Fourier Transform Raman Spectroscopy
Transcript of Fourier Transform Raman Spectroscopy
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Bruce Chase Central Research & Development Dept. E.I. du Pont de Nemours & Co. Wilmington, Del. 19898
Infrared molecular spectroscopic studies have benefited tremendously from the introduction of Michelson interferometers and the associated microcomputers necessary for data processing. The Michelson interferometer has both a multiplex and a throughput advantage when compared with a conventional grating instrument, and these advantages, coupled with the high-frequency precision available with laser referencing, have allowed measurements that were out of the question 10 years ago.
The first and most obvious property of interferometers is their large signal-to-noise (S/N) per unit measurement time. During the development of inter-ferometry for analytical vibrational spectroscopy, many researchers were intrigued by the possibility of extending 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 conventional, 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 photo-multiplier is a shot noise limited device; 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 spectrometer. 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
tor simultaneously and the detector is shot noise limited, the noise goes up by the same factor as the S/N enhancement resulting from the multiplexing process. The two effects cancel each other.
This was part of the original argument used to discard the possibility of Fourier transform (FT) Raman spectroscopy (1). Another problem noted early on was the difficulty of removing the very intense line due to quasi-elastic scattering. We will discuss that problem later in the paper. If there is to be no S/N advantage for Raman spectroscopy done with an FT instrument
ered. The Raman scattering effect is a nonresonant phenomenon. For an incident flux of 108 photons, on average, only one photon will be Raman-scattered. 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 impurities or weakly fluorescent samples can be a real problem, because their contributions to the detected signal will be in the form of a relatively constant background plus shot noise associated with
INSTRUMENTATION in the visible range, why do it? The answer becomes obvious when we rephrase the question and address the more relevant concern of the limitations 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 background 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 Raman signal. This phenomenon is easily understood when the relative efficiencies for the two processes are consid-
this background. This tendency is especially true for polymers and biological materials. In addition, photode-composition 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 spectroscopy might be more widely employed.
An additional problem with Raman spectroscopy involves the lack of a precise frequency base. Grating spectrometers suffer from lack of reproducibility 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 cm - 1
are necessary if accurate spectral subtractions are to be done. Currently, spectral subtractions in conventional Raman spectroscopy are difficult, and much effort has been expended to develop spinning dual-compartment cells to avoid the problems encountered when subtracting successive scans.
A final problem with conventional
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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 gratings are required. Under these conditions, the throughput of the instrument falls drastically. A number of nonlinear techniques have been successfully applied to the problem of high resolution, but the problems of excessive background and photodecomposi-tion still dominate the practice of Raman spectroscopy. Recent work by Jennings et al. (2) has demonstrated that FT-Raman spectroscopy successfully 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 fluorescence problem. Temporal-based techniques, 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, temporal-based techniques do hot effectively alleviate the photodecomposition problems. The traditional drench-quench method, in which the sample is irradiated 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 itself fluoresces, this approach is useless. The recent discovery of the luminescence-quenching properties of a silver surface (3) can be exploited in many cases, but the film deposition procedure is not always amenable to all samples. 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 approach has often been discussed in the past.
The krypton laser was supposed to have been the answer to the fluorescence problem because it provided a strong line at 6471 A. Unfortunately, the Raman effect itself is wavelength-dependent, and the cross section for scattering falls off as l/λ4. In addition, it often appears that excitation in the red is still sufficient to produce fluorescence at a reduced level. The excitation probably occurs through hot band-assisted processes. The overall gain in the
Raman-to-fluorescence ratio is not sufficient.
To completely avoid the excitation process, Hirschfeld suggested that Raman 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.
FT-Raman 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 wavelength, lenses would not be able to collect and refocus all wavelengths to the same point, and we would have a wavelength-dependent distribution of intensities 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 minimal, and excellent results are obtained using lenses as collection elements.
Our initial approach utilized a parabolic mirror with a hole at the apex
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LN2 cooled Ge detector
Fixed mirror
Spatial ^ filter
Beam AmmL· Sample splitter ηΤΎ Β 200-mm
\ ., 1 ^ lens
Dielectric filters
/ Parabolic collection
mirror
V Nd/YAG laser with line filter
Figure 1. Optical diagram of an FT-Raman spectrometer. LN2 = liquid nitrogen.
(Figure 1). This mirror had a diameter of 75 mm and a focal length of 29 mm, resulting in an f/1 collection system. We are currently working with ellipsoidal collectors, which will refocus the collected light onto the aperture stop of the spectrometer, and with cassegran-ian elements, which will operate in a similar fashion. The optimum collection optics will result from a compromise between maximum collection efficiency and the required degree of colli-mation.
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 wavelength, this laser is a source of interfering emission lines. If the laser is not routed outside the main beam path, or if it is not properly filtered with an interference filter, the Raman spectrum will contain a large number of neon emission lines. After modulation, conventional interferometer transfer optics take the beam to the detector.
Before striking the detector, however, the quasi-elastic scattered-light component must be removed. This was considered to be one of the major impediments 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 converter and detection system. Second, the detector can be forced into a nonlinear response region. Simply attenuating 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 than all the other lines, the distributive property of the Fourier transform process can degrade the noise performance across the entire spectrum. Random noise associated with that one strong line is indist inguishable from all other noise sources in the interferogram. This effect has been demonstrated using less than complete optical filtering of the quasi-elastic scattering component (6).
The first filter employed was an absorptive color filter constructed from the plastic in a pair of laser safety goggles. This material is specified as hav
ing an optical density of 14 at 1.06 μτη, and the transmission is good to longer wavelengths. The transition from opacity to high transmission is only gradual, so the effective range of the Raman 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 that have been considered are shown in Figure 2. We have chosen to work with a triple-stage, long-pass filter arrangement (Figure 2a). The drawbacks to the Raman notch or Chevron filter (Figure 2b) are limited aperture and limited angular acceptance. Eventual-
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.)
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Knife edge
Gratings
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ly, the double-grating spectrograph operated in zero dispersion (Figure 2c) with a knife-edge in ternal e lement 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 LN2-cooled germanium detector (Judson J16). The detector has a D* (detectivity, a measure of detector sensitivity) of 1 Χ 1011. Other possible choices for detectors include a cold-shielded InSb or PbS element. This is an area in which much can be gained from improvements in technology. Any increase in detectivity will translate directly into S/N improvement, because the Raman experiment is still detector noise limited.
FT- vs. conventional Raman Once such an instrument has been assembled, the results obtained on a conventional Raman instrument should be compared with those obtained with an interferometer. Figure 3 is a spectrum of acetanilide obtained on a Jobin-Yvon MOLE (a conventional Raman spectrometer) operated at 4 c m - 1 spectral band pass. Normal photon-counting 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.
The same sample, which was run in the FT-Raman instrument using 600 mW of 1.06-μπι radiation from a Spectron SL50 Nd-YAG laser, is shown in Figure 4. Total measurement time was also 60 min, and the instrument resolution was 4 cm - 1 . A Norton-Beer strong apodization function was used in processing the data. Phase correction was done with a Mertz algorithm, although power spectra often give equivalent results. The performance 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 pho-tomultiplier 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.
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500 1000 1500 2000 2500 3000 3500
Raman shift (cm -1)
Figure 5. Spectra of anthracene. (a) Conventional instrument, 5145 A excitation; (b) FT instrument, 1.06 μπι excitation. (Reprinted from Reference 6.)
500 1000 1500 2000 2500 3000 3500
Raman shift (cm ~1)
Figure 6. Spectra of poly(p-phenylene terephthalamide). (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 that the performance of the FT-Raman instrument is roughly equal to that of the conventional instrument. Is there any other advantage offered by FT-Raman? What can this instrument give us that was unavailable with the grating instrument? As mentioned earlier, the answer is fluorescence rejection. Operation at 1.06 μΐη avoids all background problems, and many samples that were intractable become amenable to Raman studies.
Figure 5 shows spectra of anthracene taken in the visible with conventional instrumentation (Figure 5a) and in the near-IR with the interferometer (Figure 5b). The strong fluorescence background is completely eliminated by working at 1.06 μηι. Similar results are shown in Figure 6 for a fiber of commercial interest, poly(p-phenylene terephtha lamide) . T h e spec t rum obtained in the IR (Figure 6b) shows tremendous 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. Spinning the sample will avoid this thermal problem. Another example of fluorescence rejection is shown in Figure 7. This is a spectrum of solid Rhodamine-6G taken with the interferometer. This sample, which is a laser dye when excited in the visible, shows no sign of fluorescence. To date, we have found no compound tha t shows a nonthermal
(fluorescence) background when excited by 1.06-μπι radiation.
An additional advantage of FT instrumentation in Raman spectroscopy is the high-frequency precision of the data. As mentioned before, spectral subtraction in Raman spectroscopy has
proved to be somewhat difficult because of the lack of frequency precision focus from scan to scan with a conventional grating instrument. Many ingenious experimental techniques have been developed to overcome this problem, but they all introduce extra corn-
Figure 7. FT-Raman spectrum of Rhodamine-6G. (Reprinted from Reference 6.)
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plexity. With data generated in an FT-Raman experiment, spectral subtraction should be as straightforward as it is in FT- IR.
Figure 8a shows an FT-Raman spectrum of a physical mixture of anthracene and 6is-phenyliminoterephthalal-dehyde (BPT). Figure 8b is the spectrum of pure BPT. Because this is a physical mixture with no interactions,
the subtraction should be precise. We should be able to remove all contributions to the spectrum from B P T . Figure 8c shows the subtraction results, and Figure 8d is a reference spectrum of anthracene. The agreement is excellent; relative band intensities are preserved. This aspect of FT-Raman spectroscopy 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 obvious area for future efforts. An ideal filter arrangement would allow acquisition of spectra closer to the Rayleigh line; our current limit is 200 cm - 1 . If a filter similar to the iodine cell for the argon laser could be found, low-frequency results would be improved. Much of the current work is being carried out with interferometers developed for mid-range IR spectroscopy. Overall in terferometer efficiency might be improved by designing a special-purpose interferometer for near-IR performance.
The feasibility of the technique has been demonstrated. When further improvements to sensitivity are made, the technique will have a chance of becoming truly complementary to IR, especially in the industrial analytical spectroscopy lab.
Acknowledgments
I would like to acknowledge the assistance of Donald Bly in the preparation of this manuscript.
References
(1) Hirschfeld, T.; Schildkraut, E. R. In Laser 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 University, personal communication, 1986.
(4) Rabolt, J. F. et al. Appl. Spectrosc. 1987,4/(5), 721-26.
(5) Hirschfeld, T.; Chase, B. Appl. Spectrosc. 1986, 40(2), 133.
(6) Chase, B. J. Am. Chem. Soc. 1986, 708(24), 7485.
Bruce Chase obtained his Ph.D. in chemistry from Princeton University in 1975. He joined the Central Research Department of Du Pont immediately after graduate school and has been in the spectroscopy division since that time. His work has involved the use of FT methods in vibrational spectroscopy. 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 spectroscopy.
Figure 8. FT-Raman spectra of (a) a mixture of BPT and anthracene, (b) pure BPT, (c) subtraction of spectrum b f rom spectrum a, and (d) pure anthracene. (Reprinted from Reference 6.)
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