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ANALYTICAL CHEMISTRY, VOL. 50, NO. 13, NOVEMBER 1978
AC-Coupled Inverse Raman Spectrometry Michael D. Morris," David J. Wallan, G. Patrick Ritz, and Jeanne P. Haushalter Department of Chemistry, University of Michigan, Ann Arbor, Michigan 48 109
Inverse Raman spectra are observed using a CW laser as w , and a pulsed dye laser as 02.The detector electronics are ac-coupled, allowing amplification of the pulsed component generated on the CW laser beam by the Raman process. The present system allows detection of an absorption as small as 0.005 %. Coherent Raman spectra generated by this technique are not distorted by a nonresonant background process and are obtained with better signal-to-noise ratlo than CARS spectra. The major experlmental problem is thermal blooming modulation of the CW laser signal caused by the pulsed laser. High resolution spectra of benzene are presented and spectra of aqueous nitrate solutions are presented and compared to CARS spectra. System modifications to decrease thermal blooming problems and to lower detection limits are discussed.
The problem of sample fluorescence has long plagued Raman spectrometrists and has limited the analytical applications of Raman spectrometry. Although many ad hoc techniques have been developed for removal of fluorescent impurities from samples, none have proved wholly satisfactory and none allow examination of molecules which are themselves fluorescent. Recently, coherent anti-Stokes Raman spectrometry (CARS) has emerged as a powerful technique for examination of fluorescent materials ( 1 , 2 )such as laser dyes (3) and flavins ( 4 ) and for the examination of nonfluorescent materials in fluorescent matrices. However, CARS suffers from well-known drawbacks (5, 6). First, the Raman emission is always accompanied by a coherent nonresonant background emission which is out of phase with the Raman signal. The nonresonant signal adds coherently to the Raman signal and the resulting observed signal may not resemble an ordinary Raman spectrum. In addition to distorting spectra, the nonresonant emission sets a high detection limit on CARS. A t some point, CARS signals disappear into the noise of the nonresonant background. The problem is particularly severe in aqueous solution, since the background emission of water is about ten times greater than that of benzene and certain other organic solvents ( 7 ) . There have been no really systematic studies made of CARS detection limits, but they are probably 1C-100 times higher than those obtainable with normal Raman spectrometry (6) under comparable conditions. Thus, while CARS overcomes the problems of sample fluorescence, the technique may have limited prospects for trace analysis. Because of the clear advantages and the limitations of CARS, there has been an extensive effort to develop other coherent Raman techniques which do not suffer from nonresonant background problems. Levenson and co-workers have advocated the use of the Raman-induced Kerr effect (RIKES) to circumvent the background problem (8-10). They measure the birefringence change induced in a sample by the interaction of two laser beams separated in frequency by a Raman active vibrational frequency. A major experimental problem with RIKES is the need for linearly polarized light with an extinction ratio of lo5 or greater. RIKES depends on the cancellation of two components of the third-order 0003-2700/78/0350-1796$01.00/0
susceptibility of the sample. While this cancellation appears to be good under normal Raman conditions, it has not been demonstrated under resonance Raman conditions. The Inverse Raman Effect. An alternative approach to background-free coherent Raman spectrometry is the use of the inverse Raman effect. The term inverse Raman effect refers to the absorption in a sample from a laser beam a t frequency w1 induced by interaction with another laser beam of lower frequency w2. The absorption occurs when w1 - w2 is equal to the frequency of a Raman-active vibration of the sample. Alternatively, one can measure the increase in intensity induced in beam w2 under these conditions. This induced increase is called Raman gain. The inverse Raman effect was first observed by Jones and Stoicheff in 1964 (11). In that classic paper it was pointed out that inverse Raman spectrometry could be used for acquisition of Raman spectra of fluorescent samples. However, the limitations of existing technology prevented widespread use of this technique. Yeung (12) significantly improved inverse Raman spectrometric instrumentation by substituting a dye laser for the incoherent probe sources previously used by workers in this field. He also reformulated inverse Raman theory and cleared up inconsistencies in earlier theoretical treatments. Yeung has shown that the induced absorption is given by Equation 1.
P,(O =
where P,(O) is the power density (W/cm2)in the laser beam at frequency w, (cm-') incident on the sample, P I ( / )is the power density as a function of beam interaction length, 1 (cm), P2 is the power density of the laser beam at frequency w2, Sw is the bandwidth of the Raman-active vibration (cm-'), n2 is the refractive index of the sample at a2,(du/dR) is the Raman cross-section (cm2/steradian-molecule,N is the concentration of the sample (molecules/cm3),h is Planck's constant and c is the speed of light. For the inverse Raman effect, w1 > w 2 . A similar expression describes Raman gain at 0 2 . The basic experimental problem of inverse Raman spectrometry is that it is difficult to obtain an induced absorption of more than a few percent even using very high peak power lasers such as ruby or neodymium-YAG. Thus, conventional inverse Raman measurements have shown even higher detection limits than CARS and the technique has remained fairly obscure. Recently, Owyoung and co-workers (13-15) have described a modified inverse Raman and Raman gain experiment in which one laser beam is modulated and lock-in detection is used t o extract the induced component on the other laser beam. By this technique, which they call CW stimulated Raman spectrometry, they are able t o measure much smaller absorptions or gains than is possible by the classical methods. They have reported measurements of Raman gains as low as IO-: and calculate an ultimate limit of about 4 X lo-' ( 1 3 ) . Similar limits could be obtained for absorption measurements. However, since Owyoung has used CU' lasers, the induced 0 1978 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 50, NO. 13, NOVEMBER 1978 +24V
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Ilk Figure 1. Apparatus for ac-coupled inverse Raman spectrometry. L i , L,, 200-nm f / l achromats; L,, 100-mm f / l achromat; filter; 488.0-nrn interference filter, 1-nm bandwidth
absorption or gain is limited by the low (less than 1watt) peak power of those lasers. From Equation 1,it is clear that increasing power P2 should greatly improve the detection limits of inverse Raman spectrometry. Indeed, the need for high power is the reason why most inverse Raman work has employed a ruby or neodymium-YAG laser to generate up. By using a CW laser to generate w1 and a high peak power pulsed dye laser to generate w2, it is possible to obtain fairly high Raman induced absorptions or, by interchanging the functions of the lasers, fairly high Raman gains. The induced absorption or gain will have exactly the same time dependence as the pulsed laser. Therefore, by using ac-coupled electronics in t h e detector system, it is possible to measure only the absorption or gain component on the CW beam, down t o 1 part in lo5 or smaller. The induced absorption or gain will generally be much less than 1 % , and, therefore, directly proportional to concentration. This hybrid technique, which we call ac-coupled inverse Raman spectrometry (or Raman gain spectrometry) offers experimental simplicity, good detection limits, and freedom from the problems of nonresonant background. AC-coupled Raman gain measurements were made by Owyoung and Peercy (16) in the course of a n investigation of nonlinear susceptibility of benzene with a Jamin interferometer. They reported Raman gain measurements for the benzene 992 cm-' band obtained by point-by-point measurements, using the second harmonic of a YAG laser (532 nm) and a manually-tuned CW dye laser. It is clear that ac-coupled inverse Raman spectrometry can be carried out with any pulsed laser/CW-laser pair with the proper (optical) frequency relationship. Use of a high peak power pulsed dye laser has certain advantages. First, high peak powers simplify signal acquisition since they generate relatively deep modulations of the CW beam. Second, such lasers are commercially available, reliable, and are tunable over a wide range of wavelengths. Raman gain and inverse Raman measurements give equivalent information. Because inverse Raman measurements are made a t the highest frequency incident on the sample, they provide spectral as well as spatial rejection of fluorescence. For that reason, we have made our measurements in the induced absorption rather than the induced gain mode.
EXPERIMENTAL The experimental apparatus is diagrammed in Figure 1. The CW laser was a Lexel Model 85-1 operated at 488.0 nm and fitted with an intra-cavity etalon for single longitudinal mode operation. The pulsed dye laser was a Molectron DL-200 with motor-driven grating. Dye laser power was limited to 3-5 kW by operating the nitrogen laser a t a low discharge voltage and by an aperture in the dye laser beam. The CW laser power was controlled at 25-50 mW at the laser head.
Figure 2. Detector and amplifier. D,; H-P 5082-2407; Qi and Q2,
2N5109: IC-1 SL-155OC. 1-yF capacitors are Taelectrolytic; all others are ceramic disks. 4-12 V, 4-6 V derived from 4-24 V by 78L series 3-terminal regulators (not shown) All lenses used were achromatic cemented doublets and aluminum mirrors were used in the beam steering optics. Conventional spectrophotometer cells were used as sample cells. The beam alignment system was similar to that for CARS ( 1 7 ) . A small crossing angle (lo-1.5') was used in these experiments. Although crossing the beams reduces the interaction length, it allows separation of the pulsed and CW beams without the need for monochromation beyond that provided by an argon laser spike filter. The detector and ac-coupled amplifier are shown as Figure 2. The photodiode (Hewlett-Packard,5082-4207)has an area of 0.008 cm2. The transistors used as impedance converters were chosen for low noise figure and good high frequency response. The amplifier (Plessey Semiconductor SL 1550 C) is an integrated circuit video amplifier and is a low noise gain block requiring no external components for operation. The overall gain of the circuit is about 32 db. The primary power source is two small 12-volt storage batteries (Globe-Union 1215) connected in series. Batteries are used to avoid the problem of adequate filtering and shielding of a line-operated power supply. The photodiode and amplifier components were mounted on a double-sided printed circuit board so that a good ground plane was available The circuit board was mounted in a commercial RFI-shielded box (Compac 6210-5120-2-1). The batteries were enclosed in a locally constructed brass box, since commercial RFI-shielded boxes of suitable dimensions were not readily available. These precautions were necessary to elminate interference from the burst of RF noise which accompanies each laser firing. The output of the photodiode amplifier was fed to a Princeton Applied Research gated integrator (Model 162, with Model 164 plug-in). The aperture duration of the gated integrator was set for about 50 ns, while its time constant was 1 p s According to the manufacturer, these conditions give a signal/noise ratio improvement of approximately 6 when the device is operated in the exponential averaging mode. The output of the gated integrator system was recorded on a conventional strip chart recorder. For these experiments, ACS reagent grade benzene and sodium nitrate were employed. Distilled water was used to prepare sodium nitrate solution. All solutions were filtered through 0.1-ym pore diameter membrane filters before introduction into the sample cell.
RESULTS AND DISCUSSION AC-coupled inverse Raman spectra of benzene are shown in Figure 3. Under the conditions of this experiment, the resolution is limited by the bandwidth of the dye laser, about 0.02 nm (0.8 cm-'). We observe the strong ring breathing mode a t 992 cm-' and the four nearby weaker bands commonly seen in high resolution benzene Raman spectra (28). These bands are more clearly resolved here than in CW stimulated Raman spectra. Because we cannot calibrate our dye laser with sufficient accuracy to make absolute frequency assignments, we have assumed a frequency of 991.5 cm-' for the benzene
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ANALYTICAL CHEMISTRY, VOL. 50, NO. 13, NOVEMBER 1978
Figure 3. AC-coupled inverse Raman spectra of benzene. Inset is 992 cm-' region at 1OX attenuation
L'
Figure 4. AC-coupled inverse Raman and CARS spectra of aqueous sodium nitrate solutions. (a)inverse Raman 1 M; (b) inverse Raman, 0.5 M; (c) CARS, 0.5 M
ring-breathing mode and have measured all other band positions relative to this one. Figure 3 also shows the 1171 cm-' CH deformation band of benzene. This band is about 40 times less intense than the 992 cm-' band. It is difficult to observe this band above the nonresonant background in CARS spectra, but in these experiments the band is clearly visible. Figure 4 shows the 1050 cm-' symmetric stretching vibration of aqueous sodium nitrate solutions at 1 M and 0.5 M. For comparison, a CARS spectrum of 0.5 M sodium nitrate is shown. These data demonstrate two important properties of ac-coupled inverse Raman spectrometry. First, as shown in the figure, the band intensity varies linearly with nitrate concentration. By contrast, CARS intensities have a square law dependence a t high concentrations and a linear dependence at low concentrations. Second, there is no evidence of a nonresonant signal here. The spectra are identical to conventional Raman spectra of this band. By contrast, the CARS spectra are severely distorted by background emission a t 0.5 M. Figure 4 also demonstrates the superior signal/noise ratio of ac-coupled inverse Raman spectrometry. The CARS spectrum was taken with a dual-channel gated integration system (19),with digital signal averaging of 36 laser pulses and digital ratioing of signal and reference channels. The inverse Raman spectrum was taken with a fully-analog system whose manufacturer urges that digital storage and averaging be used when the pulse repetition rate is as low as in these measurements. Moreover, no reference channel is employed here to compensate for dye laser output fluctuations. Even under these adverse conditions, the inverse Raman spectra
show better signal/noise ratios at 0.5 M than CARS spectrum taken under more nearly optimum experimental conditions. We have found it necessary to use an intra-cavity etalon on the argon ion laser. In the absence of the etalon, the laser oscillates on many longitudinal modes simultaneously. These modes are separated by 188 MHz in this laser, which has a short cavity. Consequently, with no etalon, a 188-MHz oscillation is always observed a t the detector and can obscure the Raman signal. The etalon completely suppresses multimode oscillation and removes this beat frequency. The major experimental problem with ac-coupled inverse Raman spectrometry is that thermal blooming (20) modulation always accompanies the process, because a pulsed laser is employed. Thermal blooming modulation is observed as a long pulse (typically about 50 ns) which occurs slightly after the laser pulse. The effect is caused by the defocusing of the CW beam by the transient thermal expansion and subsequent cooling induced by the pulsed laser beam. This causes the beam to slightly overflow the detector and results in a spurious signal. We have minimized the effects of thermal blooming by collimating the argon laser with lens L2 prior to presenting it to the detector focusing lens, LB. This operation decreases the convergence angle of the beam and thus decreases the change in the area of the beam caused by the defocusing. Resonance enhanced inverse Raman spectra have been obtained (22, 22). We have attempted to observe the accoupled resonance inverse Raman spectrum of cyanocobalamin using the argon ion 488.0 nm line and scanning the dye laser through a region 1500 cm to the green. Under these conditions, the cyanocobalamin 1504 cm-' band should have an intensity approximately equal to that of 0.2 M nitrate ( 2 3 ) . However, the thermal blooming signal is so large it completely obscures the Raman signal. There are several approaches to the circumvention of the thermal blooming problem in ac-coupled inverse Raman spectrometry. First. one can use a large area pin photodiode or vacuum photodiode so that beam expansion beyond the active area of the device is negligible. The major experimental problems are high junction capacitances of the pin diodes and the high bias voltages of both devices. The large bias voltage required makes battery operation impractical and introduces the problem of adequate RFI shielding of line-operated power supplies. Alternatively, one can diffuse the light by inserting a ground glass plate in front of the detector, effectively integrating the beam. A small flnumber lens can be used to focus the diffused beam into the detector. However, diffusion of the beam results in loss of the detector of about 90% of the incident light, requiring either additional amplifier gain or a photosensor with internal gain, such as an avalanche photodiode or a low gain photomultiplier tube. These approaches are under investigation and will be the subject of a later report. A t present the detection limits of our ac-coupled inverse Raman system are set by noise in the photodiode-amplifier system, The rms noise voltage is about 10 mV. Since the amplifier has a gain of 40, this corresponds to a primary photocurrent of about 250 nA. Under typical operating conditions, the dc current in the photodiode is about 2 to 5 mA. Thus, our detector system is currently limited to measurement of induced absorption of 5 X or greater. We must emphasize that this detector system is only a first design. Transistors and video amplifiers with lower noise figures than those used here are available and can be used to improve system performance. Moreover, cooling the entire system to ca. -20 "C is quite feasible and would reduce system noise at least 20 db. In addition, digital storage and averaging of the boxcar integrator output would improve the performance of
ANALYTICAL CHEMISTRY, VOL. 50, NO. 13, NOVEMBER 1978
that device. while use of a reference channel monitoring" the dye laser fluctuations would further improve system performance. With these modifications to our apparatus, measurement of induced absorption of 10 or less i q very feasible. Under these conditions, the detection limits of ac-coupled inverse Raman spectrometry should be equal to or better than those obtainable by spontaneous Raman spectrometry.
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(7) J. Nestor, T. G. Spiro, and G. Klaumin, F'roc. Natl Atad. Sci , 73, 3328 (1976) (8) D Heiman, R W Hellwarth M D Levenson, and G Martin. Phys Rev L e t t , 36, 189 (1976) (9) M D Levenson and J J Song, J Opt Soc A m , 66, 641 (1976) (10) G L Eesley. M D Levenson, and W M Tolles, IEEE J QuantumEktron , 14 45 (1978) (11) W.J. Jones and 8. P. Stoicheff, Phys. Rev. Lett., 13, 657 (1964). (12) E. S. Yeung, J . Mol. Spectrosc., 53, 379 (1974). (13) A. Owyoung, Opt. Commun., 22, 323 (1977). (14) A. Owyoung and E. D. Jones, Opt. Lett., 1, 152 (1977). (15) A. Owyoung, IEEE J . Quantum Electron., 14, 192 (1978). (16) A. Owyoung and P. S. Peercy, J . Appl. Phys., 48, 674 (1977). (17) D. J. Wallan, G. P. Ritz, and M. D. Morris, Appl. Spectrosc., 31, 475 (1977). (18) A. Langseth and R. C. Lord, J . Chem. Phys., 6, 203 (1938). (19) G. P. Ritz. D. J Wallan, and M. D. Morris, Appl. Spectrosc.. in press. (20) J. H. Whinnery, Acc. Chem. Res., 7, 225 (1974). (21) S. H. Lin, E. S. Reid, and C. J. Tredwell, Chem. Phys. Lett.. 24. 389 (1974). (22) W. Werncke, A. Lau. M. Pfeiffer, H.-J. Weigmann, .:1 Hunsalz, and K. Lenz, Opt. Comrnun., 16, 128 (1976). (23) C.-W. Tsai and M. D. Morris, Anal. Chim. Acta, 76, 193 (1975).
ACKNOWLEDGMENT The authors thank Nestor Clough, Lexel Corporation, for the loan of the argon ion laser used in these experiments. LITERATURE CITED (1) P. R. Regnier and J. P. E. Taran, Appl. Phys. Len.. 23, 240 (1973). (2) R. F. Begley, A. B. Harvey, and R. L. Byer, Appl. Phys. Lett., 25, 387 (1974). (3) L. A. Caniera and L. P. Goss, 32nd Symposium on Molecubr Specb-oscopy, Ohio State University, Columbus, Ohio, June 1977. paper WE-8. (4) P. K. Dutta, J. R. Nestor, and T. G. Spiro. Proc. Natl. Acad. Sci., 74, 4146 (1977). (5) W. M. Tolles, J. W. Nibler, J. R. McDonald, and A. B. Harvey, Appl. Spectrosc., 31, 253 (1977). (6) M. J. Levenson, Phys. Today, 30 ( 5 ) . 44 (1977).
RECEWED for review May 11, 1978. Amepted August 17, 1978. Work supported in part by National [nstitutes of Health grant GM-22604.
Computer-Controlled Dual Wavelength Spectrophotometer K. L. Ratzlaff' School of Chemical Sciences, University of Illinois, Urbana, Illinois 6 780 1
F. S. Chuang, D. F. S. Natusch, and K. R. O'Keefe" Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523
transmitted light intensities a t one wavelength for separate sample and reference cells, DWS uses light of' two different wavelengths which passes through a single sample cell. Operationally, the advantage of DWS derives from the fact that both beams experience the same sample environment. The implications of this characteristic have been considered in previous papers ( I , 2). The technique was first suggested and subsequently developed by Britton Chance for the determination of reaction rates in biological media (3-9); hobever. it was 1969 before the utility of DWS for equilibrium methods of chemical analysis was realized; a t that time, commercial DWS instruments became available (10-22) and applications were presented for analyses in the presence of spectral interferents (10-22). In the past, DWS instrumentation has been directed toward beam-modulated approaches with high modulation rates. In these techniques, the source illuminates two independent wavelength isolation devices (monochromators or filters) and the phase-separated outputs of these devices illuminate the sample. Another possible approach t o DWS involves wavelength modulation of a single monochromator. This is the method often used in derivative spectrometers. Regardless of the type of wavelength selection used, a dual wavelength spectrophotometer that is to be useful for a range of applications must incorporate a variety of features. These include the following. (1) The spectrophotometer must measure transmitted intensities at two separate wavelengths either simultaneously or alternately. If the latter, the modulation rate should be significantly faster than the half-time of the fastest reaction
The construction and evaluation of a computer-controlled dual wavelength spectrometer for use in making UV-visible molecular absorption measurements is described. An electromechanical modulator is used to step the grating of a Czerny-Turner configuration monochromator to either of two positions at a maximum rate of 2 Hz. A wavelength modulation precision of better than f2.5 X nm is achieved. The spectrophotometer is evaluated in terms of instrumental precision, linearity, and accuracy in typical measurement configurations and several applications of the instrument are described.
The measurement of electronic energy transitions in molecules in solution has found widespread application in chemistry with the major limitation of electronic transition (UV-visible) spectrometry being related to sample characteristics. Dual-beam-in-time or dual-beam-in-space and time spectrophotometers generally rely for their accuracy on the ability of the experimenter to prepare a reference solution that closely approximates the sample matrix. This ability is often quite limited when dealing with samples that are strongly scattering or that contain appreciable amounts of interferents that absorb in the wavelength region of interest. The technique of Dual Wavelength Spectrophotometry (DWS) represents a fundamental departure from conventional ratio spectrophotometry in this regard. Rather than ratioing Present address, Department of Chemistry, Northern Illinois university, DeKalb, Ill. 60115. 0003-2700/78/0350-1799$01 O O / O
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1978 American Chemical Society
