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Anal. Chem. 1984, 56,3000-3001
Computer-Controlled Instrument for the Recovery of a Resonance Raman Spectrum in the Presence of Strong Luminescence S. M. Angel, M. K. DeArmond,* K. W. Hanck, a n d D. W. Wertz Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695-8204
A problem frequently encountered in Raman spectroscopy is a large background due to sample luminescence which can be orders of magnitude greater than the Raman intensity. Selection of a laser line such that the Raman spectrum of interest does not overlap with the sample luminescence is occasionally possible; however, the appropriate laser line is not always available. One instance in which the choice of laser lines is limited is resonance Raman spectroscopy, which requires excitation into the absorption band. In many such cases, the Raman spectrum must be recovered from the luminescence. The methods suggested to solve this problem (1-6)can be grouped into two categories: pulsed techniques and differential techniques. The pulsed techniques use to advantage the difference between Raman lifetimes (vibrational), which s and luminescence are on the order of to lifetimes, which are usually on the order of to lo4 s. A pulsed laser system is used along with ultrafast gating electronics so that the signal from the photomultiplier tube is only measured during the laser pulse. Since pulsed laser systems (source and detection) are costly and complex (8),and the technique depends on a relatively long-lived emitting state, such a technique is not generally useful. A number of differential systems based on frequency modulation (3),wavelength modulation (41,and polarization modulation (5)have been described. All of these techniques depend on obtaining two spectra-one of which is the pure luminescence spectrum and the other the combined luminescence and Raman spectrum. The Raman spectrum is obtained by subtracting the former from the latter; however, the Raman signal must be above the background noise level. Of the differential techniques, the frequency modulation method is similar to the pulsed methods in that it uses the disparity between the luminescence and the Raman lifetimes to obtain a difference spectrum. Again, the complexity and cost of the apparatus limit implementation by most spectroscopists. In the wavelength modulation technique the differential spectrum is obtained by alternating between two laser lines of different wavelength. These are chosen so that the pure luminescence spectrum and the combined luminescence plus Raman spectrum are alternately measured and the signals sent to separate inputs of a differential recorder. As discussed earlier, this technique is not very useful if the choice of laser lines is limited, as in resonance Raman spectroscopy. A differential technique based on polarization modulation depends on the Raman lines being highly polarized and the luminescence being totally depolarized. This will be true for liquid samples where the Raman vibrations are all totally symmetric. In many instances resonance Raman spectra are comprised largely of highly polarized lines arising from totally symmetric vibrations. We have been attempting to obtain the resonance Raman spectra of some da metal-tris(diimine) complexes which often luminesce (9). Since the majority of the Raman peaks for these complexes are highly polarized we have constructed a computer-controlled instrument which uses polarization modulation to recover the Raman signal from the luminescence. A system has been described in the literature which uses a rotating disk polarizer, a polarized reference source, and a lock-in amplifier (5). This system is not, however, conveniently adapted for computerized data acquisition.
(a,
Our computer-controlled system simplifies the instrument and makes it more versatile, allowing a variety of measurements to be made. The purpose of this paper is to report the results we have obtained with our computer-controlled luminescence rejection instrument. EXPERIMENTAL SECTION A block diagram of the instrument is shown in Figure 1. Minor modifications were made to a Jarrell-Ash 25-300 Raman spectrometer to allow for computer control and data acquisition. These modifications consisted of connecting an Airpax Model K82924-M3 stepper motor with a Model K33504 driver board to the monochromator and interfacing the appropriate counting circuitry to a Micro Technologies Unlimited Model MTU-130 microcomputer. The computer was used for control of the instrument, data collection, and display. The analyzing polarizer (P) consists of a chopper mounted on a stepper motor (same model as above) which is controlled by the computer. The chopper is designed to pass light through either of two ports which are covered with plastic polarizers. One of the polarizers is oriented to pass light polarized parallel to the The other is oriented electric vector of the incident laser beam (Ill). to pass light polarized perpendicular to that of the incident laser beam ( I I ) . Three intensity measurements are made with each complete rotation of the polarizing chopper: I,,,I,, and Idark.The dark count is measured by rotating the chopper so that it blocks the light scattered from the sample (S). Usually, all three intensity measurements are made for each wavenumber setting and then the monochromator is stepped. The measurements are averaged for several cycles of the chopper so as to improve the signal to noise ratio. After dark count subtraction, the I,,and I , spectra are used to generate the luminescence rejected spectrum. In order to compensate for polarization effects introduced by the monochromator optics, a depolarization scrambler (D)was placed between the polarization chopper and the monochromator slit (E). This was necessary because the monochromator. optics pass light differently depending on its polarization. RESULTS AND DISCUSSION In the luminescence rejection experiment two spectra are and lI I,) using a polarizing analyzer and a pomeasured (I larized laser excitation source. Since luminescence from a liquid sample is completely depolarized, the luminescence intensity will be the same for both spectra. However, for Raman bands that are highly polarized, the Raman intensity spectrum. Therefore, it is only will be greater for the Ill necessary to subtract the two spectra (I,,- IL) to recover the Raman spectrum and reject the luminescence. It should be noted that for samples which do not luminesce, 1,/11, could be plotted to get the Raman depolarization ratios. In order to measure how well the Raman spectrum is recovered, the “degree of recovery”, R , is defined as
R.:- R2/L2
R1/L1
(1)
In this equation, R1/L1is the ratio of the Raman signal to luminescence signal before rejection and R 2 / L 2is the ratio after rejection. Figure 2a shows the normal Raman spectrum for a ;;.lo-’ Molar solution of Rhodamine 590 laser dye in ethanol. There is some evidence of a peak around 880 cm-l, but the spectrum is of little use as shown. The luminescence rejected spectrum for the same region is shown in Figure 2b. This spectrum was obtained with seven cycles of the polarizing chopper. Com-
0003-2700/84/0356-3000$01.50/0 0 1984 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 56, NO. 14, DECEMBER 1984
3001
P
MTU
Optical and electronic design: A, laser; L, lens: S, sample: P, polarizing chopper; D, polarization scrambler; E, slit: MC, monochromator; MS, monochromator stepping motor; PMT, photomultiplier; PC, photon counter; C, counter electronics. Data and control llnes are Figure 1.
I
n
C
I\ A
shown as arrows.
Wavenumber (cm-11
Resonance Raman spectra of Ru(bpy),+ with a small amount of Ru(bpy),'+: (a) normal resonance Raman spectrum taken with 514-nm excitation; (b) luminescence rejected resonance Raman spectrum taken with seven cycles of the polarizing chopper and 514-nm excitation; (c) normal resonance Raman spectrum taken with 488-nm excitation. Figure 3.
Wavtnumbcr Icm-l) Figure 2. Raman spectra of ethanol spiked with Rhodamine 590 laser dye: (a)normal Raman spectrum; (b) lumlnescence rejected spectrum taken with seven cycles of the polarlzlng chopper: (c) normal Raman spectrum of ethanol. The depolarization ratios are shown beside each Raman band.
spectrum obtained with the same exciting line is shown in Figure 3b. Several Raman bands are seen in this spectrum and the luminescence background has been essentially removed. The degree of recovery for thie spectrum can be determined by comparison with the spectrum in Figure 3c. The resonance Raman spectrum obtained for the same sample, but with 488 nm excitation, is shown in this figure. At this wavelength, luminescence is not a problem and a "pure" resonance Raman spectrum is obtained. Comparing the two spectra shows that complete recovery has been obtained in the luminescence rejected spectrum for all peaks. The broadening of the 1490-cm-l peak is due to resonance enhancement of a side band that is not enhanced with 488-nm excitation. The degree of recovery for this spectrum is about 300.
ACKNOWLEDGMENT The authors are grateful to Charles B. Boss and Robert J.
Donohoe for many helpful discussions. parison with the pure ethanol spectrum (Figure 2c) shows complete recovery of the peaks at 880 cm-l and 1100 cm-l. The LITERATURE CITED peak at 1050 cm-' is not well recovered, since this peak is (1) Van Duyne, R. P.; Jeanmaire, D. L.; Shrlver, D. F. Anal. Chem. 1974, almost totally depolarized. The two highly polarized peaks 46, 213-222. (2) Loewenschuss, A.; Moss, A. Appl. Spectrosc. 1982, 36, 183-184. show excellent recovery with a degree of recovery of about 300. (3) Galeener, F. L. Chem. Phys. Left. 1977, 4 8 , 7-11. Following the success of this technique with the standard (4) Morhange, J. F.; Hlrllrnann, C. Appl. Opt. 1978, 75, 2969-2970. samples we next turned our attention toward obtaining res(5) Argusllo, A.; Mendes, G. F.; Leite, R. C. C. Appl. Opt. 1974, 73, 1731-1732. onance Raman spectra of luminescent transition-metal com(6) Yaney, P. P. J . Opt. SOC.Am. 1972, 62, 1297-1303. plexes. Of particular interest was singly reduced tris(bi(7) Laubereau, A.; von der Llnde, D.; Kalser, W. Phys. Rev. Left. 1972, 28, 1162. pyridine)ruthenium(II) ( [ R ~ ( b p y ) ~ ](91, + ) since we had been (8) Lytl'e, F. E. Anal. Chem. 1974, 4 6 , 545A, 817A. investigating the series [Ru(bpy)Jn, where n = 2+, 1+,0, or (9) Angel, S. M.; DeArmond, M. K.; Donohoe, R. J.; Hanck, K. W.; Wertz, 1-. The singly reduced product is usually not obtained without D. W. J . Am. Chem. SOC. 1984, 706, 3688-3669. some small amount of the unreduced [ R ~ ( b p y ) ~ ] ~ While + . the small amount of unreduced "impurity" does not appear in the RECEIVED for review July 9,1984. Accepted August 27,1984. resonance Raman spectrum, it can cause problems with luAcknowledgment is made to the dohors of the Petroleum minescence. Research Fund, administered by the American Chemical Figure 3a is the resonance Raman spectrum obtained with Society, and to the National Science Foundation, Grants 514-nm excitation for [ R ~ ( b p y ) ~with ] + some [ R u ( b p ~ ) ~ ] ~ +CHE-80-14193 and CHE-81-19702, for support of this reimpurity. The luminescence rejected resonance Raman search.
