Chandrasekhara Venkata Raman The Nobel Prize in Physics 1930

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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.

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 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.

Chandrasekhara Venkata Raman was born at Trichinopoly in Southern India on November 7th, 1888. His father was a lecturer in mathematics and physics so that from the first he was immersed in an academic atmosphere. He entered Presidency College, Madras, in 1902, and in 1904 passed his B.A. examination, winning the first place and the gold medal in physics; in 1907 he gained his M.A. degree, obtaining the highest distinctions. His earliest researches in optics and acoustics - the two fields of investigation to which he has dedicated his entire career - were carried out while he was a student.

Since at that time a scientific career did not appear to present the best possibilities, Raman joined the Indian Finance Department in 1907; though the duties of his office took most of his time, Raman found opportunities for carrying on experimental research in the laboratory of the Indian Association for the Cultivation of Science at Calcutta (of which he became Honorary Secretary in 1919).

In 1917 he was offered the newly endowed Palit Chair of Physics at Calcutta University, and decided to accept it. After 15 years at Calcutta he became Professor at the Indian Institute of Science at Bangalore (1933-1948), and since 1948 he is Director of the Raman Institute of Research at Bangalore, established and endowed by himself. He also founded the Indian Journal of Physics in 1926, of which he is the Editor. Raman sponsored the establishment of the Indian Academy of Sciences and has served as President since its inception. He also initiated the Proceedings of that academy, in which much of his work has been published, and is President of the Current Science Association, Bangalore, which publishes Current Science (India). Some of Raman's early memoirs appeared as Bulletins of the Indian Associationfor the Cultivation of Science (Bull. 6 and 11, dealing with the "Maintenance of Vibrations"; Bull. 15, 1918, dealing with the theory of the musical instruments of the violin family). He contributed an article on the theory of musical instruments to the 8th Volume of the Handbuch der Physik, 1928. In 1922 he published his work on the "Molecular Diffraction of Light", the first of a series of investigations with his collaborators which ultimately led to his discovery, on the 28th of February, 1928, of the radiation effect which bears his name ("A new radiation", Indian J. Phys., 2 (1928) 387), and which gained him the 1930 Nobel Prize in Physics.

Other investigations carried out by Raman were: his experimental and theoretical studies on the diffraction of light by acoustic waves of ultrasonic and hypersonic frequencies (published 1934-1942), and those on the effects produced by X-rays on infrared vibrations in crystals exposed to ordinary light. In 1948 Raman, through studying the spectroscopic behaviour of crystals, approached in a new manner fundamental problems of crystal dynamics. His laboratory has been dealing with the structure and properties of diamond, the structure and optical behaviour of numerous iridescent substances (labradorite, pearly felspar, agate, opal, and pearls). Among his other interests have been the optics of colloids, electrical and magnetic anisotropy, and the physiology of human vision.

Raman has been honoured with a large number of honorary doctorates and memberships of scientific societies. He was elected a Fellow of the Royal Society early in his career (1924), and was knighted in 1929.

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