Laser-Induced Fluorescence Detection of NH3 in Flames with the Use of Two-Photon Excitation
U L F W E S T B L O M a n d M A R C U S ALDI~N* Combustion Center, Lund Institute of Technology, P.O. Box 118, S-221 O0 Lund, Sweden
Laser-Induced Fluorescence (LIF) has been used for the detection of ammonia molecules, NH3, in flames at atmospheric pressure. Excitation was accomplished by a two-photon step from the ground state, X, to the C' state, with the detection of the fluorescence radiation from the C' state to the .4 state. This excitation scheme was also utilized for the simultaneous flame detection of O H and NH3. NO could also be si- multaneously detected with the laser beam resulting from frequency mixing of the IR beam and the doubled dye laser beam from a Nd:YAG- based laser system. Index Headings: Analysis for combustion processes; Flame spectroscopy; Fluorescence.
INTRODUCTION
Several different laser-based spectroscopical tech- niques have, over the last decade, been developed into important tools for studying combustion processes. These techniques are characterized by having features such as nonintrusiveness and high temporal and spatial resolu- tion and by being species selective (see Ref. 1 and ref- erences therein). One of the most used techniques for combustion diagnostics is Laser-Induced Fluorescence (LIF), in which a species is excited from its initial ground state to an excited state. By analysis of the fluorescence emitted during the subsequent deexcitation, information about the species in terms of number density, gas tem- perature, and flow velocity can be acquired. The LIF technique has been used for the detection of a number of flame species with the use of both one- and multi- photon excitation schemes (see, for example, Refs. 2 and 3) and it has been successfully applied to multiple-point visualization 4 and multiple-species detection) ,6
One molecule of interest in combustion that has so far not been detected in a flame environment with the use of laser-induced fluorescence is ammonia, NH3. LIF de- tection from the first excited state of this species has not been possible since it suffers from strong pre-dissocia- tion. However, two-photon excitation to a newly iden- tified system has been used for LIF detection in cell experiments. 7 Also two-photon-induced stimulated emis- sion (SE), recently demonstrated as a tool in combustion diagnostics, s was reported for ammonia in Ref. 7.
The ammonia molecule has been the subject of an increased interest in combustion situations, due to its impact on NO formation from fuel-bound nitrogen, and this interest has resulted in several theoretical and ex- perimental investigations of ammonia flames. 9,m Another reason for the interest in ammonia in combustion situ- ations is its presence as a pollutant from industrial pro-
Received 7 December 1989. * Author to whom correspondence should be sent.
cesses as well as the injection species for NO suppression in exhaust gases. 11 Techniques for measuring NH3 in combustion and high-temperature systems have mostly been based on using IR or UV absorption, ~2,~3 but also Raman scattering 14 and photo-acoustic detection 15 have been reported.
In the present paper we present (the first, to our knowl- edge) LIF detection of ammonia molecules in a flame. The detection was accomplished by a two-photon exci- tation to the C' state. The stimulated emission from this transition was also detected, and its potential for diag- nostic utilization was investigated together with some comparison with the properties of LIF. Experiments were also performed in order to examine to what degree spec- tral coincidences of the frequency-doubled and frequen- cy-mixed laser beams from a Nd:YAG based-laser system would make it possible to detect some of the species NO, OH, O, and NH3 simultaneously.
In the next section of this paper some spectroscopic features of ammonia are described, and the experimental arrangement is then discussed. The NH3 flame mea- surements, together with the simultaneous detection scheme, are then described, followed by a summary and conclusion.
NH 3 SPECTROSCOPY
The spectroscopy of ammonia has for a long time been the subject of numerous investigations (see, for example, Refs. 16 and 17). The molecule has a pyramidal ground state X, whereas the excited states are all planar. 18 Duncan 19 classified the excited states of ammonia, rang- ing from 220 to 115 nm, into four different progressions. Most bands show long progressions due to the symmetric out-of-plane bending vibration, ~2- The first excited state, the A state, has absorption bands between 215 and 170 nm but suffers from strong predissociation, preventing detection of NH3 by conventional LIF. However, laser- induced fluorescence detection after an X - A excitation of ND32° has been reported. The second series between 167.5 and 140 nm has been the subject of the present inves- tigation. In this spectral region, the B - X and the C - X bands are the most pronounced in conventional absorp- tion, with the former being the stronger. 21
In the late seventies, a new electronic state was ob- served by Nieman and Colson 22,23 in this spectral region by three-photon excitation followed by ionization detec- tion. This state, the C' state, is nearly isoenergetic with the C state and escaped identification in conventional VUV absorption due to the transition's being one-photon forbidden. 24
Together with the B state, the C' state has been excited in a two-photon process, and the fluorescence originating
0i × FIG. 1. Energy-level diagram illustrating the relevant two-photon ex- citation schemes for the laser-induced fluorescence/stimulated emis- sion detection of ammonia. (The excitation is illustrated by excitations to v~ = 2 and v~ = 7 in the C' and B states, respectively.)
in the B-A and the C'-A transitions has been detected. The latter transitions, named the "Schusters bands," were first seen more than a century ago, 25 and have been attributed to transitions between excited states of the ammonium radical, NH4. However, recently these bands were proposed instead to originate from the C'-A tran- sition of ammonia. 2s This suggestion was also supported by Ashford et al., 24 who made detailed spectroscopic as- signments.
An energy-level diagram showing the pertinent energy levels and some excitation schemes is illustrated in Fig. 1. The B and C' state absorb in the same spectral interval, but since both of these states show a strong Av = 0 propensity rule, the corresponding fluorescence radiation occurs at widely different wavelengths, 720 and 565 nm, respectively, as is also illustrated in Fig. 1. This means that individual excitation scans can easily be recorded without spectral interference. In Fig. 1 the excitation is exemplified by transitions from the ground state to v2 = 7 and ~2 = 2 in the B and C' states, respectively.
EXPERIMENTAL
The experimental setup included a Quantel Data- chrome Nd:YAG laser, YG, 581-10, operating with a mix- ture of Rhodamine 610/640 dyes in the wavelength region 600 to 620 nm and with Rhodamine 590 in the wavelength region 570 to 580 nm, producing pulse energies of around 80 mJ in the dye laser output. Subsequent doubling in K D * P crystals to 305 nm and to 287 nm yielded pulse energies of around 10 mJ, while doubling followed by mixing with the residual 1.06-~m radiation from the YAG laser, also in a K D * P crystal, yielded around 3 mJ/pulse at 226 nm, the wavelength used for the detection of NO and O. We performed both the doubling and the mixing processes in a Quantel WEX system, making well-con- trolled wavelength scans possible while retaining 80% of the maximum available energy. The total UV power
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FIG. 2. (a) Laser-excitation spectrum of the C' state of ammonia in a cell at room temperature with detection at 565 nm. (b) Laser-exci- tation spectrum of the B state of ammonia in a cell at room temperature with detection at 720 nm.
changed typically by a factor of two during a 20-nm scan. In all work presented here, the dye laser was operated with an intra-cavity prismatic beam expander specified to give the dye laser a linewidth of 0.08 cm -1, while the YAG laser has a specified linewidth of 1 cm -1. After spectral isolation, the UV laser radiation was focused (f = 200 mm) into either a stainless-steel cell connected to a vacuum system or into an NHJO2 flame at atmospheric pressure, stabilized on a water-cooled, porous-plug burn- er (diameter = 40 mm). A UV Nikon f = 105 mm, [/4.5 quartz lens, positioned perpendicular to the laser beam, collected part of the fluorescence and imaged it onto the entrance slit of a Monospec 27 spectrograph equipped with three interchangeable gratings, yielding dispersions of 1.5, 6.0, and 24.0 nm/mm. For the detection, a PARC OMA III intensified and gateable diode-array system, with the detector positioned at the exit of the spectro- graph, was used throughout the measurements. The OMA was operated either in the normal data acquisition mode, as when emission spectra were recorded, or--as for the height and excitation scans--in the Y:T mode, which permits changes in a segment of a spectrum to be mon- itored as a function of time. For the power-dependence measurements, we positioned a Newport Corporation Model 935-10 variable attenuator in the beam, making possible continuous changes in the laser power (mea- sured with an Ophir 10A-P power meter) without af- fecting the beam shape or its direction. In the experi- ments on stimulated emission, the laser beam was recollimated after the cell by a second lens and then directed onto the entrance slit of the spectrograph through suitably chosen filters absorbing the UV laser beam.
MEASUREMENTS
Results for Ammonia. Two different wavelength re- gions were investigated by recording of excitation scans. First the region between 300 and 310 nm, covering the C' (v2 = 1), C'(2), and C'(3) bands emitting at 565 nm and the B(6) and B(7) bands emitting at 720 nm, was examined, and secondly the region between 280 and 290
LIF and SE emission spectra from the C' state with excitation at (a) 305.10 nm, (b) 305.12, and (c) 305.22 nm.
nm, covering the C'(6) band, also emitting around 565 nm, was investigated.
Figure 2 shows excitation spectra for the 300-310 nm region recorded with the use of a cell filled with NH3 at atmospheric pressure and at room temperature. The flu- orescence was detected at 565 nm for the C' bands, il- lustrated in Fig. 2a, and at 720 nm for the bands origi- nating in the B state, seen in Fig. 2b. In the cell experiments it was found that the LIF signal intensity from the C' state, despite being induced by a two-photon process, was extremely strong. No detailed investigations of detection limits were performed, but fluorescence from 1 Torr of NH3 in the cell could easily be detected with the diode-array detector using a single laser pulse. We estimated the fluorescence intensity from the B state to be weaker by a factor of 100, also taking differences in the detector quantum efficiency into account. Stimulated emission (SE), induced at 305 nm and occurring in cold NH 3 at pressures above 200 Torr and above atmospheric pressures, was also detected at 565 nm. The emission spectra for both SE and LIF were studied for several excitation wavelengths. In Fig. 3 three different LIF and SE emission spectra with only slightly different excita- tion wavelengths in the C' state [305.10 nm (a), 305.12 nm (b), and 305.22 nm (c)] are shown in which the SE signal intensity was attenuated by a factor of 10E7. The broad double heads in the LIF spectra are the thermally equilibrated P branch on the longer wavelength side and the Q and R branches at the shorter wavelengths. The appearance of much narrower SE emission is probably caused by emission from only the directly excited rota- tional states, indicating the difference in time scale of the LIF and SE emission. As can be seen, both the LIF and SE spectral profiles were very dependent on even a small change in the excitation wavelength. The reason for this is not fully understood, but differences in the predissociation rates for the rotational states may partly explain this phenomenon.
In order to investigate the possibility of detecting am- monia molecules in a flame, initially the fluorescence from the C' bands was induced. An excitation spectrum, recorded 2 mm above the reaction zone in an ammonia/ oxygen flame at atmospheric pressure and with an equiv- alent ratio of ¢ = 2.4, is shown in Fig. 4a, and the cor- responding cell spectrum in Fig. 4b. The difference in LIF peak intensity between cold NH3 and NH3 in a flame was roughly a factor of 50.
Neither fluorescence radiation from the B state nor stimulated emission could be detected in the flame. Hence we exclude the possibility of interference of SE in the LIF measurements in flames.
To demonstrate the two-photon excitation technique for the detection of ammonia as a tool in combustion chemistry, we recorded height scans in an NHJO2 flame at atmospheric pressure for different stoichiometric con- ditions. Figure 5 shows such height scans for ¢ = 0.8, 1.7, and 2.4. The scan starts ~0.6 mm above the burner sur- face due to the geometrical cutoff of the focused beam by the burner head. The ammonia is, as can be expected, completely consumed in the lean flames, while there is a constant fraction of ammonia beyond the reaction zone in the richer flames.
When one is using high power lasers, especially in the UV, care has to be taken to ensure that the induced signal originates in the probed species alone. Several examples of photo-induced effects with the use of high-power lasers for flame diagnostics have been reported earlier (e.g., for 0, 27 and for soot/hydrocarbons28). However, in this ex- pe r imen t - i n which no parent molecule could decompose into NH 3 and in which laser-induced recombination pro- cesses seemed very unlikely--photochemical effects were not expected to play a role. This assumption was also confirmed by power-dependence measurements, which
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FIG. 6. Excitation spectra of NO (dashed curve), OH (dotted curve), and NH~ (solid curve) for the investigation of spectral coincidences. The wavelength scale is that of the fundamental dye laser.
were performed on NH3 in the cell for both LIF and SE and in the flame for LIF. In cold NH3 the power depen- dence was 1.8 and 3.9 for the LIF and for the SE, re- spectively, and 2.0 for the flame LIF.
For the same equivalence ratio, height scans recorded at different power levels were found to be "self-similar," that is, they overlapped in all points when auto scaled, showing that the signal originates in a process whose order is independent of height--which is further evidence that only NH3 contributed to the signal.
Simultaneous Multispecies Detection. The C' absorp- tion bands around 305 nm may be used for the simul- taneous detection of NH~ and OH, since the OH radical has the v" = 0 to v' = 0 vibration band in the X2II-A2X electronic transition in this spectral region. However, ammonia molecules also have a two-photon absorption band in the C'(6) state around 287 nm. Although the fluorescence is weaker than at 305 nm because of an increasing rate of predissociation at higher P2 values} 4 this result is of special interest since not only does the OH radical have the v" = 0 to v' = 1 vibration band in the same electronic transition as mentioned above, but also NO and O could be detected when a fundamental dye laser wavelength at 2 x 287 nm was used. This result was achieved by using a Nd:YAG-based laser system, where the frequency-doubled dye laser beam could be further frequency-mixed with the YAG residual infrared radiation at 1.06 /~m, generating radiation around 226 nm, at which atomic oxygen, O, and NO also absorb. We have recently investigated the possibility of simulta- neously detecting OH, NO, and 0 by using spectral co- incidences. 6 It would be especially interesting to find a spectral overlap between NH3 and NO, since NH3 is a strong NO suppressor. Figure 6 shows excitation scans, recorded in a flame, for NO (dashed curve), OH (dotted curve), and NHa (solid curve), where the NH3 and OH spectra were recorded with the frequency-doubled ra- diation at 287 nm, and the NO spectrum with the fre- quency-mixed radiation at 226 nm. The fundamental wavelength of the dye laser was scanned over the same spectral region, 572-575 nm, for all three species. It was found that although there is a partial overlap between NH3, OH, and NO at 575.88 nm and consequently a
possibility of yielding emission from all three species for one fixed dye laser wavelength, it was not possible to record them simultaneously at the same height from a flame with identical flame conditions, i.e., at the same equivalent ratio. The NH3 spectrum is recorded in a rich flame, ¢ = 2.4, while the OH and NO are recorded in a lean flame, ¢ = 0.8. Seen in the ammonia excitation spec- trum in Fig. 6 is also the tail of the preceding C'7 band starting at 280 nm and extending into the C'6 band to give a continuous absorption between 280 and 282 nm, where--at the corresponding mixed wavelength when the limited laser bandwidth is considered--atomic oxygen has three two-photon resonances: 2 3P2-3 ~Po.1.2, 2 3P1-3 3P0,1.2, and 2 3P0-3 ~P0,1,2. From our earlier investigations, 6 we know that the first and second O transitions overlap with NO but not with OH, while the third and weakest transition overlaps with both NO and OH. At the position of the second peak, the NH3 signal is at about 0.3, and in the third peak at about 0.2, of the band head intensity in Fig. 6.
The emission spectra recorded when an excitation wavelength k = 575.85 nm was used are shown in Fig. 7. The well-known ~, band, 2II-2Z, of NO is seen between 226 and 310 nm. The 2II-2Z (v" = 0, 1 v' = 0, 1) of OH is emitted between 280 and 310 nm, whereas the C'(6)-A transition of ammonia occurs around 565 nm.
DISCUSSION AND SUMMARY
As has been demonstrated in this paper, it is possible, with the use of a two-photon excitation scheme, to detect ammonia molecules in a flame environment. The results presented here are qualitative. However, when one is using a measuring technique for diagnostic purposes it is essential to consider the possibility of quantitative determinations. With laser-induced fluorescence, the main problem is to correct for radiativeless transitions from the excited state, the so-called quenching. Several suggestions for avoiding or correcting this phenome- non-e .g . , saturation, 29 direct measurements of the quenching rate, 3° or use of predissociative states,31--have been presented. In the case of ammonia, all excited states, including the B and C' states, are predissociative2 2 For example, the observed lifetime of the different ~2 states
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Emission wavelength (nm) FIG. 7. Emission spectra of NO, OH, and NH3 with the use of excitation wavelength at 575.85 nm. (The spectra were recorded in flames with different equivalent ratios.)
of the B state in ammonia is 6.1 ps. 33 (The situation is further complicated in the C' state since the decay rate varies strongly with v2 and rotation. ~3) This means that, in principle, the quenching effect does not prevent the quantitative determination of ammonia distributions in flames. We have, however, not pursued this approach further in this work.
The possibility of simultaneously detecting several species by using one single laser shot can be an advantage in intercorrelation studies. We have demonstrated that it is possible to simultaneously excite and detect NH3, OH, and NO, with the possible extension to atomic oxy- gen. Actually, as was also mentioned in Ref. 6 and re- cently demonstrated in Ref. 34, these species could be extended to yet one more. Oxygen molecules absorb at the atomic oxygen wavelengths when in a flame environ- ment, which would thus make it possible to simulta- neously detect five species with a time resolution of ~ 10 ns.
As was stated in the previous section, NO, OH, and NH3 could be simultaneously spectrally excited, but they could not be detected in the same flame at the same height. The main reason for this was that a flame at atmospheric pressure was used, which meant that under lean conditions when most NO and OH was expected, 9 NH3 was consumed within the first 0.5 mm above the burner. Under rich conditions, the NH3 height profile flattens out to a constant, nonzero level, which is in agree- ment with the results of Ref. 13. At larger stoichiometric values, however, the concentrations of NO, as well as of OH, have decreased by more than an order of magni- tude25 An additional effect that decreases the fluores- cence signal intensity from species absorbing around 226 nm is the strong absorption at this wavelength due to the X-A transition of NH3. For circumvention of this problem, investigations could also be made with the use of a two-photon excitation process at 452 nm for NO, a wavelength at which NH3 does not absorb strongly. How- ever, there may still be a possibility of detecting NH3 at this wavelength since the X-C' (3) transition has a three- photon absorption in this spectral region. This excitation scheme, followed by ionization detection, was the original detection scheme of the C' state. 22,2~ It is not known, however, to what degree this excitation is followed by laser-induced fluorescence.
The simultaneous detection scheme may thus not be of optimal use in studies of premixed laminar flames as presented here. The great advantages would be in tur- bulent flames, where also simultaneous visualization of these species using planar laser-induced fluorescence would be of interest.
In the experiments on stimulated emission, several comparisons between NH3 and our earlier work on CO 3s could be made. Both signals are very strong above thresh- old, but in neither case could signals be observed in a flame environment. However, the pressure dependence of these species seems quite different. As was stated above, the NH3 signal appeared above 200 Torr, which can be compared with the SE from CO, which was detected only below 200 Torr. It should also be noted that the power dependence of the SE signal in NH~ was almost twice what would be expected for a true two-photon excitation process. This indicates that, although the signal was very strong, we were working in the regime of exponential growth where the power dependence can obviously be much higher than quadratic.
In summary, the two-photon excitation scheme fol- lowed by the LIF detection of ammonia in flames seems very applicable, with several possibilities of multiple- species detection. The stimulated emission technique, on the other hand, does not seem to be applicable to the detection of ammonia molecules in flames, whereas in low-temperature diagnostics, e.g., emission analyses, it seems quite adequate.
ACKNOWLEDGMENTS
This work was financially supported by the Swedish National Board for Technical Developments, STU, and the Swedish Energy Admin- istration, SEV.
1. A. C. Eckbreth, Laser Diagnostics for Combustion Temperature and Species (Abacus Press, Cambridge, Massachusetts, 1987).
2. K. Kohse-H5inghaus, R. Heidenreich, and Th. Just, in Proceedings o[ the Twentieth International Symposium in Combustion (The Combustion Institute, Pittsburgh, Pennsylvania, 1984), p. 1177.
3. M. Alden, H. Edner, P. GrafstrSm, and S. Svanberg, Opt. Commun. 42, 244 (1982).
4. R. K. Hanson, in Proceedings of the Twenty-First International Symposium on Combustion (The Combustion Institute, Pitts- burgh, Pennsylvania, 1986), p. 1677.
5. J. B. Jeffries, R. A. Copeland, G. P. Smith, and D. R. Crosley, in Proceedings of the Twenty-First International Symposium on Combustion (The Combustion Institute, Pittsburgh, Pennsylvania, 1986), p. 1709.
6. U. Westblom and M. Alden, Appl. Opt. 28, 2592 (1989). 7. C. R. Quick, Jr., J. H. Glownia, J. J. Tiee, and F. L. Archuleta,
Proc. SPIE Int. Soc. Opt. Eng. 380, 156 (1983). 8. M. Alden, U. Westblom, and J. E. M. Goldsmith, Opt. Lett. 14,
305 (1989). 9. J. A. Miller, M. D. Smooke, R. M. Green, and R. J. Kee, Comb.
Sci. and Techn. 34, 149 (1983). 10. C. J. Dasch and R. J. Blint, Comb. Sci. and Techn. 41, 223 (1984). 11. R. K. Lyon, U.S. Patent No. 3900554, August 1975.; idem., Int. J.
Chem. Kin. 8, 315 (1976). 12. J. Wolfrum, Appl. Phys. B 46, 221 (1988). 13. C. J. Fisher, Comb. and Flame 30, 143 (1977).
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14. G. H. Miller and A. J. Mulac, J. Quant. Spectr. Rad. Transf. 25, 53 (1981).
15. A. Olafsson, M. Hammerich, J. Billow, and J. Henningsen, App]. Phys. B49, 91 (1989).
16. J. K. Dixon, Phys. Rev. 43, 711 (1933). 17. A. E. Douglas and J. M. Hollas, Can. J. Physics. 39, 479 (1961). 18. G. Herzberg, Electronic Spectra and Electronic Structure o[ Poly-
atomic Molecules (Van Nostrand, Princeton, 1966). 19. A. B. F. Duncan, Phys. Rev. 47, 822 (1935). 20. S. Koda, P. A. Hackett, and R. A. Back, Chem. Phys. Lett. 28, 532
(1974). 21. A. E. Douglas, Disc. Far. Soc. 35, 158 (1963). 22. G. C. Nieman and S. D. Colson, J. Chem. Phys. 68, 5656 (1978). 23. G. C. Nieman and S. D. Colson, J. Chem. Phys. 71, 571 (1979). 24. M. N. R. Ashfold, C. L. Bennett, R. N. Dixon, P. Fielden, H. Rieley,
and J. Stickland, J. Mol. Spectr. 117, 216 (1986). 25. A. Schuster, Rep. Brit. Assoc. 38 (1872). 26. J. K. Watson and W. A. Majewski, J. Mol. Spectr. 115, 82 (1986). 27. J. E. M. Goldsmith, Appl. Opt. 24, 3566 (1987).
28. P. E. Bengtsson and M. Alden, "Optical Investigation of Laser- Produced C2 in Premixed Sooty Ethylene Flames," Comb. and Flame, in press.
29. A. P. Baronavski and J. R. McDonald, Appl. Opt. 16, 1897 (1977). 30. D. Stepowski and M. J. Cottereau, Appl. Opt. 18, 354 (1979). 31. P. Andresen, A. Bath, W. GrSger, H. W. Liilf, G. Meijer, and J. J.
ter. Meulen, Appl. Opt. 27, 3656 (1988). 32. M. N. R. Ashfold, R. N. Dixon, and R. J. Stickland, Chem. Phys.
88, 463, (1984). 33. M. N. R. Ashfold, R. N. Dixon, N. Little, R. J. Stickland, and C.
M. Western, J. Chem. Phys. 89, 1754 (1988). 34. I. J. Wysong, J. B. Jeffries, and D. R. Crosley, Opt. Lett. 14, 767
(1989). 35. D. I. Maclean and H. G. Wagner, in Proceedings of the 11th In-
ternational Symposium on Combustion (The Combustion Insti- tute, Pittsburgh, Pennsylvania, 1967), p. 871.
36. U. Westblom, S. Agrup, M. Alden, H. M. Hertz, and J. E. M. Goldsmith, "Properties of Stimulated Emission for Diagnostic Pur- poses," Appl. Phys. B, in press.
