In the Laboratory
Stimulated Raman Spectroscopy of Small Molecules A Physical Chemistry Laboratory Experiment Christopher A. Grant and J. L. Hardwick Department of Chemistry, University of Oregon, Eugene, OR 97403
There has been a great deal of recent and continuing interest in the introduction of lasers into the modern undergraduate teaching laboratory (1–6). In the physical chemistry laboratory, in particular, a great deal of emphasis has been placed on the use of lasers to introduce chemistry majors to modern experimental techniques and to communicate to them the excitement associated with the new tools available to research chemists. A new generation of Nd:YAG lasers has recently become commercially available,1,2 enabling a variety of experiments that are technically unfeasible or difficult to interpret using the more commonly employed nitrogen laser. These lasers are self-contained, requiring no external cooling water and only 110-volt AC current. They produce ca. 10 mJ of doubled (532 nm) light per ~6 ns pulse at a repetition rate of 1–20 Hz, permitting observation of a variety of nonlinear optical phenomena. We describe here the use of such a laser to observe and analyze stimulated Raman scattering (SRS) and coherent anti-Stokes Raman scattering (CARS) in neat organic liquids. Raman spectroscopy (7) was first described in the 1920s, and has been discussed previously in this Journal (8–11). Observation of stimulated Raman scattering (12–14), on the other hand, is a more recent development (15, 16), having become practical only after the evolution of sufficiently powerful lasers (17). In these stimulated processes, nonlinear phenomena amplify the ordinarily weak Raman effect to the point where it is easily visible to the eye (indeed, both eye protection and prudence are required to avoid injury). By observing this process, students are able to examine some details about the vibrational energy levels of the scattering molecules and gain insight into the special ways that a very intense radiation field can interact with a molecule.
Experimental Details For our experiments, acetonitrile (CH3CN) and benzene (C6 H6 ) were used as the scattering media. These liquids have the advantages of being strong Raman scattering media and of being readily available in perdeuterated form. These latter isotopomers illustrate convincingly that one of the molecules (acetonitrile) scatters via a hydrogen stretching vibration, while in the other (benzene) a skeletal mode dominates the spectrum. Benzene was one of the materials in which SRS was first observed (15). Therefore, a moderately large body of literature describing its SRS spectrum is available (15, 17, 20–23), and it has historical interest as well as scientific merit. The Stokes shift of benzene is comparatively small, so that several lines may easily be observed in the visible part of the spectrum. On the other hand, benzene is a significant health hazard, and other laboratories may choose not to undertake the risk associated with its use. Fortunately, the Stokes shifts of a large number of organic solvents have recently been reported (24, 25), which should allow each laboratory to choose materials that strike an acceptable balance between pedagogical and safety considerations. The experimental setup is deceptively simple (Fig. 1). Light from a doubled New Wave Research MiniLase-20 Nd:YAG laser was focused with a short (50 mm) focal length lens into a cell containing an organic liquid. An Intertech FD 532 Nd:YAG laser produced similar results. The cell
Safety Considerations •
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318
Care should be used when handling the acetonitrile (CH3CN) and benzene (C6H6) employed in the experiments described below. Since both of these liquids are toxic, and benzene is carcinogenic, all transfers must be performed in a fume hood and the cells must be adequately stoppered. There is, in addition, a potential flammability hazard from both of these chemicals. The laser used in this experiment is dangerous. A Class IV laser is, by definition, a health and safety hazard. Any laser powerful enough to produce nonlinear optical effects is also powerful enough to destroy a substantial portion of the human retina in a few nanoseconds. Do not look directly into the laser beam or its specular reflection. Appropriate laser goggles should be worn when the laser is firing, and the laser radiation should be suitably contained. Readers unaccustomed to dealing with high-power lasers are urged to consult appropriate laser-safety documentation (18, 19).
Figure 1. Schematic of experimental apparatus. The dashed line indicates the light path. The mirror may be swung into place in order to collect a reference spectrum using the Fe–Ne lamp.
Journal of Chemical Education • Vol. 74 No. 3 March 1997
In the Laboratory
Table 1. SRS Line Positions and Shifts found for Acetonitrile and Benzene Air Wavelength (Å)
Vacuum Wavenumber (cm{1)
5320.6
18789.6
S1
6308.6
15846.9
2942.7
v. strong
S2
7746.5
12905.5
5884.1
moderate
Pump beam
SRS Shift (cm{1)
Intensity
v. strong
CH3CN
CD3CN S1
5995.7
16674.0
2115.5
v. strong
S2
6866.8
14558.8
4230.8
moderate
A1
4781.9
20906.3
2116.7
weak
C6H6
Figure 2. Spectrum of the second Stokes line of benzene (C6D6).
used for acetonitrile was 100␣ mm long and 18␣ mm in diameter; the cell used for benzene was 100␣ mm long and 8␣ mm in diameter. Raman-shifted laser light emerged more or less colinearly with the laser light on the other side of the cell. The laser beam, with its frequency-shifted components, was then attenuated by reflecting the light off a pair of uncoated microscope slides; the main portion of the laser radiation was discarded in a beam dump. The suitably attenuated light was focused using a pair of lenses and viewed with a monochromator. Calibration of the spectra was done using a hollow-cathode Fe-Ne lamp as a reference (26). The laser pulse energy was about 10 mJ; this energy produced easily observable stimulated Raman scattering. Attenuating the laser power with neutral density filters extinguished the stimulated Raman spectrum at a pulse energy of about 1 mJ. Any of a variety of monochromators may be used to view the scattered light. If the light is introduced into a larger spectrograph, its spectrum may be recorded either photographically or electronically. The spectrometer/detector combination we used was a 3/4 m Czerny-Turner mount grating spectrometer (Spex instruments) coupled to an inexpensive CCD array (EDC-1000 Computer Camera, Electrim Corp.). This arrangement allowed us to record about 25 Å of the spectrum at a single shot; a typical spectrum is illustrated in Figure 2. For all but the very dimmest lines, only a single laser pulse was needed to produce an acceptable spectrum. The light produced was sufficiently bright to record even with a prism instrument such as a medium Hilger spectrograph. Recording the spectrum using a scanning monochromator would be possible but less convenient, since the scan rate must be slowed down to accommodate the low repetition rate of the laser. The shifted light was bright enough to cause an apparent color change in the laser light: the scattered light appeared yellow rather than green. Viewed through a greenblocking filter (or a pair of laser safety goggles) the shifted laser light scattered from a piece of white paper appeared orange or red. Viewing the beam scattered from a piece of paper (not a specular reflection of the beam) through a hand-held spectroscope typically revealed two or three Stokes-shifted lines to the red of the green exciting line and
S1
5617.3
17797.3
992.3
v. strong
S2
5948.6
16806.0
1983.5
v. strong
S3
6321.6
15814.5
2975.1
strong
S1(ν1)
6356.2
15728.4
3061.2
weak
S4
6742.9
14826.3
3963.3
v. weak
A1
5053.5
19782.7
993.1
weak
C6D6 S1
5602.5
17844.4
945.2
v. strong
S2
5915.5
16899.9
1889.6
v. strong
S3
6265.6
15955.8
2834.2
strong
S4
6659.7
15011.6
3778.0
weak
A1
5066.0
19733.9
944.4
weak
perhaps a much weaker anti-Stokes line to the violet. (The power of the laser used is such that direct viewing of even a diffuse reflection is potentially dangerous; the reader is urged not to undertake direct inspection of reflections unless he or she has sufficient experience with lasers to judge whether the reflection is of low enough intensity to be viewed safely.) The only unavoidable experimental difficulties we encountered involved the photochemical generation of particulate matter in samples that had not been changed over a period of several weeks. Benzene was especially prone to form “soot” in the cell, blocking the laser beam and obliterating the signal. Results The SRS activity of the four compounds examined is listed in Table 1. With acetonitrile, it was possible to see two Stokes shifted lines, but the anti-Stokes shifted line was visible only with CD3 CN. With benzene four Stokes and one anti-Stokes lines were observed with both C6H 6 and C6 D6. Interestingly, with C6H 6, a Stokes-shifted line representing a different molecular vibration (the ν1 C–H stretch) was observed to be shifted by 3061 cm{1 from the exciting line (23, 27). We were unable to see the corresponding line in C6 D6, which should be shifted by 2292 cm{1 . The uncertainty in wavelength of ±0.1 Å was determined largely by the resolution of the CCD camera. The uncertainty in vacuum wavenumber varied from ±0.3 cm{1 at 7000 Å to ±0.6 cm{1 at 5000 Å.
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In the Laboratory Interpretation The interpretation of the experiment centers on four distinct levels of understanding: (i) the relation of the first Stokes-shifted line to the vibrational mode of the molecule; (ii) the mechanism for producing stimulated Raman scattering (as opposed to the more common incoherent process); (iii) the mechanism for producing the second and higher Stokes lines; and (iv) the mechanism for producing the coherent anti-Stokes shifted lines. The explanation of the experiment may therefore be tailor-fit to the background and level of sophistication of the students.
The Stokes Shift Ordinary (incoherent) Raman scattering occurs when a photon collides inelastically with a molecule and departs with slightly more or less energy than when it arrived. If the photon departs with slightly less energy, the excess energy is deposited in the molecule, and the change in energy of the photon mirrors the change in energy of the molecule. The scattered light is shifted to a slightly longer wavelength than that of the incident light (a Stokes shift). Conservation of energy leads to νp = νS + νM where the subscript p indicates the incident pump radiation, S indicates the scattered radiation, and M indicates the molecular excitation. The difference in energy is proportional to the difference between the vacuum wavenumber ν p of the incident light and that of the Stokes-shifted light, ν S: ∆E = hcν p – hcνS This is exactly the same as the difference in energy between the initial and final states of the molecule, as illustrated in Figure 3. The energy level diagram looks very much like that observed in a laser-excited fluorescence spectrum, except that the upper state need not be a stationary state of the Hamiltonian; this short-lived state is best described as a collision of a molecule with a photon and is often referred to as a virtual state of the molecule. By measuring the wavelengths of the incident and scattered light the student can, therefore, determine the difference in energy between the two molecular states involved. This energy difference is usually found to correspond to a vibrational excitation of the molecule, so that a Raman spectrum yields the same kind of information as an infrared spectrum. The details of the two kinds of spectra may be different, however, because infrared and Raman spectra usually have different selection rules and different inten-
Figure 3. Two hypothetical mechanisms for producing a second Stokes line.
320
sity distributions. Thus, states observed in a Raman spectrum need not be observable by infrared spectroscopy and vice versa. For the cases in point, benzene has a center of symmetry and therefore obeys the mutual exclusion rule for Raman and infrared spectra: vibrations that are Raman active are not infrared active, and vibrations that are infrared active are not Raman active. Acetonitrile, which does not possess a center of symmetry, obeys no such rule, and the symmetric C–H stretching vibration, responsible for the stimulated Raman spectrum, is also strong in the infrared absorption spectrum. The observed Stokes shifts for these samples are reported in Table 1. These are in agreement with the values reported for ν1 for acetonitrile and ν2 for benzene (27). A comparison of the Stokes shifts for CH3CN and CD3CN reveals that they differ by a factor very close to (mH/mD)1/2; this is the result expected if the vibration involves only the motion of hydrogen atoms. The difference between the shifts for C6H6 and C6D 6 is much smaller, as anticipated for the ring breathing vibration of benzene, which involves primarily a motion of the carbon skeleton.
Stimulated Raman Scattering If the incoming light is very intense, a sizable fraction of molecules is excited into the virtual state and the population of the virtual state approaches that of the ground state. In such a case, there may be a larger number of molecules in the virtual state than in the final state, and a population inversion is said to exist with respect to these two states; this is exactly the condition responsible for gain in active laser media. This population inversion can, in turn, lead to stimulated emission from the virtual state down to the final state. The sample randomly produces a few photons of Stokes-shifted light, and the photons that happen to come out in the right direction (so that they strike molecules which have been excited by the laser to a virtual state) will stimulate those molecules to produce light with the same frequency and phase. Naturally, the higher the instantaneous laser power, the more easily this inversion can occur. Typically, a few millijoules of power are needed, and this power is deposited in the sample within the space of a few nanoseconds. In addition, an inversion is favored by a higher energy of the molecular vibration (since the Boltzmann population of the final state is lower), by a large cross-section for Raman scattering (which depends on the polarizability of the molecule and how it changes when the vibration is excited), and by a narrow Raman line width (which concentrates the gain into a narrow spectral region). The Second Stokes-Shifted Line The second Stokes-shifted line should be shifted from the exciting line by about twice as much as the first Stokes line. There are two conceptual mechanisms for producing a second Stokes-shifted line, which can easily be distinguished experimentally (Fig. 3). In the first model, the original laser light is scattered inelastically to populate a second excited vibrational state, shifting the frequency by almost twice the amount of the first Stokes shift (the shift would not be quite doubled, since the molecular vibration is not harmonic). In the second mechanism, the coherent light produced by the first Stokes shift is rescattered, shifting the frequency a second time by exactly the same amount. The former mechanism depends on a property of the molecule; the latter depends largely on the properties of the light. Careful measurements of the higher Stokesshifted lines reveal that the lines are shifted, to within experimental accuracy, by integral multiples of the first Stokes shift, confirming the second mechanism (28).
Journal of Chemical Education • Vol. 74 No. 3 March 1997
In the Laboratory The Anti-Stokes Lines If the molecule starts off in an excited vibrational state, the photon can gain energy from the molecule. Once again, the difference in energy of the departing photon equals a difference in the vibrational energy of the molecule. The scattered light is this time shifted to a shorter wavelength (an anti-Stokes shift). Incoherently Stokes-shifted light is ordinarily much stronger than the corresponding antiStokes shifted light simply because there are many more molecules in the ground state than there are in any excited state. In ordinary Raman scattering, the mechanism for producing Stokes-shifted lines and anti-Stokes-shifted lines is the same; only the states are different. The mechanism for producing coherent anti-Stokes Raman scattering is, however, entirely different from the mechanism for producing stimulated Stokes scattering. In fact, the anti-Stokes light is not a stimulated process at all; rather, it is a four-wave mixing␣ phenomenon in which the oscillating electric field of one photon (the green YAG laser) is modulated by the molecules perturbed by the field of another photon (the stimulated Stokes-shifted light). This is a specific example of coherent anti-Stokes Raman spectroscopy (CARS), a technique that has gained wide popularity with the advent of modern lasers because of its high sensitivity compared with conventional Raman spectroscopy (29, 30). Energy is conserved in this process according to the equation ν pump + νpump = νStokes + νanti-Stokes while momentum is conserved by
hν pn p + hν pn p = hνSn S + hνaSn aS
compounds (14, 24). This experiment, while hardly an exhaustive treatment of the topic, offers students an easily implemented introduction to this rapidly expanding field of modern chemistry. Acknowledgments The authors are grateful to New Wave Research, Inc., for the loan of a laser used in this work. This work was supported in part by grant SG-94-063 from the Special Grant Program in the Chemical Sciences of the Camille and Henry Dreyfus Foundation. We also acknowledge with gratitude the helpful discussions with Bruce S. Hudson during the course of this work. Notes 1. New Wave Research Inc., Sunnyvale, CA 94089. 2. Intertech Inc., Middletown, RI 02842.
Literature Cited 1. 2. 3. 4. 5.
6. 7. 8. 9. 10. 11.
where ni is the refractive index of the medium at νi. Since the refractive indices are not all the same, the four photons will propagate at fixed, nonzero angles with respect to one another (the phase-matching condition) (31). That is, the anti-Stokes radiation does not emerge colinearly with the pump beam, but rather in a narrow cone. Similar mechanisms also contribute slightly to the intensity of the higher order Stokes-shifted lines (21): νS1 + ν S1 = ν S2 + νp
12. 13. 14. 15. 16. 17. 18. 19.
The description of four-wave mixing theory given here is necessarily superficial. More detailed treatments may be found in a variety of nonlinear optics textbooks (32, 33); Shen (34) is perhaps the most comprehensive.
20. 21. 22. 23.
Discussion This is an experiment that is remarkably easy to implement but is also quite flexible, allowing it to be used in teaching students at widely different levels of preparation. It produces a result in the form of color-shifted laser light too conspicuous to ignore, yet which cannot easily be understood without learning some of the fundamentals of Raman spectroscopy, laser gain, and molecular vibrations. The anti-Stokes lines are particularly subtle, depending on a thorough understanding of the wave properties of light. Stimulated Raman scattering has found a variety of applications, including production of coherent light of varying frequencies and the transfer of a signal from one laser source to another having a different frequency (25). Recently, SRS has begun to be used as an analytical technique for identification of and noninvasive monitoring of organic
24. 25. 26. 27.
28. 29. 30. 31.
32. 33. 34.
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