and 6 should be omitted at this time.)
5. Turn on the cooling water valve for the laser (big blue knob).
6. Turn on the laser, as follows:
a. Flip the large power switch located on the wall to the on position.
b. Turn the lasers power supply, located on the floor, to the on position by turning the key.
c. Press the ON button of the lasers power supply.
7. Turn the spectrometers motor speed control on. Note: when running the motor do not apply the brake while it is operating at high speeds.
1. Set up the mercury lamp so that it shines into the slit on the side of the spectrometer. Ultraviolet light from the Hg lamp is harmful; do not look at when on, and keep it covered to avoid accidents.
2. Cover the lamp and the slit (use aluminum foil and black cloth) so that the only light entering the spectrometer is coming from the mercury lamp.
3. Perform a ``pallpark'' calibration, as follows:
a. Set the entrance slits to about 0.5 mm.
b. Make sure that the shutter in front of the PMT is closed, and open the top section of the spectrometer cover above the slits.
c. Hold a piece of paper in front of the second (internal) slits, and run the grating motor. At some point a pair of bright yellow lines should appear. Note the grating counter reading; this gives the ``ballpark'' position for subsequent calibration. You should also obtain ballpark positions for green and purple lines.
d. Replace the cover on the spectrometer.
4. Run the data collection software to obtain results. This software is designed to collect data from the spectrometer and photomultiplier. The photomultiplier sends data to the computer via a cable connected to the back of the computer. The Raman software records the relative intensity of the incoming electromagnetic radiation through a range of wavelengths. The intensity is measured in counts per second, which is the amount of electromagnetic radiation striking the photomultiplier per second. The software is capable of presenting the results in graph and table format.
a. Turn on the computer and log on.
b. On the desktop, double-click the icon named Raman. If this icon does not exist, open the file C: ogram Files DevStudio MyProjects raman Release raman.exe.
c. At the top left of the Raman window, click on File, then click on New. Note: all instruments should be turned on at this point.
d. A prompt will appear asking for the measurement time. The default value is 1000 seconds, and the maximum value is 50,000 seconds. Enter an appropriate time for measuring the sample and click the button labeled Step 2.
e. Another prompt will appear asking for the counter value. Enter the counter value found on the spectrometer that will be used as the starting point for measurement. It is imperative that the spectrometer motor speed be constant for the duration of the measurement. In order to assure this, start the motor at a value some distance away from the desired starting value. Then run the motor at a constant speed towards the desired value.
f. Click on the Start button as the desired value is reached.
g. A window will appear and graph the results. The x-axis measures time, and the y-axis measures counts per second. To see this data in table format, click on the View button at the top of the screen, then click on the Listview button. The counts per second is labeled as Frequency (Hz) in this window.
h. Once the desired stopping value is reached, press the Stop button at the top of the Raman window.
i. To save the data as a text file, click on File, then click on Save.
5. Compare the results of the mercury lamp calibration to the known values of mercury emission lines. From this comparison, a correlation can be found between the spectrometers counter value and the actual wavelength. NOTE:
a. The are several types of mercury lamps, with different spectra.
b. 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 slits. The maximum count rate should be below 1 counts/s.
c. Some strong lines from the lamp may place second order diffraction peaks in your spectra.
Sample Measurement Using the Laser:
1. Set the current through the laser to 40 Amps using the current knob on the lasers power supply.
2. Turn the fine tuning knob on the back of the laser so that the laser beam is barely visible.
3. Set up the sample and optical equipment so that the laser beam passes vertically through the sample.
4. Use lenses to collect and focus the scattered light onto the slit on the side of the photomultiplier.
5. Turn the fine tuning knob so that the laser beam is at maximum intensity.
6. Run the data collection software to obtain results, as described above.
1. Turn the current for the laser down to 20 Amps.
2. Turn off the laser.
a. Press the OFF button on the lasers power supply.
b. Turn the key on the power supply to the off position.
c. Flip the large power switch to the off position.
d. Allow cooling water for the laser to run for at least ten minutes after turning the laser off.
3. Turn off all other equipment.
4. Close all of the water valves.
Examine the dependence of spectra on incident wavelength and polarization.
Note the character of the lines (rotational, vibrational).