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Bruce Chase Central Research & Development Dept. E.I. du Pont de Nemours & Co. Wilmington, Del. 19898
Infrared molecular spectroscopic stud ies have benefited tremendously from the introduction of Michelson interfer ometers and the associated microcom puters necessary for data processing. The Michelson interferometer has both a multiplex and a throughput ad vantage when compared with a conven tional grating instrument, and these advantages, coupled with the high-fre quency precision available with laser referencing, have allowed measure ments that were out of the question 10 years ago. The first and most obvious property of interferometers is their large signalto-noise (S/N) per unit measurement time. During the development of interferometry for analytical vibrational spectroscopy, many researchers were intrigued by the possibility of extend ing this effort to Raman spectroscopy, in which the noise is also a problem. In retrospect, this approach was wrong, because it does not address the real problems associated with convention al, linear Raman spectroscopy, and it can lead to a false conclusion. For routine Raman spectroscopy, the standard instrumentation is a visible laser, a double monochromator, and a photon-counting detector. A photomultiplier is a shot noise limited de vice; the noise is proportional to the square root of the light intensity falling on it. To a first approximation, the noise level of such an experiment will not benefit from a multiplexing spec trometer. The S/N ratio for a detector noise limited system increases as the square root of the number of resolution elements, but when we allow all of the resolution elements to strike the detec 0003-2700/87/0359-881A/$01.50/0 © 1987 American Chemical Society
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tor simultaneously and the detector is shot noise limited, the noise goes up by the same factor as the S/N enhance ment resulting from the multiplexing process. The two effects cancel each other. This was part of the original argu ment used to discard the possibility of Fourier transform (FT) Raman spec troscopy (1). Another problem noted early on was the difficulty of removing the very intense line due to quasi-elas tic scattering. We will discuss that problem later in the paper. If there is to be no S/N advantage for Raman spec troscopy done with an FT instrument
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ered. The Raman scattering effect is a nonresonant phenomenon. For an inci dent flux of 108 photons, on average, only one photon will be Raman-scat tered. Fluorescence, on the other hand, is a resonant phenomenon. If there is an impurity at the parts-per-million level with a fluorescent quantum yield of 0.1, under the same conditions 10 fluorescent photons will be produced. Therefore highly fluorescent impuri ties or weakly fluorescent samples can be a real problem, because their contri butions to the detected signal will be in the form of a relatively constant back ground plus shot noise associated with
INSTRUMENTATION in the visible range, why do it? The answer becomes obvious when we re phrase the question and address the more relevant concern of the limita tions of Raman spectroscopy. Problems with conventional Raman What are the current limitations for Raman spectroscopy when performing a linear, nonresonant experiment? It is only indirectly a problem of signal to noise. The most common problem in Raman spectroscopy involves the background. In the absence of a back ground signal, the Raman experiment is extremely sensitive; one is able to detect single photons arriving at the photomultiplier. The presence of a background, however, masks the signal of interest and increases the observed noise. Fluorescence, either from the sample or from impurities contained in the sample, will often totally mask the Ra man signal. This phenomenon is easily understood when the relative efficien cies for the two processes are consid-
this background. This tendency is es pecially true for polymers and biologi cal materials. In addition, photodecomposition can occur, destroying the sample before data can be obtained. If ways can be found to avoid the two problems of excessive background and photodecomposition, Raman spectros copy might be more widely employed. An additional problem with Raman spectroscopy involves the lack of a pre cise frequency base. Grating spectro meters suffer from lack of reproducibil ity in the frequency base from scan to scan. This error may only amount to a few tenths of a cm - 1 , but experience in infrared (IR) spectroscopy has shown that frequency precisions of 0.01 c m - 1 are necessary if accurate spectral sub tractions are to be done. Currently, spectral subtractions in conventional Raman spectroscopy are difficult, and much effort has been expended to de velop spinning dual-compartment cells to avoid the problems encountered when subtracting successive scans. A final problem with conventional
ANALYTICAL CHEMISTRY, VOL. 59, NO. 14, JULY 15, 1987 • 881 A
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Raman instruments is the difficulty in obtaining high-resolution data. To work at high spectral resolution, very narrow slits and high dispersion grat ings are required. Under these condi tions, the throughput of the instru ment falls drastically. A number of nonlinear techniques have been suc cessfully applied to the problem of high resolution, but the problems of exces sive background and photodecomposition still dominate the practice of Ra man spectroscopy. Recent work by Jennings et al. (2) has demonstrated that FT-Raman spectroscopy success fully addresses the problem of limited resolution in gas-phase studies. In such an experiment, however, the Rayleigh line is relatively weak and there is no fluorescent background. Many approaches have been taken in an attempt to minimize the fluores cence problem. Temporal-based tech niques, in which the difference in time scale between Raman scattering and fluorescence is exploited, have been promising, but they are not universally applicable. In addition, temporalbased techniques do hot effectively al leviate the photodecomposition prob lems. The traditional drench-quench method, in which the sample is irradi ated with the laser for an extended time in an effort to photobleach the system, is fine if the background arises from an impurity, but if the sample it self fluoresces, this approach is useless. The recent discovery of the lumines cence-quenching properties of a silver surface (3) can be exploited in many cases, but the film deposition proce dure is not always amenable to all sam ples. Nonlinear experiments such as coherent anti-Stokes Raman scattering provide a high degree of fluorescence discrimination, but again, they are not universally applicable—especially to solid samples. Because the fluorescence and the photodecomposition processes have certain minimum energies associated with them, the most logical approach would be to reduce the energy of the photons striking the sample to a value lower than the threshold for excitation. In this way, the first excited electronic state (of the sample or of an impurity) would never be populated. This ap proach has often been discussed in the past. The krypton laser was supposed to have been the answer to the fluores cence problem because it provided a strong line at 6471 A. Unfortunately, the Raman effect itself is wavelengthdependent, and the cross section for scattering falls off as l/λ 4 . In addition, it often appears that excitation in the red is still sufficient to produce fluores cence at a reduced level. The excitation probably occurs through hot band-as sisted processes. The overall gain in the
882 A • ANALYTICAL CHEMISTRY, VOL. 59, NO. 14, JULY 15, 1987
Raman-to-fluorescence ratio is not suf ficient. To completely avoid the excitation process, Hirschfeld suggested that Ra man spectroscopy performed with a Nd-YAG laser might be the answer. This laser operates at 1.06 μιη or 9395 cm -1 , which should be well below the threshold for any fluorescence process. Unfortunately, the cross section for Raman scattering at 1.06 μια is down by a factor of 16 from that at 5145 À. An additional problem is the lack of good detectors (i.e., shot noise limited) comparable to a photomultiplier. F T - R a m a n instrumentation
To compensate for the loss in cross section and the poor detectors, a multiplexing instrument would be required. This is the basic argument for attempting FT-Raman spectroscopy. No drastic improvement in performance over a conventional system operating in the visible is expected unless there is a background present. Then the FT-Raman instrument operating at 1.06 /urn should allow the acquisition of spectra, whereas the instrument operating at 5145 À fails completely. The basic FT-Raman instrument is similar to a conventional grating instrument in that the scattered light must be collected and then passed through a spectrometer. The collection optics in an FT-Raman experiment serve the same purpose as in a conventional Raman experiment. We need to collect as much light as possible for analysis. There are, however, some constraints. The half-angle divergence of the collected beam must not exceed the resolution requirements of the interferometer, or the collected beam must be passed through the limiting aperture stop of the interferometer. This condition is easily met for low-resolution experiments (1-4 cm - 1 ). The second constraint involves the type of optical element employed. Normally, collection of scattered light is accomplished using lenses that have a wavelength-dependent chromatic aberration. In the visible region, the chromatic aberrations are usually acceptable because the entire Raman spectrum may cover only 0.23 μτα. In the near-IR the Raman spectrum would span close to 1 μια in wave length, lenses would not be able to col lect and refocus all wavelengths to the same point, and we would have a wave length-dependent distribution of in tensities across the detector. However, recent work by Rabolt, Zimba, and Hallmark (4) has shown that with the proper choice of optical materials the chromatic aberration problem is mini mal, and excellent results are obtained using lenses as collection elements. Our initial approach utilized a para bolic mirror with a hole at the apex
LN2 cooled Ge detector Fixed mirror Beam splitter
Spatial filter
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Figure 1. Optical diagram of an FT-Raman spectrometer. LN2 = liquid nitrogen.
(Figure 1). This mirror had a diameter of 75 m m and a focal length of 29 mm, resulting in an f/1 collection system. We are currently working with ellipsoi dal collectors, which will refocus the collected light onto the aperture stop of the spectrometer, and with cassegranian elements, which will operate in a similar fashion. The optimum collec tion optics will result from a compro mise between maximum collection effi ciency and the required degree of collimation. Once the scattered radiation has been collected and collimated, it is then passed into the modulator. For this spectral region, a quartz beam splitter normally is used. Most interferometers use a helium neon laser for referencing. In some cases, this laser passes down the center of the main beam splitter, colinear with the IR beam. Because the detectors employed in the near-IR are sensitive to the helium neon wave length, this laser is a source of interfer ing emission lines. If the laser is not routed outside the main beam path, or if it is not properly filtered with an in terference filter, the Raman spectrum will contain a large number of neon emission lines. After modulation, con ventional interferometer transfer op tics take the beam to the detector. Before striking the detector, how ever, the quasi-elastic scattered-light component must be removed. This was considered to be one of the major im pediments to FT-Raman spectroscopy (5), because the intensity of this line is high relative to the Raman lines. A large amount of. light at one frequency can cause several problems. First, the one very strong line can completely fill up the dynamic range of the A/D con verter and detection system. Second, the detector can be forced into a non linear response region. Simply attenu ating the intensity of this line some
what to avoid saturating the detection system is not enough. If one line in the spectrum is much stronger t h a n all the other lines, the distributive property of the Fourier transform process can de grade the noise performance across the entire spectrum. Random noise associ ated with t h a t one strong line is indis tinguishable from all other noise sources in the interferogram. This ef fect has been demonstrated using less t h a n complete optical filtering of the quasi-elastic scattering component (6). T h e first filter employed was an ab sorptive color filter constructed from the plastic in a pair of laser safety gog gles. This material is specified as hav
ing an optical density of 14 at 1.06 μτη, and the transmission is good to longer wavelengths. T h e transition from opac ity to high transmission is only gradual, so the effective range of the R a m a n spectrum for this type of filter starts at 800 c m - 1 Stokes shift. It is not suitable for routine use, but it is an ideal filter for initial experiments and alignment. Several other types of filters t h a t have been considered are shown in Fig ure 2. We have chosen to work with a triple-stage, long-pass filter arrange ment (Figure 2a). T h e drawbacks to the R a m a n notch or Chevron filter (Figure 2b) are limited aperture and limited angular acceptance. Eventual-
Gratings
Knife edge Figure 2. Filter arrangements for rejection of quasi-elastic scattered component. (a) Multistage filter, (b) Chevron filter, (c) polychromator filter. (Reprinted with permission from Reference 5.)
884 A • ANALYTICAL CHEMISTRY, VOL. 59, NO. 14, JULY 15, 1987
ly, the double-grating spectrograph op erated in zero dispersion (Figure 2c) with a knife-edge i n t e r n a l e l e m e n t might prove to give the highest degree of laser line rejection coupled with the best transmission close to the laser line. Current cost and optical alignment constraints are a drawback. The detector used in our laboratories is an LN 2 -cooled germanium detector (Judson J16). The detector has a D* (detectivity, a measure of detector sen sitivity) of 1 Χ 10 11 . Other possible choices for detectors include a coldshielded InSb or P b S element. This is an area in which much can be gained from improvements in technology. Any increase in detectivity will translate di rectly into S/N improvement, because the Raman experiment is still detector noise limited. FT- vs. conventional Raman Once such an instrument has been as sembled, the results obtained on a con ventional Raman instrument should be compared with those obtained with an interferometer. Figure 3 is a spectrum of acetanilide obtained on a JobinYvon MOLE (a conventional Raman spectrometer) operated at 4 c m - 1 spec tral band pass. Normal photon-count ing detection was used, and the laser was a Spectra Physics 171 krypton ion unit operating at 6471 À. The power level at the sample was 80 mW. Total measurement time was 60 min. T h e same sample, which was run in the F T - R a m a n instrument using 600 mW of 1.06-μπι radiation from a Spectron SL50 N d - Y A G laser, is shown in Figure 4. Total measurement time was also 60 min, and the instru ment resolution was 4 c m - 1 . A NortonBeer strong apodization function was used in processing the data. Phase cor rection was done with a Mertz algo rithm, although power spectra often give equivalent results. The perfor mance of the two instruments appears to be roughly comparable when the higher power level is employed in the infrared. More power incident on the sample compensates for the loss in cross section going from 6471 Â to 10,600 A. The F T instrument performs quite well in comparison with the conventional instrument despite the loss in scattering cross section. One of the possible reasons is the increased quantum efficiency (Q.E.) of the detector. A photomultiplier often has a Q.E. of 0.10, whereas the germanium diode has a Q.E. of 0.70. The interferometer has a higher throughput than the grating instrument, and the efficiency is greater. These effects combine with the multiplex advantage to help compensate for the loss from decreased cross section as well as the poor sensitivity of the detector relative to a photomultiplier.
Figure 3. Spectrum of acetanilide obtained on a conventional Raman instrument.
Figure 4. Spectrum of acetanilide obtained on an FT-Raman instrument.
886 A • ANALYTICAL CHEMISTRY, VOL. 59, NO. 14, JULY 15, 1987
500 500
1000
1500
2000
2500
3000
1000
1500
2000
2500
3000
3500
1
3500
Raman shift (cm ~ )
-1
Raman shift (cm ) Figure 5. Spectra of anthracene.
Figure 6. Spectra of poly(p-phenylene terephthalamide).
(a) Conventional instrument, 5145 A excitation; (b) FT instrument, 1.06 μπι excitation. (Reprinted from Reference 6.)
(a) Conventional instrument, 5145 A excitation; (b) FT instrument, 1.06 /im excitation. (Reprinted from Reference 6.)
It is clear from Figures 3 and 4 t h a t the performance of the F T - R a m a n in strument is roughly equal to t h a t of the conventional instrument. Is there any other advantage offered by FT-Ra man? What can this instrument give us that was unavailable with the grating instrument? As mentioned earlier, the answer is fluorescence rejection. Oper ation at 1.06 μΐη avoids all background problems, and many samples t h a t were intractable become amenable to Ra man studies. Figure 5 shows spectra of anthracene taken in the visible with conventional instrumentation (Figure 5a) and in the near-IR with the interferometer (Fig ure 5b). T h e strong fluorescence back ground is completely eliminated by working at 1.06 μηι. Similar results are shown in Figure 6 for a fiber of com mercial interest, poly(p-phenylene ter e p h t h a l a m i d e ) . T h e s p e c t r u m ob tained in the IR (Figure 6b) shows tre mendous background rejection when compared with that taken in the visible (Figure 6a). There is a broad base-line feature observed, but this is not due to fluorescence. It comes from sample heating by the laser. The hot sample acts as a black body radiator, and the detection system is so sensitive that the black body emission is detected. Spin ning the sample will avoid this thermal problem. Another example of fluores cence rejection is shown in Figure 7. This is a spectrum of solid Rhodamine6G taken with the interferometer. This sample, which is a laser dye when excit ed in the visible, shows no sign of fluo rescence. To date, we have found no compound t h a t shows a nonthermal
(fluorescence) background when excit ed by 1.06-μπι radiation. An additional advantage of F T in strumentation in R a m a n spectroscopy is the high-frequency precision of the data. As mentioned before, spectral subtraction in Raman spectroscopy has
proved to be somewhat difficult be cause of the lack of frequency precision focus from scan to scan with a conven tional grating instrument. Many inge nious experimental techniques have been developed to overcome this prob lem, but they all introduce extra corn-
Figure 7. FT-Raman spectrum of Rhodamine-6G. (Reprinted from Reference 6.)
888 A • ANALYTICAL CHEMISTRY, VOL. 59, NO. 14, JULY 15, 1987
plexity. With data generated in an F T R a m a n experiment, spectral subtrac tion should be as straightforward as it is in F T - I R . Figure 8a shows an F T - R a m a n spec trum of a physical mixture of anthra cene and 6is-phenyliminoterephthalaldehyde (BPT). Figure 8b is the spec trum of pure B P T . Because this is a physical mixture with no interactions,
the subtraction should be precise. We should be able to remove all contribu tions to the spectrum from B P T . Fig ure 8c shows the subtraction results, and Figure 8d is a reference spectrum of anthracene. T h e agreement is excel lent; relative band intensities are pre served. This aspect of F T - R a m a n spec troscopy may do more than anything else to popularize its usage.
Conclusion
Many aspects of this experiment still require development. Optimization of collection optics and detectors is an ob vious area for future efforts. An ideal filter arrangement would allow acquisi tion of spectra closer t o the Rayleigh line; our current limit is 200 c m - 1 . If a filter similar to the iodine cell for the argon laser could be found, low-fre quency results would be improved. Much of the current work is being car ried out with interferometers devel oped for mid-range IR spectroscopy. Overall i n t e r f e r o m e t e r efficiency might be improved by designing a spe cial-purpose interferometer for nearIR performance. T h e feasibility of the technique has been demonstrated. When further im provements to sensitivity are made, the technique will have a chance of becom ing truly complementary to IR, espe cially in the industrial analytical spec troscopy lab. Acknowledgments
I would like to acknowledge the assis tance of Donald Bly in the preparation of this manuscript. References
(1) Hirschfeld, T.; Schildkraut, E. R. In La ser Raman Gas Diagnostics; M. Lapp, Ed.; Plenum: New York, 1974, pp. 379-88. (2) Jennings, E.; Weber, Α.; Brault, J. W. Appl. Opt. 1986, 25(2), 284. (3) Van Duyne, R., Northwestern Universi ty, personal communication, 1986. (4) Rabolt, J. F. et al. Appl. Spectrosc. 1987,4/(5), 721-26. (5) Hirschfeld, T.; Chase, B. Appl. Spec trosc. 1986, 40(2), 133. (6) Chase, B. J. Am. Chem. Soc. 1986, 708(24), 7485.
Figure 8. FT-Raman spectra of (a) a mixture of BPT and anthracene, (b) pure BPT, (c) subtraction of spectrum b from spectrum a, and (d) pure anthracene. (Reprinted from Reference 6.)
Bruce Chase obtained his Ph.D. in chemistry from Princeton University in 1975. He joined the Central Re search Department of Du Pont imme diately after graduate school and has been in the spectroscopy division since that time. His work has involved the use of FT methods in vibrational spec troscopy. His areas of research include applications of diffuse reflectance, IR emission, and the study of artifacts in FT data processing. In 1985 he and the late Tomas Hirschfeld demonstrated the feasibility of FT-Raman spectros copy.
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