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Two-photon excitation of atomic oxygen in a flame
Volume 42, number 4
OPTICS COMMUNICATIONS
15 July 1982
TWO-PHOTON EXCITATION OF ATOMIC OXYGEN IN A FLAME M. ALDI~N, H. EDNER, P. GRAFSTROM and S. SVANBERG Department o f Physics, Lurid Institute o f Technology, Box 725, S-220 07 Lund, Sweden
Received 30 March 1982
Atomic oxygen has been detected in a lean acetylene/oxygen flame using the 2p4 3P2-2p 33p 3p two-photon transition at 226 nm and fluorescence detection at 845 nm.
Free atoms are of fundamental importance in combustion processes, since they enter several flame reactions. They are especially important in chemiluminescent processes, giving the light emission from organic flames [1,2]. Saturated laser-induced fluorescence utilizing resonance absorption of laser light would be a natural approach for the sensitive detection of atomic species in flames. However, oxygen like other important species such as C and N has its resonance transition deep in the vacuum ultraviolet spectral region ('~130 nm) preventing flame spectroscopy. Apart from atmospheric absorption, the generation of tunable laser radiation in this region is troublesome. Thus, less direct techniques have to be applied. Atomic oxygen has been studied in a flame environment using Raman spectroscopy [3] and CARS [4], respectively. Those measurements yielded a detection limit of about 1%. While sacrifying the spatial resolu-
C2H2/02
/ ~
F/1.9
FILTERS
FL..E "( I ~ . ' -
~
f=50mm L~'T'~226 nm " PELLIN ~ / ' ~ 2:ndA.S BROCA ~/ ~ PRISM ~ f /
...1
tion, higher sensitivity was reached, using intracavity absorption by the forbidden aurora lines connecting levels within the ground 2p 4 configuration [5]. Twophoton excitation of the 2p33p 3p level at 226 nm, monitored by the 845 nm fluorescence to the 2p33s 3S 2 level, is a further alternative, which is also more sensitive, with the drawback that the quenching rate has to be known. This detection scheme has been studied by Bischel et al. [6] in measurements on a well-defined plasma discharge. A N d : YAG-pumped, frequency-doubled dye laser emitting at 31.4 nm was Raman.shifted to the two-photon excitation wavelength at 226 nm. In our measurement the same type of scheme has been applied to a realistic combustion situation in a measurement on an acetylene/oxygen flame with an equivalence ratio of 0.57 (T = 3000 K). At this temperature the ground state contains about 60% of the
[ GRATING
I COOLED [ ~1 BOXCAR
[.O.00.RO- ~
I RA;;NR
,.TEGRATOR
CHART
RECORDER
[
SNFTER.
Fig. 1. Experimental set-up used in the two-photon excitation of atomic oxygen in a flame. 244
0 030-4018/82/0000-0000/$ 02.75 © 1982 North-Holland
Volume 42, number 4
OPTICS COMMUNICATIONS
atoms. The experimental set-up used in our flame measurement on O is shown in fig. 1. The exciting UV light, of pulse energy 130/aJ, was obtained by second anti-Stokes shifting of frequencydoubled dye laser pulses at 277 nm. The light was focused through a f = 50 mm CaF 2 lens into the flame. The fluorescent light at 845 nm was collected with F]l.9 optics and filtered by coloured-glass filters and a small grating monochromator, so that 0.4% of the emitted light reached the cathode of a cooled RCA 31034 photomultiplier tube with a known anode sensitivity. Finally, the signal was fed to a boxcar integrator with a 50 ns gate. An experimental curve obtained by scanning the dye laser over the line profile is shown in fig. 2 together with a level scheme, where the transitions involved are indicated. The experimental line width was normally about 0;12 A due to the laser bandwidth, and hence the upper-state fine structure is not resolved. A check of the quadratic intensity dependence of tlie two-photon signal was performed in order to ensure, that the short-wavelength, strongly focused laser radiation did not disturb the measurements by causing appreciable photoionization [7] or by creating oxygen atoms by dissociation. Such effects would be revealed as a deviation from strict quadratic intensity dependence. In fig. 3 the quadratic dependence is ascertained. Using the theoretical calculations by Pindzola [8], yielding a generalised two-photon cross section of 2 × 10 -43 cm 4 s, we made a first rough estimate of
2p~ 3p I \ at,5 . m 2 p 3 3 $ 351
2x226_~
.....
/~'~22~7
2~.e
22~5., 22~0 X~ir
15 July 1982
signat
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x
50
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I
I
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I
I
100 arb. units I
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Fig. 3. Experimental verification of the quadratic intensity dependence of the two-photon signal. A least-squares-fit is also included. the number density of oxygen atoms, taking the experimental geometry and system efficiency into account, yielding n = 1015 cm - 3 X k/(2 X 1010 s - l ) . The value is here given in terms of the quenching rate k and "normalised" to an estimated k value of 2 X 1010 s-1 corresponding t o a homogeneous line width of 0.1 c m - 1 . This number density corresponds to an oxygen mole fraction of 0.05% in the flame. According to ref. [1 ], the oxygen mole fraction is predicted to be around 10% in such a flame. The reason for this large discrepancy is not known. However, Bischel et al. [6] have observed a similar deviation. The main uncertainty, apart from the unknown quenching rate k, is due to the estimate of the effective excitation volume. If we disregard the discrepan. cy and use the theoretically predicted concentration 10%, our present detection limit for atomic oxygen in a flame is estimated to be around 0.1%. This value can easily be improved by a factor of 10. However, in order to make more precise measurements it is necessary to bring down the line width of the laser in order to be able to determine the homogeneous width of the transition and hence the quenching rate k. Similar experiments for atomic carbon are in preparation in this laboratory. Lifetime and quantum-beat measurements on C, O, N and H using a state reached in two-photon absorption as a platform for a subsequent laser excitation are also planned for samples of unperturbed atoms.
Fig. 2. Level scheme and experimental recording of the twophoton transition. 245
Volume 42, number 4
OPTICS COMMUNICATIONS
The authors are grateful to Th. Hbgberg, AB VOLVO for discussions and general interest in this work, which was financially supported by the Swedish Board for Technical Developments.
References [1 ] A.G, Gaydon and H.G. Wolfard, Flames, their structure, radiation and temperature (Chapman and Hall, London, 1979).
246
15 July 1982
[2] A.G. Gaydon, The spectroscopy of flames (Chapman and Hall, London, 1974). [3] C.L Dasch and LH. Bechtel, Optics Lett. 6 (1981) 36. [4] R.E. Teets and LH. Bechtel, Optics Lett. 6 (1981) 458. [5 ] S.J. Harris and A.M. Weiner, Optics Lett. 6 (1981) 142. [6] W.K. Bischel, B.E; Perry and D.R. Crosley, Chem. Phys. Lett. 82 (1981) 85. [7] W.K. Bischel, J. Bokor, D.L Kligler and C.K. Rhodes, IEEE J. Quant. Electron. 15 (1979) 380. [8] M.S. Pindzola, Phys. Rev. A 17 (1978) 1021.
OPTICS COMMUNICATIONS
15 July 1982
TWO-PHOTON EXCITATION OF ATOMIC OXYGEN IN A FLAME M. ALDI~N, H. EDNER, P. GRAFSTROM and S. SVANBERG Department o f Physics, Lurid Institute o f Technology, Box 725, S-220 07 Lund, Sweden
Received 30 March 1982
Atomic oxygen has been detected in a lean acetylene/oxygen flame using the 2p4 3P2-2p 33p 3p two-photon transition at 226 nm and fluorescence detection at 845 nm.
Free atoms are of fundamental importance in combustion processes, since they enter several flame reactions. They are especially important in chemiluminescent processes, giving the light emission from organic flames [1,2]. Saturated laser-induced fluorescence utilizing resonance absorption of laser light would be a natural approach for the sensitive detection of atomic species in flames. However, oxygen like other important species such as C and N has its resonance transition deep in the vacuum ultraviolet spectral region ('~130 nm) preventing flame spectroscopy. Apart from atmospheric absorption, the generation of tunable laser radiation in this region is troublesome. Thus, less direct techniques have to be applied. Atomic oxygen has been studied in a flame environment using Raman spectroscopy [3] and CARS [4], respectively. Those measurements yielded a detection limit of about 1%. While sacrifying the spatial resolu-
C2H2/02
/ ~
F/1.9
FILTERS
FL..E "( I ~ . ' -
~
f=50mm L~'T'~226 nm " PELLIN ~ / ' ~ 2:ndA.S BROCA ~/ ~ PRISM ~ f /
...1
tion, higher sensitivity was reached, using intracavity absorption by the forbidden aurora lines connecting levels within the ground 2p 4 configuration [5]. Twophoton excitation of the 2p33p 3p level at 226 nm, monitored by the 845 nm fluorescence to the 2p33s 3S 2 level, is a further alternative, which is also more sensitive, with the drawback that the quenching rate has to be known. This detection scheme has been studied by Bischel et al. [6] in measurements on a well-defined plasma discharge. A N d : YAG-pumped, frequency-doubled dye laser emitting at 31.4 nm was Raman.shifted to the two-photon excitation wavelength at 226 nm. In our measurement the same type of scheme has been applied to a realistic combustion situation in a measurement on an acetylene/oxygen flame with an equivalence ratio of 0.57 (T = 3000 K). At this temperature the ground state contains about 60% of the
[ GRATING
I COOLED [ ~1 BOXCAR
[.O.00.RO- ~
I RA;;NR
,.TEGRATOR
CHART
RECORDER
[
SNFTER.
Fig. 1. Experimental set-up used in the two-photon excitation of atomic oxygen in a flame. 244
0 030-4018/82/0000-0000/$ 02.75 © 1982 North-Holland
Volume 42, number 4
OPTICS COMMUNICATIONS
atoms. The experimental set-up used in our flame measurement on O is shown in fig. 1. The exciting UV light, of pulse energy 130/aJ, was obtained by second anti-Stokes shifting of frequencydoubled dye laser pulses at 277 nm. The light was focused through a f = 50 mm CaF 2 lens into the flame. The fluorescent light at 845 nm was collected with F]l.9 optics and filtered by coloured-glass filters and a small grating monochromator, so that 0.4% of the emitted light reached the cathode of a cooled RCA 31034 photomultiplier tube with a known anode sensitivity. Finally, the signal was fed to a boxcar integrator with a 50 ns gate. An experimental curve obtained by scanning the dye laser over the line profile is shown in fig. 2 together with a level scheme, where the transitions involved are indicated. The experimental line width was normally about 0;12 A due to the laser bandwidth, and hence the upper-state fine structure is not resolved. A check of the quadratic intensity dependence of tlie two-photon signal was performed in order to ensure, that the short-wavelength, strongly focused laser radiation did not disturb the measurements by causing appreciable photoionization [7] or by creating oxygen atoms by dissociation. Such effects would be revealed as a deviation from strict quadratic intensity dependence. In fig. 3 the quadratic dependence is ascertained. Using the theoretical calculations by Pindzola [8], yielding a generalised two-photon cross section of 2 × 10 -43 cm 4 s, we made a first rough estimate of
2p~ 3p I \ at,5 . m 2 p 3 3 $ 351
2x226_~
.....
/~'~22~7
2~.e
22~5., 22~0 X~ir
15 July 1982
signat
x
a r b . units
x
50
/ I
[L..r ,.,,~,t,l 2 I
I
I
50 I
I
I
I
I
100 arb. units I
I
I
:~
Fig. 3. Experimental verification of the quadratic intensity dependence of the two-photon signal. A least-squares-fit is also included. the number density of oxygen atoms, taking the experimental geometry and system efficiency into account, yielding n = 1015 cm - 3 X k/(2 X 1010 s - l ) . The value is here given in terms of the quenching rate k and "normalised" to an estimated k value of 2 X 1010 s-1 corresponding t o a homogeneous line width of 0.1 c m - 1 . This number density corresponds to an oxygen mole fraction of 0.05% in the flame. According to ref. [1 ], the oxygen mole fraction is predicted to be around 10% in such a flame. The reason for this large discrepancy is not known. However, Bischel et al. [6] have observed a similar deviation. The main uncertainty, apart from the unknown quenching rate k, is due to the estimate of the effective excitation volume. If we disregard the discrepan. cy and use the theoretically predicted concentration 10%, our present detection limit for atomic oxygen in a flame is estimated to be around 0.1%. This value can easily be improved by a factor of 10. However, in order to make more precise measurements it is necessary to bring down the line width of the laser in order to be able to determine the homogeneous width of the transition and hence the quenching rate k. Similar experiments for atomic carbon are in preparation in this laboratory. Lifetime and quantum-beat measurements on C, O, N and H using a state reached in two-photon absorption as a platform for a subsequent laser excitation are also planned for samples of unperturbed atoms.
Fig. 2. Level scheme and experimental recording of the twophoton transition. 245
Volume 42, number 4
OPTICS COMMUNICATIONS
The authors are grateful to Th. Hbgberg, AB VOLVO for discussions and general interest in this work, which was financially supported by the Swedish Board for Technical Developments.
References [1 ] A.G, Gaydon and H.G. Wolfard, Flames, their structure, radiation and temperature (Chapman and Hall, London, 1979).
246
15 July 1982
[2] A.G. Gaydon, The spectroscopy of flames (Chapman and Hall, London, 1974). [3] C.L Dasch and LH. Bechtel, Optics Lett. 6 (1981) 36. [4] R.E. Teets and LH. Bechtel, Optics Lett. 6 (1981) 458. [5 ] S.J. Harris and A.M. Weiner, Optics Lett. 6 (1981) 142. [6] W.K. Bischel, B.E; Perry and D.R. Crosley, Chem. Phys. Lett. 82 (1981) 85. [7] W.K. Bischel, J. Bokor, D.L Kligler and C.K. Rhodes, IEEE J. Quant. Electron. 15 (1979) 380. [8] M.S. Pindzola, Phys. Rev. A 17 (1978) 1021.