Raman Spectroscopy for the Undergraduate Physical and Analytical Laboratories Douglas B. ~ a l l o w a ~Edward ', L. ciolkowskiz and Richard F. ~ a l l i n ~ e r ~ Wabash College, Crawfordsville, IN 47933 Raman spectroscopy, while continuing to gain importance in modern chemical and biochemical research (I), remains an experimental technique not o k n encountered by undergraduate chemistry students. Although the theoretical and experimental aspects of Raman spectroscopy have been discussed in this Journal on several occasions (2-7) and some instrnmental technique articles have appeared (8, 91,only a few undergraduate-level experiments, including both dry lab (10, 11)and laboratory (12,13) types, have been presented. Both of the labratory experiments (12, 13) presented in this Journal require research-grade Raman equipment (i.e., expensive ion lasers, double monochromators, and sensitive photomultipliers) to implement. The conventional wisdom seems to be that the Raman experiment requires such expensive equipment, thus placing the experiment beyond the budgetary scope of most teaching-oriented departments. This is simply not the case. A small and relatively inexpensive, yet versatile, pulsed nitrogen laser has recently bewme commercially available, and this laser has shown promise for use in the undergraduate teaching laboratory (14, 15).The goals of this paper are to describe the assembly of a simple Raman
I
GATED INTEGRATOR
I
PHOTOMULTIPLIER HOUSING AND HIGH VOLTAGE SUPPLY
MONOCHROMATOR
, ,, ,, ,, ,, ,,
FAST
COLLECTION OPTlCS
9 : ,,
;
,,
'\
I NnROGEN LASER
2
Figure 1. Raman apparatus schematic (top view). The dashed lines indicate light paths and the dotted lines indicate electrical connections.
Fgure 2. Sample lminaton scheme (sde vew) h = heght of tne w leclion lenses and the monochromator entrance s t (located oehind the cuvette in this picture) spectrometer that uses this nitrogen laser and other commonly available or easily punchaseable components and to discuss the ways that this instrument can be used to enhance the educational experience in undergraduate physical chemistry and analytical chemistry laboratories. We present the set-up of the experimental apparatus, display several spectra acquired with this apparatus to highlight the capabilities of the system, and discuss possible uses of this instrument in the teaching laboratory. The Experimental Setup The experiment is schematically illustrated in Figure 1. The nitrogen laser (Laser Scientific Inc., Model VSL337) issues a train of -1-ns, 200-pJ pulses at repetition rates from 1-20 Hz. The compact size (approximately 10 in. x 4 in. x 3 in.) and ease of use (turnkey operation, sealed gas chamber, no special electrical requirements, no daily optical alignment) make the VSG337 ideal for use by novice ooerators. One simificant advantage in using a nitrogen laser for this experiment is that t h e am an ~ G a t t e r i ~ e f ficiencvscales roughly as the fourth Dower of the excitation frequehzy, makingtde Raman intensities from this 337.1 nm source approximately 12 times larger than would be obtained using a common helium-neon alignment laser (632.8 nm) with equivalent average power of about 2 mW. The VSL337 costs approximately $3000, is safe to use electrically and optically (although students should be taught to use laser safety glasses!), and has a replaceable gas cartridge for convenient maintenan~e.~ 'Present address: Department of Chemistry. University of Wisconsin, Madison. WI 53706 2Present address: Universitv of North Carolina, Chapel Hill, NC 27514
3~uthor to whom correspondence concerning this article should be addressed at Wabash College. Volume 69 Number 1 January 1992
79
The N2 laser beam was directed vertically through the s a m p l e cell using a q u a r t z right-angle prism ( a n aluminized mirror should also work) and was focused in the sample cell using a 100-mm focal length quartz (not glass) lens (Fig. 2). The sample cell was a quartz fluorescence cuvette with five polished faces. A melting point capillarv (a common Raman samule cell) did not work well in-this experiment because theAcurvedcapillary surface scattered and reflected a significant amount of laser light into the monochromator, swamping the Raman signal. The samole was verticallv illuminated through the center of the cuvette in order to match the vertical slits of the monochromator, thereby maximizing the scattered light collection efficiency.The signal decreased by a factor of approximately six when horizontal sample illumination (peruendicular to the slits) was used. The samule cell and rieht gngle prism were mounted on a n inexpe&e ry translation staee to facilitate imapine the scattered lieht onto the narrow monochromator entrance slit. The Raman scattered radiation was collected with a simple plano-convex glass lens (40 m m diameter, W1.75) and focused onto the monochromator entrance slit (typicalIv 150 um wide x 20 mm hieh) with an W6 elass lens. The c"o11ection optics were mouLted on an inexpensive optical rail for convenient positioning of the collection lenses along the samplelslit axis. The GCAiMcPherson EWE-700 wavelength-drive single monochromator had a 350-mm focal length and a n W6.8 aperture; any reasonable quality quarter- or third-meter single monochromator should do nicely for this experiment. No cut-offfilters were needed in front of the entrance slit to block inelastic (Rayleigh) scattering interferences. We used 150-wm slitwidths for the ganged e n t r a n c e a n d exit s l i t s t o yield a s p e c t r a l bandwidth of approximately 0.3 n m (-26 cm-' a t 337.1 nm) for the spectra displayed in this paper, although usable data was obtained with slit widths down to 75 pm (0.15 nm resolution) with more extensive signal averaging! The GCAI McPherson 701-50 PMT Module provided high voltage (-1000V) to the Hamamatsu R372 photomultiplier. Another sienificant advantage of the near-ultraviolet nitrogen laser is that the waveiengths of the tered light (338-370 nm) are near the maximum of the spectral response curve for several very inexpensive sideon ~hotomultioliers.such a s the R372 or the RCA 1P28. he extremeiy red response of these reasonably priced PMT's is the most serious impediment to using a helium-nwn laser a s the excitation source for this experiment.
-
- -
-
-
ama an-scat:
Signal Processing Two devices were used to process the Raman simal from the PIMT. First, a picoammeLr (Pacific ~nstrumentsModel 124 DigitalPhotometer, $1500; the analog version costs $1000) was used to directly read the PMT anode current. The picoammeter output voltage was sent to a strip chart recoider or to the anaiog-tu-diital converter interfive of a personal computer. Second, a gated integrator (Stanford Research Svstnms Model SR250 in n Model SH2AO mainframe, 1 $4500) was used to process the PMT signal. In this exoeriment. a microscoue cover slide was inserted into the laser beam just before the focusing lens inorder to deflect a small uonion of the 337.1 nm radiation toward a fast photodiodePwhose output provided a trigger signal for the eated inteerator? After the eated inteerator is triegere;, a user-ckrollable delay is executed before a variable width gate is opened to monitor PMT signal. Since Raman scattering only occurs when the laser is on, the gated integrator provides a marked enhancement in signal-to-noise ratio by discriminating against PMT darkcurrent. We used a 26-116 delay and a 2-11s gate width for this
-
-
~
-
80
Journal of Chemical Education
-
~
-
experiment. The integrator averaged a set number of pulses before exporting the averaged signal to a recorder or a PC. We used a n ADALAB interface card (Interactive Microware Inc., S t a t e College, PA) to send the data (pic~,ammcteror gated integrator voltage^ in real time into a graphics program .INPERPI.OI: dI1M International, oroColumbus. OH!on a Zenith PC. The HYPERPLOT - gram was used to sum consecutive individual spectra and can be used to smooth spectra, although none of the spectra presented in this paper are smoothed. The gated integrator approach, which teaches students the basics of time-resolved spectroscopic detection, i s strongly suggested a s the preferred pedaeoeical wav to do this expenment. If the c&mercia&ate> integrator is too expensive, Evans and Associates (Berkeley, CA) markets a circuit board which can be readily assembled into a homemade gated integrator for around $1000 (16).Astudent in a n advanced analytical course a t Wabash has constructed this device as a special project. Raman Spectra The Raman spectra of several neat organic liquids (cvclohexane, carbon tetrachloride. chloroform. and benzine) acquired with the instrument described'above are shown in Figure 3. The monochromator was scanned from 338.0 to 3580 nm, corresponding to a range of vibrational frequency (defined a s the frequency of the Raman-scattered radiation minus the frequency of the laser radiation) of -120 to 1770 cm-'. The fluorescence cuvette sample cell permits resolution of low frequency Raman lines frbm the Rayleigh scattering, even using a s m a l l single monochromator. Note in Figure 3 that the lowest frequency CCI4 fundamental a t 218 cm-' is easily resolved. Thus, one can measure vibrational frequencies using this Raman instrument that are well below the NaCl window absorpt i o n limit for infrared s u e c t r o s c o ~ v . When t h e monochromator is calibrated using t h e h o g e n laser wavelength (i.e., setting the zero of Raman frequency a t the laser line), the Raman peak positions can be determined to f 3 cm-'. The relative peak intensities are somewhat skewed from their actual values, depending on the polarization properties of the scattered radiation of each peak, for severai reasons. The nitrogen laser output is not strongly polarized, the monochromator gratings preferentially reflect the polarization parallel to the grooves, and no polarization scrambler was used prior to the entrance slit (as is commonly done in research-grade instruments) to ensure equal spectrometer response for the different polarization components of Raman scattered radiation. In general, the nontotally symmetric (depolarized) vibrational bands are more intense relative to the totallv svmmetric bands for this instrument than is actually foGd. This is most easilv seen in the CCL suectmm (Fie. 3). in which the two lowest frequency bands (iepolarized Lending modes a t 218 cm-' and 315 cn") appear nearly a s intense a s the 4The laser used to collect the Raman spectra in this paper had an old gas cartridge and was operating well below power specifications for a new cartridge. We were unable, due to low signal intensity, to increase the resolution sufficientlyto resolve the well-known isotopiccomponents of the symmetric stretching band of CC14. This experiment is easily done with higher power lasers. The fast photodiode was mnstructed after a diagram by D. J. Dere in the Spectra Physics-QuantaJRay Nd:YAG laser manual. Construction of this device, which can be assembled for a p proximately $50, provides good practical experience for the instrumental analvsis student. 'Tne pnotod ooe trcggermg proced~reis necessary s nce tne VSL337 ooes not prome a TT.. o ~ t pPJ ~ tse syncnronos wctn the aser pulse.
I
LASER I
330
h\
340
SCATTERED WAVELENGTH
-
(nm)
Fiaure 4. Raman soectrurn lsinole scan) ~.Stokes ~ .and ,~ ~ , -o -f ~CCI.- in the arkstokes spectrai regions (-800 cm" to +800cm-' relative to laser line). Same experimental conditions as in Figure 3. "Laser ;i" indicates peak due to Rayleigh scattering of laser line.
Figure 3. Raman spectra of cyclohexane, carbon tetrachloride, chloroform, and benzene using the apparatus shown in Figure 1. Typical conditions: Laser repetition rate = 10 Hz, monochromator slits = 150 pm, monochromator scan rate = 0.05 nmls, PMT high voltage = -lOOOV, gated integrator average = 30 pulses, computer interface acquired 2000 data points over 338-358 nm scan range. The four spectra are not on the same intensity scale. The CCI, and CHCI, spectra are single scans, while the C6H, and C6H1, spectra are the sums of three individual scans. totally symmetric (polarized) stretching mode at 459 em-'. The real intensities of the depolarized bending modes are approximately half that of the totally symmetric stretch. I t is straightforward to acquire the Raman spectra of a wide variety of neat organic liquids using this apparatus. Cyclohexane (see Fig. 3) was selected as one sample for this paper due to its relatively modest (among organic liand its Raman quids) Raman scattering cross section (17), spectrum was quite easy to obtain. This instrument does not, however, have the sensitivity to ohtainRaman spectra of species in aqueous solution. We just barely observed (above the PMT dark signal) the totally symmetric vihration of the sulfate ion, a fairly strong Raman scatterer among inorganic ions, in a saturated aqueous %SO4 solution. Additionally, gas-phase Raman spectroscopy is not possible on this instrument due to the low sample density of gases. Demonstrating Stokes and Anti-Stokes Scattering An important Raman concept t h a t is easily demonstrated with this apparatus is the difference hetween Stokes and anti-Stokes Raman scattering processes. Figure 4 shows both the Stokes and anti-Stokes Raman bands of CCla. The Stokes bands, whose transitions start in the ground vibrational quantum level and end in an excited vibrational level, appear on the high wavelength side
~
of the laser line (lower absolute frequency, positive Raman shiR); the anti-Stokes hands, whose transitions start in thermally populated excited vibrational levels and end in the mound vibrational level. amear on the low waveleneth sidc(higher absolute freqkeicy, negative ~aman-ihi%j: The intensities of the anti-Stokes hands decrease markedly as the vibrational frequency (Av) increasse, due to the e-Av'kTBoltzmann population dependence of the initial quantum state. Since kT (h = Boltzmann constant = 0.695039 ern-'IK) is approximately 210 cm-' a t room temperature, the anti-Stokes bands become vanishingly weak for Av > 600 mi'. One can use the Stokeslanti-Stokes intensity ratio for a given Raman mode to calculate the sample temperature using
From the peak areas of the three lowest freauencv bands in the Figure 4 spectrum, we calculate a sample temperature of 280 f 20 K, which compares to the measured room temperature of 290 K. An interesting experiment would be to devise a variable-temperature cell holder and compare the sample temperature determined from the Stokes and anti-Stokes Raman intensities to t h e actual (thermocouple) temperature. Analytical Capabilities of Raman Spectroscopy The Raman spectra of a series of benzene/chloroform mixtures (Fig. 5) show the analytical capabilities ofRaman spectroscopy. The totally symmetric benzene ring-breathinrc mode a t 992 cm-'(near 348 nm) is labelled B. The four prominent lower frequency peaks are all due to chloroform. As the percentage (by volume) of benzene in the mixture is varied (samples were prepared using a graduated pipette) from 5.7% to 28.6%, the benzene Raman peak becomes more intense relative to the chloroform Raman peaks. The plot of the relative integrated peak areas of the 992 cn-' benzene band and the spectrally isolated 366 cn-' chloroform hand (near 341 nm, labelled C) gives a linear calibration curve (Fig. 6). This experiment demonstrates the excellent selectivity (due to Volume 69 Number 1
January 1992
81
and the collection optics can be adjusted until a focused orange line is seen on the monochromator entrance slit. When a PMT simal is observed. the positions are svstematieally adju&d to obtain the madmum signal. m his takes some patience and several iterations through all of the positional degrees of freedom to attain the largest possible simal.8 With the sample cell and UD-diredine~ r i s m on a tr&lation stage, the kignment is easier because the most sensitive adjustment is the position of the sample and beam perpendicular to the sampldslit axis and the translation stage maintains a constant relationship between the beam and the sample during this adjustient. After the dye signal is optimized, without moving the sample, themon&hromator is scanned around the totally symmetric benzene Raman line a t 992 cn-' (349 n m from the nitrogen laser line). This Raman peak should be detected, and only slight movements in the positions of the cell and collection optics should be required to maximize the Raman intensity. The dye solution can then be replaced by other samples with no further optical adjustment. ~
-
340
344
348
SCATTERED WAVELENGTH
(nm)
Fioure 5. Raman soectra of chloroformbenzene mixtures. Volume oe-rcent of benzene: 5.7%. lower: ~. 14.3%. middle:. 28.6%. -~. "Doer. ~, , ,~~ - Same expermenla con0 loons as In Flg~re3. The oenzene pea6 s labe ed 6 , wmle me otner peaks are from cnoroform.Tne spectra were sca eo lo have rodgnly tne same mtenstty in cnloroform pea6 C. ~
~~
~
~
narrow spectral lines) and reasonable sensitivity of Raman spectroscopy as a n analytical tool. Sample fluorescence is a potentially serious interference in Raman spectroscopy since both are shifted to lower frequency from the laser line. The magnitude of the interference depends strongly on the wavelength of the fluorescent emission. This situation is illustrated by taking the Raman spectra for solutions of highly fluorescent laser dyes in Gnzene. The Raman spec&& of benzene with a small amount of rhodamine 6G (R6G. emission maximum = 560 um) is not noticeably different from that of the neat solvent, due to the large wavelength difference between the Raman scattering and the fluorescence. However, the Ramau features of benzene are completely obscured by the fluorescence signal from a trans-stilbene (emission maximum = 410 nm). An important practical suggestion for this experiment is to use the fluorescence from a benzeneJR6G solution, with an absorbance of approximately 1.0 in a 1.00 cm cell at 534 nm, to optimize the position of the sample cell and the collection optics. For the Raman experiment, one needs to be sure that the focused laser beam intersects the sample a t the point in space "viewed" by the collection optics and the monochromator. This complex optical probG since lem is made much simpler using the R ~ solkion, the oranw fluorescence is easily visible to the naked eve. With the-monochromator set at560 nm and the slits n&rowed, the three-dimensional positions of the sample cell
~
Physical Chemistry Experiments Many experiments in the physical chemistry laboratory are possible using the Raman instrument described in this paper. At Wabash. we have developed a vibrational sp&troscopy experiment in the juiior-level physical chemistry laboratory, in which the students collect the spectra of small m b ~ e c u ~ (gas e s phase or liquid) and analyze the data according to the valence force field equations in Herzberg (18). The ability to acquire Raman as well as infrared data improves this laboratory immensely. Students learn a great deal about vibrationalmotion, force constants, and selection rules in this experiment. Another interestine ohvsical chemistrv experiment is the observation of the Boltzmann populakoniaw from the Stokes and anti-Stokes Raman intensities. This experiment reauires a clever thermostatted cell design and careful intensity t e a e r o u ~lab or measurements: it would be a ~ o r o ~ r i afor an individual special project. '& ekperiment involving the acquisition of the Raman spectra of benzene solutions of vaiious fluorescent laserAdyesshows scattering and electronic emission on the same instrument and leads to an interesting discussion ofthese two spectroscopicprocesses.
-. -
-
A
Analytical Experiments This Raman instrument has applications in the analytical laboratory as well. An experiment such as the one shown in Figures 5 and 6 demonstrates the quantitative analytical capabilities of Raman spectroscopy, an aspect of vibrational spectrscopy (infrared and Raman), in general, that is often overlooked by analytical chemists. From this experiment, it can be shown that Raman, while not a tremendously sensitive method, is quite selective. Another good analvtical ex~erimentis a com~onent~erformance haracter$ation oithe Raman instrument. ~ ; c han effort highlights the component nature of analvtical instruments, which is often hidden in modern instrumental packages. Students learn how the properties of a particular component help determine the overall properties of the instrument, and students can systematically study the relationship between the analytical signal and the adjustment of some specific variable. Examples of this type of experiment might include the determination of signal
-
,
0
10
VOI % Benzene
20
30
Figure 6. Plot of the data from Figure 5. The ordinate is the relative peakarea of benzene band B to chloroform band C.
82
Journal of Chemical Education
~-
Single lens imaging might be used in place of the dual lens system described - in~the~text -~ ~ in an attemot to simolifv . , the- ootical~ alion-, merit, o ~tne t redLced cgnt m lemon ejflclericyc o d~ prove a ser.ow proo em n trying to measuretne wean Raman scatter ng. ~
~
~~
~
-
maenitude and suectral resolution as a function of the monochromator slit width, the measurement of the observed sienal-to-noise (SIN) ratio of a Raman band as a function Gf PMT voltage, and the observation of the interaction between observed S/N ratio and the RC filterinn (with the picoammeter) or signal averaging (with the gated integrator) of the signal processing electronics. If the data are collected on a personal computer, the student can explore the effects of Santsky-Golay smoothing or Fourier filtering on the spectra. Raman spectra, with characteristically sharp peaks, are quite sensitive (as compared to UVNis snectra. for examule) to the ~arametersof these data massaging methods. Finally, summed spectra should show an increase in S/N ratio as the square root of the number of individual spectra in the sum. Conclusion In conclusion, we hope to have demystified Raman spectroscopy to a certain extent by showing how a straightforward instrument can he constructed and by showing the types ofexperiments that can be performed by students using the instrument. Several crucial practical hints and suggestions have been offered to assist the novice user., Finally, we have shown that there are many places in the physical and analytical chemistry under-
graduate laboratories in which Raman spectroscopy would be an interesting and pedagogically useful teaching tool. The authors welcome any questions or comments concerning the set-up and use of this instrument. Literature Cited
3. la) Bulkin. B. J. J. Cham. Edire. 1969,16, A781-A800. ibl Bulkin, B. J. J. Chem. Edue 1969,46,A858-A868. 4. Hoskina. L. C. J. Chem. Edm. 1916,52,568572. 5. Stmmmen, 0.P: Nakamob, K. J Chern. Educ. 1977.54.47G478. 6. Shaw, C. F J. Chem. Edue.. 1981.58.34L348. 7. Lee.6.Y J. Chrm. Educ 1985.62.561-566. 8 . M d a m e e , R. W:. van Venmgv, J. R.: Hams, W . C.J. Chem. Educ 1977, M , 79. 9. Behlow H. W.:Petersen. J. D. J.Chem. Edac 1985.62. 163. lo. DeHaan, F P.; Thibesult. J. C.; Otteson. U. K. J. c h e r n : ~ d u c1974,52,261365. 11. Ho8kina.L. C.J. Chem. Educ 1977.54.642643. 12. H o s h s , L. C. J C h m . Educ 1984,61.460462. 13. Hageman. H.; Sekha7.Y R.; BiU, H.J. Chrm. Educ 1987,64,456453. 14. ial Steehler J. K J Cham. Educ 1990.. 67. A37-A40 ibl Sfeehler. J. K. J Chem. Edue. 1990,67,A65-A71. 15. Dailinger, R. F "Using Lasen in the Undergraduate Laboratarf; Sympo~iumon Lasers in Analytical Chemistry; 201st AmericanChemical Society National Meeting: Boala", MA May 1990. 16. Schee1ine.A. J. Chem Educ. 1984,61.1110-1113. 17. Weber, A , Ed. Topics in Current Physics:Springer-Verlag: Berlin, lW9; Vol. 11, p
.
.
.
7w
18. Hersberg, G. Mokcrrlor Spectra and Mokeulor Sfmelure N Infmrpd and Ramon Spadm ofPolyotornic Mokcuks; Van Nostrand:New York. 1946: pp 168-186.
Volume 69 Number 1 January 1992
83
