-r/ BNIX Chase Central Research 8 Development Dept. E.I. du Pont de Nemours 8 Co. Wilminalon. 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 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 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 spectrmcopy, the standard instrumentation is a visible laser, a double monochromator, and a photon-counting detector. A photomultiplier 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 detec0003-2700/87/0359-881A/501.50/0 @ 1987 American Chemical Society
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 pmsibility of Fourier transform (FT) Raman spectroscopy ( I ) . 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 he Raman-scattered. Fluorescence, on the other hand, is a resonant phenomenon. If there is an impurity a t 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 he in the form of a relatively constant hackground plus shot noise associated with
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.
this background. This tendency is especially true for polymers and biological 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 spectroscopy might he 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-' 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
Roblems with mventional 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-
ANALYTICAL CHEMISTRY, VOL. 59, NO, 14, JULY 15. 1987
881A
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882A
Raman instruments is the difficulty in obtaining high-resolution data. T o work a t 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 photodecomposition 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, temporalbased 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 wavelengthdependent, and the cross section for scattering falls off as l/X4. 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
ANALYTICAL CHEMISTRY, VOL. 59, NO. 14, JULY 15, 1987
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 pm or 9395 cm-', which should be well below the threshold for any fluorescence process. Unfortunately, the cross section for Raman scattering at 1.06 pm is down by a factor of 16 from that at 5145 An additional problem is the lack of good detectors (i.e., shot noise limited) comparable to a photomultiplier.
A.
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 pm should allow the acquisition of spectra, whereas the instrument operating a t 5145 A 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-'). 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 pm. In the near-IR the Raman spectrum would span close to 1 pm 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 a t the apex
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rigure 1. upticai aiagram or an r I-naman spectrometer. LN2 = liquid nihcgen.
(Figure 1). This mirror had a diameter of 75 mm and a focal length of 29 mm, resulting in an f l l collection system. We are currently working with ellipsoidal 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 collection optics will result from a compromise between maximum collection efficiency 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 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 he 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 AD converter and detection system. Second, the detector can be forced into a nonlinear response region. Simply attenuating the intensity of this line some884A
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 indistinguishable 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-
J.
ing an optical density of 14 a t 1.06 wn, 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 a t 800 em-’ 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 2h) are limited aperture and limited angular acceptance. Eventual-
* Knife
Figure 2. Filter anangements for rejection of quasl-elastic scattered compone (a1 Munistage liller. (b) Chewon finer, (e) polychromatwfilter. (Repintmi wllh p”missi0n horn Relerenca 5.)
* ANALYTICAL CHEMISTRY, VOL. 59. NO. 14, JULY 15, 1987
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ly, the double-grating spectrograph OK erated in zero dispersion (Figure 21 with a knife-edge internal element 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 LNn-cooled germanium detector (Judson J16). The detector has a D* (detectivity, a measure of detector sensitivity) of 1 X 10". Other possible choices for detectors include a coldshielded 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. c@nvenUonalRaman 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 JobinYvon MOLE (a conventional Raman spectrometer) operated at 4 cm-I spectral band pass. Normal photon-counting detection was used, and the laser was a Spectra Physics 171 krypton ion unit operating at 6471 A. The power level a t the sample was EO mW. Total measurement time was 60 min. The same sample, which was run in the FT-Raman instrument using 600 mW of 1.06-pm 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-l. A NortonBeer 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 8, to 10,600 A. The FT instrument performs quite well in comparison with the conventional instrument despite the loss in scattering cross section. One of the passible 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. 886A
Raman shit(cm-')
Flgure q. uwctrum of acetanilide obtaim VII an FT-Raman
ANALYTICAL CHEMISTRY, VOL. 59, NO. 14. JULY 15. 1987
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Flgure 5. Spectra of anthracene.
Figure 6.~Spectra of poly(pphenylene terephthalamide).
la) ConventioMl instrument. 5145 A excitation: (b) FT inshument. 1.06 vrn excitation. (Repinled hom Reference 6.)
(a) Conventionai instrument. 5145 A excitation; (b) FT instrument. 1.06 #rn excitation. (Reprintedham 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 reiection. ODerrltion at 1.06 Irm avoids all backgr&nd problems, and many samples that were intractable brwmc amenable t u Raiman studies. Figure Sshuwsspectra nf anthracene tnken i n t h t \iiible with wnventiunal instrnmeiit3tion (Figure 5 3 ) and in the iitar-lN with the interteronieter tFigurr 5111.The strong fluorescence backyr~iuiid is completely eliminated by working at 1.06 pm. Similar results are shown i n Figure ti tor a fiber uf cum. mercial interest, pd)\p-phenylene ierephihalamide,. The spectrum ull. tained in the I R t Figure 61)) shows tremenduus background rejection when compared with that taken in ihc visible (Figure Gal. There is a broad base-line feature obsened, but this is nut due tu fluorescence. It comes irum sample heating hy the laser. The hut sample acts as a black body radiatm, and the dete
