Pulsed excitation and asynchronous detection in Fourier transform Raman spectroscopy

. ~5~ ELSEVIER

Journal of MOLECULAR STRUCTURE Journal of Molecular Structure 347 (1995) 207-216

Pulsed Excitation and Asynchronous Detection in Fourier Transform Raman Spectroscopy Kelly Asselin and Bruce Chase Corporate Center for Analytical Sciences Dupont Experimental Station Wilmington DE 19880-0328

1. INTRODUCTION We are currently approaching the ten year anniversary of the development of Fourier transform Raman spectroscopy. This decade has seen tremendous advances in the sensitivity, ease of use, and utility of this technique. There have been significant improvements in the interferometers 1, detectors 2 and optical filters3, 4, resulting in dramatically improved instrument performance. The only aspect of the measurement which has not changed significantly since the first spectra were obtained is the excitation source. A Nd/YAG laser operating at 1.064 micrometers has proven to be an excellent choice, which provides reasonable sensitivity, and minimization of fluorescence background in almost all cases, so there has been little need to explore alternate lasers. Now, with the basic instrumentation well optimized, there is the opportunity to explore some of these other options. Alternate lasers may differ in either lasing wavelength or time domain behavior. Various groups have explored alternate wavelength lasers at both shorter and longer wavelengths from the Nd/YAG system. Using diode lasers or Ti/Sapphire lasers and an innovative semiconductor filter, Shulte 5 has obtained FT-Raman spectra of rhodopsin in the 800 NM region, with very good sensitivity and excellent low frequency response. However, the superior performance of CCD based dispersive multichannel detection systems in this optical range will make interferometry the second method of choice. Operation further into the red would at first seem to be counterproductive, since the u4 scattering cross-section will be further decreased, but in cases where fluorescence is still a problem, this approach can be beneficial. Chase and Asselin 6 have demonstrated that operation with a Nd/YAG unit lasing at 1.339 micrometers can effectively eliminate fluorescence which was still present at 1.064 micrometers. The use of this source as an alternate laser will likely be an attractive secondary laser for routine FTRaman instruments. The other possibility for different lasers involves the use of pulsed, Q switched or mode-locked lasers. Initially, the concept of using pulsed sources in interferometry would seem to be counterproductive. The worst type of noise one can have in an interferometric experiment is source fluctuation noise. When this 0022-2860/95/$09.50 © 1995 Elsevier Science B.V. 207-216

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noise source is dominant, the multiplex gain actually drops below unity, producing degraded performance relative to a dispersive system operating in the same region. A laser source flashing on and off would seem to be the ultimate in source fluctuation. There are several approaches which can circumvent this problem. One obvious route would be to employ a step-scan interferometer. This avoids the problem of Fourier frequencies being mixed up with pulsing frequencies. Such an instrument has been discussed by Johnson 7. However, if a rapid scan instrument is to be used, the problem of interference between the two frequencies must be addressed. The first method involves using a mode-locked laser with a repetition rate much faster than the response time of the detector. Under these circumstances, the pulsed signal appears quasi-CW to the detector, since it cannot respond to the rapid fluctuations, and ordinary signal processing of the interferogram can be accomplished. The drawback to this approach is that all potential temporal information has been lost. An alternate approach is to use a pulsed laser, such as a Q-switched Nd/YAG. The pulse rate of such a system is significantly slower than the mode locked laser, with a range of from 10 Hz to 5 KHz. This requires that the pulsing of the laser and the data acquisition be synchronized so that the detector "sees" the sample repetitively at the same time relative to a laser pulse. Such a system has been demonstrated 8 and the results are encouraging, especially in the area of thermal background suppression. The drawback to this approach is the synchronization required between the laser pulsing and the D/A converter. Masutani and co-workers 9 have developed an approach to this problem called asynchronous sampling, which utilizes a boxcar integrator/low pass filter combination to effectively convert a pulsed signal train to the continuous signal which would have been observed in the absence of pulsing. As such, this approach is suitable for any interferometric system where the source (or the sample) is being pulsed (or perturbed) at a characteristic frequency. 2. BACKGROUND The basic premise of asynchronous sampling is that a synchronous signal may be sampled by a boxcar integrator followed by a low pass filter, and if the sampling frequency is at least twice the highest frequency present (Nyquist criteria), the output will be equivalent to the initial synchronous signal. If the sampling frequency is less than twice the highest frequency present in the signal, then there will be aliasing of frequencies. The function of the low pass filter is to remove higher frequencies which arise from the sampling process. For the FTRaman experiment this means that the pulsed laser must be operating at a frequency at least twice the highest audio frequency present in the interferogram. This audio frequency is equal to the two times the highest optical frequency times the mirror velocity. In order to operate using lasers with KHz repetition rates, mirror velocities of less than 0.25 cm/sec are needed. A more complete description of the process of asynchronous sampling is available from the author as supplementary material. Masutani and co-workers 9 have demonstrated the feasibility of asynchronous sampling for both FT-Raman and modulated infrared spectroscopies. Optimizing the instrumentation requires that the behavior of this sampling with respect to several parameters be examined. For example, the pulse rate, sampling width, low pass filter setting, delay between the pulse and the

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sampling, and laser power can all affect the results. In addition, the linearity of signal averaging needs to be established. 3. EXPERIMENTAL The FT-Raman spectra were recorded using a Nicolet 910 spectrometer, fitted with a high purity germanium detector. All spectra were measured at a nominal resolution of 4 cm -1 The laser employed was a Quantronix 416 laser and Qswitching was accomplished using a Model 351 Q switch driver. Power levels could be adjusted by varying the angle of a quartz plate pickoff in the beam. The output of the detector was passed to a Stanford Research Systems SR250 boxcar integrator and then through a Stanford Research Systems SR650 dual filter. This signal was then directed back into the conventional signal processing train of the Nicolet spectrometer. Samples were examined in 5 mm NMR tubes, preselected for low background when excited at 1.064 micrometers. Average power levels were measured using a thermal laser power meter. 4. RESULTS/DISCUSSION The equivalent performance of asynchronous sampling is shown in figure 1. The upper trace is the FT-Raman spectrum of thioctic acid taken with a CW source at 100 milliwatts and 5 minutes measurement time. The lower trace is the same sample but with pulsed excitation and asynchronous sampling. The average power was 100 milliwatts and the laser repetition rate was 3 KHz. There is clearly no serious loss in sensitivity obtained with this mode of excitation. Figures 2 and 3 confirm the observation made by previous workers with respect to pulsed excitation and thermal backgrounds. Figure 2 shows the FTRaman spectrum of MoO3 obtained with CW excitation at 200 milliwatts. The rising background to higher Stokes shift is due to thermal emission from a sample being heated by the laser. The same sample excited with a pulsed laser yields the spectrum shown in figure 3 which shows a significant reduction in detection of these thermal photons. This effect is not universal since it depends on the interaction between heating by the laser pulse and the rate of thermal dissipation in the sample. The degree of thermal background reduction is dependant on the laser pulse rate and the delay before sampling. In order to evaluate the effect of changes in the operating parameters mentioned above, a standard sample of cyclohexane was used. The S/N was calculated for the strong stretching mode at 2920 cm "1, where the noise was obtained by scanning the sample with the laser off. An RMS noise was calculated over a 400 cm-1 band centered at 3000 cm -1. For asynchronous sampling to function there is a lower limit to the pulse rate of the laser. At pulse rates higher than this there should be no effect on the S/N. This assumes that there is no change in photon flux at the sample. However, at constant average power, the photons/pulse will decrease as the repetition rate increases.

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FJgure 4 shows the varJation in S/N with laser pulse rate. There is no detectable loss Jn sensJtJvJty as the pulse rate increases.

Effect of Pulse Rate on the Signal to Noise Ratio

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Figure 4 The width of the sampled pulse from the detector can be varied at the boxcar. Since this detector has a relatively long time constant (10 microseconds) the temporal behavior of the detector signal is dominated by this rather any inherent time response of the Raman scattering. One would expect that varying the width of the sampling would not have any effect on the S/N until the width becomes comparable to the exponential decay of the detector signal. Figure 5 shows this to be correct.

213

Effect of Sampling Width on the Signal to Noise Ratio .000 1 0 4

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In a similar fashion, increasing the delay between the laser pulse and the sampling has no effect until the time delay approaches the time constant of the detector. Then the S/N starts to decrease as shown in figure 6. It is also reassuring that when the delay is adjusted to sample before the laser pulse, there

Effect of Sampling Delay on the Signal to Noise Ratio

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The behavior of the S/N with increasing laser power is shown in figure 7. At powers greater than 350 milliwatts, there is a distinct non-linear response. Since in the pulsed experiment, the detector is forced to deal with peak power, rather than average power, this non-linearity is most likely due to detector (or preamplifier ) saturation, a condition which is not encountered with C W excitation until much higher powers.

Effect of Laser P o w e r on the Signal to Noise Ratio 6000

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P o w e r (milliwatts) Figure 7 Figure 8 shows the effect of signal averaging. The linear response is quite good until 900 scans. Then a roll off in the S/N improvement is seen. This is probably due to a coherent noise source related to the sampling electronics in the boxcar integrator. When these results are combined, an optimum set of operating parameters can be defined. The question then becomes, can we quantitate the difference in S/N for the pulsed experiment versus the CW experiment. Three samples were examined, sulfur, anthracene, and cyclohexane. The S/N was calculated using the strongest band in the spectrum, and determining the noise by taking a scan with the laser off. RMS noise values were from a 400 cm -1 band centered on the frequency of the strongest band. These measurements were repeated four times to provide a standard deviation on the S/N determination. Three different experiments were done. First the measurements were made using a CW laser with conventional detection electronics. Then the boxcar integrator/low pass filter was inserted into the detection signal train. A cw laser was used for illumination and the triggering for the boxcar was supplied from a frequency generator. This case simulates asynchronous detection with a CW source. Finally, the measurements were made using asynchronous detection with a pulsed source. The results are summarized in Table I.

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Effect of Number of Scans on the Signal to Noise Ratio

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Figure 8 Table I Method

Sample

S/N

std deviation

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sulfur sulfur sulfur

2.04xl 04 5.32xl 03 1.84xl 04

4.18xl 02 1.54xl 02 1.8xl 03

CW CW/ext pulse pulsed

anthracene anthracene anthracene

3.47xl 04 1.59xl 04 3.59xl 04

8.47xl 02 5.03xl 02 1.72xl 03

CW CW/ext pulse pulsed

cyclohexan e cyclohexan e cyclohexane

9.08xl 03 2.62xl 03 7.00xl 03

5.90xl 02 4.6xl 01 1.65xl 03

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It is clear for all three samples that the asynchronous detection scheme by itself increases the noise figure. The additional noise most likely arises from timing jitter in the boxcar integrator. When the pulsed source is coupled with the asynchronous detection, almost all of the lost S/N is recovered. This is understandable, when the difference between peak power and average power is considered. For the pulsed experiments, all of the photons are delivered in a burst resulting in much higher instantaneous signal detected. If the source of increased noise in the asynchronous detection method can be identified and reduced, there is the potential for significant improvement in the sensitivity of the measurement. 5. CONCLUSIONS As shown by Masutani and co-workers, the asynchronous detection scheme is well suited for FT-Raman spectroscopy with Q-switched lasers. A thorough study of the effect of operating parameters has shown that time resolved studies can certainly be performed in the 10 nanosecond regime and longer, presuming a detector with a suitably short time constant can be found with the requisite sensitivity. The linearity of signal with laser power is acceptable, but non-linear response tends to be seen at lower average powers than seen with CW excitation. There are still undefined noise sources in the asynchronous detection method which limit signal averaging, and which limit the achievable S/N. Once these sources have been identified and fixed, significant improvements in the sensitivity of the FT-Raman measurement can be expected.

i Chase, B. Appfied Spec. 1994, 48(7), 14A-19A. 2 Cutler, D. J. Spectrochim. Acta 1990, 46A, 131-151. 3 Lewis, E. N.; Kalasinsky, V. F.; Levin, I. W. Applied Spec. 1989, 43, 156-159. 4 Radziszewski, J. G.; Michl, J. Appfied Spec. 1990, 44, 414-418. 5 Schulte, A.; Lenk, T. J.; Hallmark, V. M.; Rabolt, J. F. Appl. Spectrosc. 1991, 45, 325-330. (~ Asselin, K.; Chase, B. Appl. Spectrosc. 1994, 48, 699-701. 7 Jas., G.; Wan, C.; Johnson, C. K. Spectrochim. Acta 1994,(in press). Cutler, D. J.; Petty, C. J. Spectrochim. Acta. 1991, 47A, 1159-1169. Sakamoto, A.; Furukawa, Y.; Tasumi, M.; Masutani, K. Appl. Spectrosc. 1993,

47, 1457-1461.