Anal. Chem. 1902, 54, 634-637
LITERATURE CITED L'vov, E. V. "Atomlc Absorption Spectrochemical Analysls"; Adam Hllger: London, 1970. Torsl, G.; Tessari, G. Anal. Chem. 1973, 45, 1812-1816. Paverl-Fontana. S. L.; Torsi, G.; Tessarl, G. Anal. Chem. 1974, 4 6 , 1032- 1038. Fuller, C. W. Analyst (London) 1974, 99, 739-744. Aggett, J.; Sprott, A. J. Anal. Chim. Acta 1974, 7 2 , 49-58. Campbell, W. C.; Ottaway, J. M. Talanta 1974, 21, 837-844. Tessari, G.; Torsl, G. Anal. Chem. 1975, 47, 842-849. Johnson, D. J.; Sharp, E. L.; West, T. S.;Dagnall, R. M. Anal. Chem. 1975, 4 7 , 1234-1240. Fuller, C. W. Analyst (London) 1975, 100, 229-233. Paverl-Fontana, S.L.; Torsl, G.; Tessarl, 0. Ann. Chlm. (Rome) 1976, 66,691. Sturgeon, R. E.; Chakrabartl, C. L.; Langford, C. H. Anal. Chem. 1978, 48, 1792-1807. L'vov, E. V.; Katskov, D. A.; Krugllkova, L. P.; Polzlk, L. K. Spectrochlm. Aqta, Part 8 1976, 3 1 8 , 49-80. Katskov, D. A. Zh. Prikl. Spectrosk. 1976, 2 6 , 598. Fuller, C. W. Analyst (London) 1978, 101, 798-802. Torsl, G.; Tessari, G. Anal. Chem. 1978, 4 8 , 1318-1324. Van de Broek, W.; de Galan, L. Anal. Chem. 1977, 49, 2176-2186. Tessarl, G.; Torsi, G. Ann. Chlm. (Rome) 1978, 68. 967-989. ZsakB, J. Anal. Chem. 1978, 50, 1105-1107. L'vov, E. V. Spectrochim. Acta, Part 8 1978, 3 3 8 , 153-193. Smets, E. Spectrochlm. Acta, Part 8 1980, 3 5 8 , 33-42. Eklund, R. H.; Holcombe, J. A. Talanta 1979, 2 6 , 1055-1057. Salmon, S. G.; Davis, R. H., Jr.; Holcombe, J. A. Anal. Chem. 1981, 5 3 , 324-330.
(23) (24) (25) (26) (27) (28) (29) (30) (31) (32) (33) (34) (35) (38) (37) (38) (39)
Salmon, S. 0.; Holcombe, J. A. Anal. Chem. 1978, 50, 1714-1716. Salmon, S. 0.; Holcombe, J. A. Anal. Chem. 1979, 5 1 , 648-650. Savitzky, A.; W a y , M. J. E. Anal. Chem. 1964, 3 6 , 1627-1638. Eeaty, M.; Earnett, N.; Grobenski, At. Spectrosc. 1980, 1 , 72. Gaydon, A. G. "Dlssoclation Energies and Spectra of Diatomic Molecules", 3rd ed.; Chapman and Hall: London, 1968. Holcombe, J. A.; Rayson, G. D.; Akerllnd, N., Jr. Spectrochim. Acta, In press. Lussow, R. 0.; Vastoia, F. J.; Walker, P. L., Jr. Carbon 1967, 5.591. Hart, P.; Vastola, F. J.; Walker, P. L., Jr. Carbon 1987, 5 , 363. Vastola, F. J.; Hart, P.; Walker, P. L., Jr. Carbon 1964, 2 , 65. Lalne, N. R.; Vastola, F. J.; Walker, P. L., Jr. J. Phys. Chem. 1983, 6 7 , 2030. Puri. E. R.; Slngh, S.;Mahajan, 0. P. J. Indian Chem. SOC.1980, 3 7 , 171. Cookson, J. T., Jr. I n "Carbon Adsorption Handbook"; Cheremisinoff, P. N., Ellerbusch, F., Eds.; Ann Arbor Sclence: Ann Arbor, MI, 1978; p 249. L'vov, B. V.; Eayunov, P. A.; Ryabchuk, G. N. Spectrochim. Acta, Part 8 1981, 3 6 8 , 397. Regan, J.; Warren, J. Analyst (London) 1976, 101, 220. Fuller, C. W. At. Absorpt. Newsl. 1977, 16, 106. McLaren, J. W.; Wheeler, R. C. Analyst (London) 1977, 102, 542. Regan, J.; Warren, J. At. Absorpt. Newsl. 1978, 17, 89.
RECEIVED for review August 6, 1981. Accepted January 11, 1982. This work was supported in part by National Science Foundation Grant No. CHE78-15438 and the Robert A. Welch Foundation.
Time-Resolved Rejection of Fluorescence from Raman Spectra via High Repetition Rate Gated Photon Counting 1.L. Gustafson" The Standard Oil Company (Ohio), 4440 Warrensviiie Center Road, Warrensviiie Heights, Ohio 4 4 128
F. E. Lytle Department of Chemistty, Purdue U n i v e r s i ~ ,West La fayette, Indiana 47907
High repetition rate gated photon countlng was used In conJunction with a synchronously pumped cavlty dumped dye laser to reject undesirable fluorescence background from Raman spectra. The system performance was evaluated by using benzene doped with rubrene (7,= 16.5 ns). The degree of background rejection was determined by comparing the level of fluorescence In a tlme-resolved experiment to that for contlnuous excitation using the 992-cm-' solvent peak as an Internal standard. The resulting Raman to fluorescence enhancement was 35. The same technlque was used to reject lmpurlty fluorescence from ultravloiet exclted preresonance Raman spectra.
The problem of fluorescence interference in Raman spectrometry has led to many attempts to reduce this residual background (1-8). In general, two experimental approaches have been taken to reduce the background interferences. The first of these involves the use of nonlinear techniques where the Raman signal is carried on a collimated beam. The second approach which has been used to reduce the fluorescence contribution to Raman spectra is based on the temporal difference between scattered and emitted photons. In the former category, such methods as coherent antistokes Raman spectrometry (CARS) (9-13), inverse Raman spectrometry (14-20),and Raman gain spectrometry (21-26) are capable of producing Raman spectra of fluorescing ma0003-2700/82/0354-0634$01.25/0
terials. For the normal Raman spectroscopist, however, these techniques present some formidable obstacles. The signal intensity depends on the electric field strengths of more than one laser. In addition, the CARS signal depends on the square of the normal Raman cross section and on the square of the number of scattering molecules (27). This nonlinearity with respect to concentration makes it difficult to use differencing techniques for studying perturbations due to molecular interactions (28,29). Also, these techniques require a considerable change in the physical layout of a spontaneous Raman experiment. Often the spectroscopist does not have the leisure or the facilities to conveniently switch from a conventional spontaneous Raman experiment to a coherent Raman technique. The technique described here used pulsed excitation combined with synchronous optical detection. The major advantage of this method is that it is relatively transparent to an existing spectrometer. Several authors have reported schemes using this approach. Yaney describes a system based on a 100-ns Q-switched Nd:YAG laser and fast photon counting for the duration of the pulse (2). The system operates at 450 Hz and works best for samples with long fluorescence lifetimes. Van Duyne et al. employ a system which uses a mode locked argon ion laser as the pulsed source and a time-to-amplitude converter with a level discriminator for gated detection of the Raman signal ( 4 ) . Their system is limited primarily to the measurement of Raman spectra of samples with short fluorescence lifetimes due to the fiied high 0 1982 Amerlcan Chemlcal Soclety
ANALYTICAL CHEMISTRY, VOL. 54, NO. 4, APRIL 1982 I-
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The two pulse generators and stepper motor were replaced with an Avtech Model AVM-1-S1 pulse generator which provides continuously variable pulse widths from 200 ps to 5 ns at repetition rates as high as 5 MHz. The pulse generator was triggered by a fast photodiode after the design of Harris et al. (32) in order to minimize jitter in the arrival of the gating pulse. A variable time delay of -30 ns was controlled by a 0 to -15 V signal to the input of the pulse generator. The fast pulse gated the photomultiplier signal with either a Watkins Johnson M1D or M2EC double-balanced mixer. The resultant signal was processed with a Princeton Applied Research Model 1121 ampWier/discriminator and Model 1109 photon counter. A Houston Instruments Omniscribe recorder provided a permanent record of the Raman spectra or the time profile of the signal.
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repetition rate of the source. Nonlinearities in the detection electronics introduce problems which have been discussed previously (30, 31). Finally, Harris et al. demonstrate the utility of sampled photon counting for rejecting fluorescence background from Raman spectra (1). Their instrument uses a mode locked cavity dumped argon ion laser as the excitation source and a sampling oscillocicope as the gated photon counter. Their instrument achieves a Raman to fluorescence enhancement of 34 and 13.5 for fluorophors with 71 = 4.5 ns and rf = 16.5 ns, respectively. Although the system provides good enhancements, the repetition rate of the source must be kept low in order to match the duty cycle of the detection electronica. In this case the sampling oscilloscop~memory hold-off reduces the ciource repetition rate to -40 kHz. This repetition rate limit reduces the average laser power substantially. This precludes obtaining Raman spectra for low cross section scatterers in reasonable periods of time. Recently, high repetition rate gated photon counting has been used for determining fluorescence lifetimes (31). This method can operate at the high repetition rates of mode locked cavity dumped lacier sources. Such sources are becoiming increasingly accessible to the Raman spectroscopist because these laser systems provide a convenient means of generating ultraviolet excitation (29). In this report, high repetition rate gated photon counting is demonstrated for the rejection of fluorescence from visible and ultraviolet excited Rarman spectra. EXPERIMENTAL SECTION Chemicals. The’benzene used was J. T. Baker Spectrophotometric Grade. The rubrene was purchased from Aldrich Chemical. The dl-tryptophan was obtained from Nutritional Biochemical and the disodium salt of adenosinemonophosphoric acid from Sigma Chemical. All solutions were passed through ultrafilters to remove particulates and then bubbled with argon for 10 min prior to use. Source. A synchronously pumped cavity dumped dye laser consisting of Coherent Radiation Model CR-10 argon ion laser, Model 467-SE modelocker, Model 599-04 dye laser, and Model 7200 cavity dumper ]providedvariable repetition rate, pulsed laser excitation. Tunable ultraviolet excitation was generated with an Interactive RvadiationModel 5-1 second harmonic generator as described previously (29). The visible excitation frequency of 17 500 cm-’ was choiaen to accommodate the fluorescence model system and the photomultiplier gain response. Ultraviolet excitation was 33 300 cm-*. The pulse widths in both cases were -10 ps. Detection. A block diagram of the experimental configwation configuration is shown in Figure 1. A conventional Rarman spectrometer (Instruments, SA Model HG 2000) was modified to accommodate the high repetition gated photon counting. An RCA 8850 photomultiplier was used because of the high gain requirement. The previously described high repetition rate gated photon counting method (31)was changed in thie following fashion.
RESULTS AND DISCUSSION Power Reduction. A detailed analysis of the signal-tonoise considerations in time-resolved Raman spectrometry has been presented previously (I, 4). The result obtained for the theoretical ratio of the time-resolved signal-to-noise, Sm, to that for continuous excitation, Sew, is given by
where a is the reduction in the average laser power in going from a continuous to time-resolved experiment, E is the enhancement in the ratio of Raman to fluorescence intensity between time-resolved and CW excitation, and FR is the fraction of the total Raman signal collected in the time-resolved experiment. In previous work ( I ) the power reduction factor, a,was the limiting factor in the signal-to-noise considerations. The sampling oscilloscope detection circuitry limited the cavity dumped mode locked argon ion laser source to a 4O-kHz repetition rate, providing a power reduction factor of 2 X lo4 (I). For the same laser system in conjunction with the high repetition rate gating richeme a would equal 0.25 a t 5 MHz. The calculation of the power reduction factor for the synchronously pumped cavity dumped (SPCD) dye laser is not quite as straight forward. When operating near the peak in the gain of rhodamine 6G (- 16700 cm-l), single line excitation from an argon laser at 5145 %, will produce -900 mW of continuous power. The SPCD dye laser will produce -200 mW at 5 MHz a t the riame frequency. Therefore, a equals 0.22. However, in the wings of the gain profile synchronous excitation is considerably more efficient than is continuous excitation. For example, a t 17500 cm-l the CW dye laser produces -20 mW whLereas the SPCD dye laser is capable of generating -80 mW at 5 MHz. In this case a is 4. In the extreme edges of the gain profile a actually becomes infinite due to the extended tuning range available when using synchronous excitation. In a similar fashion, the power reduction factor increases for the time-resolved experiment when frequency doubling the dye laser. Frequency doubling efficiency is proportional to the peak power-average power product. For the CW dye laser this is simply the average power squared. In the case of the frequency doubled SPCD dye laser the high peak power (1-10 kW) of the picosecond pulses provides improved doubling efficiency by a factor of lo3to lo4. Therefore a increases to lo3to lo4when using ultraviolet excitation. This analysis may be somewhat unrealistic because the optimum configuration for obtaining ultraviolet excited Raman spectra from a “continuous” laser source involves the use of the frequency doubled SPCD dye laser operating as a quasi-continuous source (29). For this reason a will be taken as equal to one for the examples involving ultraviolet excitation in the remainder of the paper. In the following SIN calculations three values will be given which correspond to LY values of 0.25,4, and 1. These represent typical values obtained for the SPCD dye near maximum gain,
636
ANALYTICAL CHEMISTRY, VOL. 54, NO. 4, APRIL 1982
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WAVENUMBER SHIFT Flgure 2. Time-resolved rejection of fluorescence from Raman spectra: M rubrene in benzene. Top curve represents continuous detection; bottom curve taken with high repetition rate gated photon counting scheme. Both spectra obtained with 100 mW average power and 1 s Integration time (band-pass = 2 cm-I).
near the wings of the gain profile, and when frequency doubled, respectively. Enhancement. The enhancement in the ratio of Raman to fluorescence signal between time-resolved and continuous excitation is dependent upon the timing capabilities of the detection electronics (1). Harris et al. (1) obtained subnanosecond time resolution (700 ps) using an Amperex XP 2020 photomultiplier sampled with a 1ns window from the sampling oscilloscope. This time resolution resulted in an enhancement of Raman to fluorescence of 115 ( f 6 ) for a solution of rubrene ( ~ =f 16.5 ns) in benzene. The time jitter was attributed to the transit time spread limit of the photomultiplier. In this study an RCA 8850 photomultiplier, with a transit time spread of -400 ps, provided an enhancement of 35 (f5) for the model system, M rubrene in benzene (Figure 2). This corresponds to a time resolution of -2.1 ns, which is considerably greater than what would be anticipated on the basis of transit time spread alone. The observed enhancement was also independent of the width of the gating pulse in the region from 200 ps to 5 ns. These results indicate that the primary source of time uncertainty is the walk effect associated with leading edge discrimination (33). When a single level discriminator is used, large amplitude pulses from the photomultiplier will cross the threshold earlier in time than will small amplitude pulses. This is shown in Figure 3. A photon occurring at tohas an amplitude probability distribution bounded by the maximum and minimum amplitudes represented by curves (a) and (b), respectively. With the discriminator at v d , an observation window centered at towould represent both signals as photons. Now consider a window centered at tp Large amplitude pulses which occur later in time, at to,will appear as events within the time window centered at tl,thus increasing the effective time jitter. The extent of this effect is dictated by the analogue rise time and the distribution of pulse heights for a given photomultiplier. A partial compensation of this effect can be accomplished by increasing v d , but this is done only with a concomitant decrease in the number of “true” events which are observed (i.e., the fraction of Raman collected). In the previous work by Harris et al. (1) the analogue rise time (10-90%) of the X P 2020 photomultiplier was 1.7 ns. With the discriminator level adjusted to the midpoint of the leading edge of the signal, the timing jitter was reduced to a value comparable to the transit time spread of the photomultiplier. The relatively slow analogue rise time of the RCA 8850 (-2.7
to
TIME (nsec) Flgure 3. The walk effect. Curves (a)and (b) represent the maximum and the minimum In the pulse height distribution, respectively. With the discrlminator at V,, the full distribution will be accepted as photon events when sampled at t o . Later large amplitude events will affect the timing jitter when sampled at t i . (See text.)
ns) exhibits a greater time jitter due to the walk effect. The discriminator level is adjusted to approximtely one-third the leading edge of the photomultiplier response. The resultant observed time jitter is comparable to what is anticipated strictly from rise time considerations. Fraction of Raman. The optimum Raman collection efficiency is a function of the pulse width used to excite the sample. Previous work with a 1-ns pulse limited the fraction of Raman to -10% for maximum signal to noise ( I ) . For the best rejection ratio, the time window will always be biased toward early times resulting in an optimum operating point corresponding to a collection efficiency of -67% (1). Whenever the pulse width of the excitation is much less than the timing capabilities of the detection, such an operating point is observed. In this work the 10-ps pulses from the SPCD dye laser were considerably shorter than the timing uncertainty of the detection electronics. The fact that the optimum operating position occurs when -67% of the Raman signal is collected facilitates positioning the time window. A strong scatterer, such as benzene, is placed in the sample compartment and the intensity of a vibrational band or the Rayleigh line is monitored. The window is adjusted to find the peak of the scatter signal in time, and then adjusted early in time to provide -67% the number of counts observed at the maximum. Once this position is determined, that portion of the experiment is optimized and should require no further modification. The improvement in the theoretical signal-to-noise ratio obtained for the model system of rubrene in benzene can now be calculated from eq 1. Near the peak of the gain in the SPCD dye laser (a = 0.25) Sm/Scw = 2.4 (f0.3) and near the edge of the gain (a = 4) STR/Scw = 9.7 (f0.5). As is demonstrated in Figure 2, using the same laser power and the same counting time, the actual improvement in the signal-to-noise ratio is 5.0 (*0.7) which agrees favorably with the theoretical value for a = 1. The demonstration of this technique using ultraviolet excitation is presented in Figure 4 for a solution of AMP excited at 300 nm in the presence of a background interference due to tryptophan fluorescence (q= 4.5 ns). In this example the spectra have been normalized and plotted on the same scale to demonstrate the enhancement in the Raman signal above the fluorescence (10 f 2). A substantial background still remains in the time-resolved spectrum but the enhancement is sufficient to allow computer-assisted background subtraction. In the presence of overwhelming fluorescence such subtraction procedures are not capable of resolving the signal from the background. Assuming a = 1, the signal-to-noise
ANALYTICAL CHEMISTRY, VOL. 54, NO. 4, APRIL 1982
LITERATURE CITED
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Flgure 4. 'rime-resolved relectlon of fluorescence from Raman spectra: 10' M AMP in the presence of IO-' M tryptophan. Solld curve represents time-resolved detection; dashed curve corresiponds to continuous detectlon, Both spectra were obtained wlth 2 mW average power and 1 s Integration time. Counts have been nornnalized to emphasize the enchancement in the Raman signal above the background (band-pass = 10 cm-').
ratio is improved by a factor of 2.6.
CONCLUSIONS Time-resolved rlejection of fluorescence from Raman spectra has been demonstrated using high repetition rate gated photon counting. !several improvements in the experiment can be made to improve ,the enhancement in the Raman to fluorescence ratio. The most significant of these is the use of a fast rise time photomultiplier to reduce the walk effect. hlicrochannel plate mulltiplier tubes have very fast rise times and sufficient gain to be useful for this application. Enhanceiments of 100 should be readily accomplished with this moclification. Enhanceinents from 10 to 100 are quite reasonable for many troublesome samples. In most cases such a reduction in the fluorescence background provides a spectrum that is readily handled by conventional background subtraction techniques. This improvement is accomplished by a method quite transparent to the existing spectrometer. This is especially true for those who anticipate using a frequency doubled SPCD dye laser asi a source for ultraviolet excited Raman spectra.
-
ACKNOWLEDGMENT T. H. Bmhaw is acknowledged for many helpful discussions and for help in obtaining the ultraviolet spectra. R. L. Swofford is also acknowledged for several useful discussions.
(1) Harris, J. M.; Chrlsmen, R. W.; Lytle, F. E.; Tobias, R. S. Anal. Chem. 1976, 48, 1937. (2) Yaney, P. P. J . Opt. SOC.Am. 1972, 62, 1297. (3) Arguello, C. A.; Mendes, G. F.; Leite, R. C. C. Appl. Opt. 1974, 73, 1731. (4) Van Duyne, R. P.; Jeanmarle, D. L.; Shriver, D. F. Anal. Chem. 1974, 46, 213. (5) Funfschilling, J ; Wllllums, D. F. Appl. Spectrosc. 1976, 30, 443. (6) Nemanich, R. J.; Solin, S. A.; Doehler, J. Rev. Sci. Instrum. 1976, 47, 741. (7) Gaieener, F. L. Chem. Phys. Lett. 1977, 48, 7. (6) Burgess, S.; Shepardl, I. W. J . Phys. €1977, 70, 617. (9) Maker, P. D.; Terhune, R. W. Phys. Rev. [Sect.] A 1965, 737, A801. (IO) Begley, R. F.; Harvey, A. B.; Byer, R. L.; Hudson, 9. S. J . Chem. Phys. 1974, 67,2468. (11) Toiles, W. M.; Nibler, J. W.; MacDonald, J. R.; Harvey, A. B. Appl. Spectrosc. 1977, 3 7 , 253. (12) Carrerla, L. A.; Goss, L. P.; Malloy, T. G. J . Chem. Phys. 1978, 6 8 , 280. (13) Chabay, I.; Klaumlnzer, G. K.; Hudson, B. S . Appl. Phys. Lett. 1976, 28, 27. (14) Lau, A.; Werncke, W.; Pfelffer, M.; Lenz, K.; Weigmann, H.J. Sow. J . Quantum Electron (8?gl. Trans/.)1976, 6 , 402. (15) Werncke, W.; Lau, A.; Pfelffer, M.; Welgmann, H. J.; Hunsaly, G.; Lenz, K. Opt. Commun. 1976, 16, 128. (16) Yeung, E. S . J . Mol. Spectrosc. 1974, 53, 379. (17) Lin, S. H.; Reid, E. S.;Tredwell, C. J. Chem. Phys. Lett. 1974, 29, 389. (18) Morris, M. D.; Wallan, D. J.; Ritz, G. P.; Haushalter, J. P. Anal. Chem. 1978, 50, 1796. (19) Morris, M. D.; Wallan, D. J. Anal. Chem. 1979, 57, 182 A. (20) Haushalter, J. P.; Ritz, G. P.; Wallan, D. J.; Dien, K.; Morris, M. D. Appl. Spectrosc. 1980, 3 4 , 144. (21) Owyoung, A. Opt. Commun. 1977, 22, 323. (22) Owyoung, A.; Jones, E. D. Opt. Lett. 1977, 7, 152. (23) Owyoung, A. I E E J . Quantum Electron. 1978 QE- 74, 192. (24) Heritage, J. P.; Bergman, J. G.; Pinczuk, A,; Worlock, J. M. Chem. Phys. Lett. 1979, 67, 229. (25) Bergman, J. G.; Herltage, J. P.; Plnczuk, A,; Worlock, J. M.; McFee, J. H. Chem. Phys. Lett. 1979, 68, 412. (26) Levine, B. F.; Rethea, C. G. Appl. Phys. Lett. 1960, 3 6 , 245. (27) Long, D. A. "Raman SDectroscopy"; . . McGraw-Hill: New York. 1977; pp 340-242. (28) Tobias, R. S.; Bushaw, T. H.; English, J. C. Indlan J . Pure Appl. Phys. 1978, 18, 401. (29) Bushaw, T. H.; Lytle, F. E.; Tobias, R. S . Appl. Spectrosc. 1978, 32, 585. (30) Haugen, G. R.; Wallen, R. W.; Lytle, F. E. Rev. Sci. Instrum. 1979, 50, 64. (31) Gustafson, T. L.; Lytle, F. E. Appl. Spectrosc. 1960, 3 4 , 185. (32) Harris, J. M.; Barnes, W. T.; Gustafson, T. L.; Bushaw, T. H.; Lytle. F. E. Rev. Scl. Instrum. 1980, 51, 988. (33) Poultney, S. K. "Advtrnces in Electronics and Electron Physics"; Martin, L., Ed.; Academic Press: New York, 1972; Vol. 31, pp 79-81.
RECEIVED for review August 28, 1981. Accepted January 4, 1982. Portions of this research were supported through funds provided by the National Science Foundation under Grants MPS 75-05907 and the National Institutes of Health.
