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Infrared emission spectra of ammonia-oxygen and hydrazine flames
Spcctrochhnica
Acta, 1959, pp. 606 to 026. Pergamon
Press Ltd.
Printed
In Qrest Britain
Infrared emission spectra of ammonia-oxygen and hydrazine flames* R. C. LORD and C. H. SEDERHoLMt Spectroscopy
Laboratory
and Department
of Chemistry, Massachusetts Cambridge 39, Mass.
(Received
27 B’ebrumy
Institute
of Technology,
1959)
Abstract-The infrared emission spectra of a hydrazine decomposition flame and of an ammonia-oxygen diffusion flame have been studied with high-resolution grating spectrometers spectra of the hydra&e flame were recorded at in the spectral range 600-4000 cm- 1. Emission various heights in the flame and at two burning pressures. High-resolution absorption spectra of ammonia and hydrazine vapor have been recorded on the same scale for comparison. Emission spectra have also been recorded from two areas of the ammonia-oxygen diffusion flame, one in the ammonia-rich part of the flame and the other in the region of intense yellow radiation due to the NH, radical. An oxy-hydrogen diffusion flame was examined to establish The high-resolution absorption the spectrum emitted by hot water vapor and by the OH radical. spectrum of ammonia was again recorded on the same scale. There are many emission lines from the hydrazine decomposition flame which can be assigned neither to ammonia nor to hydra&e. These lines are pointed out and discussed, but no definite assignment seems possible from the data because of the large number of short-lived species that might exist in this flame, all containing only nitrogen and hydrogen, and hence all having their infrared bands in similar spectral regions. Nearly all of the emission lines from the ammonia-oxygen flame have been assigned to water, ammonia or the OH radical. Several lines of uncertain assignment appear near 1100 cm-l. These may be due to ammonia, but no correlation exists between them and the ammonia absorption spectrum such as is found in other regions. The most striking new lines are found in the region 2700-3050 cm-l, where a series of twenty-three almost evenly spaced lines occur which cannot be assigned to known lines of water, ammonia or OH. The new lines have tentatively been ascribed to the NH, radical. In the emission of both flames, the R-branch of the band due to the inversion vibration at 950 cm-* in ammonia is easily visible, but the P-branch is not. This phenomenon is attributed to an effect, previously observed in diatomic molecules, which increases the intensity of either the P-branch or the R-branch of a band at the expense of the other branch. THE ready availability of infrared grating spectrometers of high aperture and moderate resolving power improves the prospect of successful study of the compositjon and kinetics of flame reactions by means of their infrared emission spectra. Prompted by an interest in detecting the NH, radical through its infrared emission, the writers have studied two flames in which this radical is known to be present from its visible emission spectrum. These are the ammonia-oxygen diffusion flame and the hydra&e decomposition flame. * This article is based on the Ph.D. thesis of C. H. SEDERHOLM, submitted to the Chemistry, Massachusetts Institute of Technology, January 1959. Acknowledgement financial support by the U.S. National Science Foundation (Grants G-1892 snd G-6230). t U.S. National Science Foundation Pre-doctoral Fellow, 1955-1959.
605
Department is made
of of
&. C. LORD
and
C. H. SEDERHOLM
The ammonia-oxygen diffusion flame has been studied in the visible region [lJ, but only recently have Dows et al. [2] investigated its infrared emission, using They found no emission to. be attributed to low-resolution prism equipment. NH, or NH. The emission spectrum of the hydrazine decomposition flame has been photographed by HALL and WOLPHARD [3], who found that the spectrum was produced by NH, and NH. No tiared’spectrum of this flame has been reported up to now, although it appears to offer an excellent possibility for observing NH, and NH because of the freedom of the flame from OH and water. It was hoped to study spectra originating at various heights in the flame and to gain information thereby about the course of the decomposition reaction. Study of the spectrum of the ammonia-oxygen flame was undertaken first. Despite the negative resuIts of DOWS et al. [2] it seemed possible that the considerably higher resolution achieved by the grating spectrometer might enable detection of NH, and perhaps NH. This study, which included the observation of some new bands and their tentative assignment to NH,, was completed before we learned of the recent work of TANNER and KING [4]. They report a low-resolution infrared absorption spectrum (ascribed to NH,) in the products of the flash photolysis of hydrazine.
Experimental Spectrometers Two small grating spectrometers of aperture f 4.0 were used [5, S]. One employed cooled lead-sulfide and lead-telluride photoconductive detectors and was used at frequencies above 2000 cm-l. The other covered the range 2000-600 cm-l with a Golay detector. In the absorption spectra reported below for comparison purposes, the spectral slit &idth employed was 0.5 cm-r. In the emission spectra, however, energy considerations forced the use of wider slits, which varied from a minimum of about 1 cm-r in the high-frequency portion of the ammonia-oxygen flame spectrum to as much as 4 cm-l for certain regions below 2000 cm-i in the hydrazine flame. Spectra were recorded on strip charts and the figures below were traced directly from the original records. At frequent intervals the mechanical slit width or amplifier gain was changed ; discontinuities in the curves make clear where these changes were made. Flume systems (a) Ammonia-oxygen diffusion Jame. The burner used was identical to the one described by Dows et al. [Z]. It was mounted with the ammonia-oxygen interface parallel to the optical path. The mounting permitted delicate horizontal motion of [I] [2] [3] [4] [5] [63
H. D. A. K. R. R.
G. A. R. N. C. C.
WOLFHARD and W. G. PARKER, Proc. Phyys. Sot. Dow, G. C. PIMENTEL and E. WHITTLE, J. Chem. HALL and H. G. WOLFHARD, Traw. Paraday Sot. TANNER and R. L. KINQ, Nuture 181, 963 (1958). LORD and T. K. MCCUBB~N, JR., J. Opt. Sot. Am. LORD and T. K. MCCUBEIN, JR., J. Opt. Sot. Am.
606
(A) 62,
(1949); Ibid. (1955). 52, 1520 (1956). Phys.
722 23,
499
45, 441 (1955). 689 (1957).
47,
65,
2 (1952).
Infrared
emission
spectra
of ammonia-oxygen
and hydrazine
flames
the burner perpendicular to the optical path, so that radiation from various sections of the flame beginning 1 cm above the top of the burner could be focused on the entrance slit by the conventional source optics. The source optics and spectrometer were enclosed in a housing which was purged with dry nitrogen to eliminate atmospheric absorption. The burner was placed about 1 cm from a rock-salt window through which the radiation entered the housing. Spectra were recorded both of the radiation from the bright yellow portion of the flame near the ammonia-oxygen interface and of that from the dim portion on the
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INTRODUCTION HYDRAZINE
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1. Apparatus
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BELLOWS VALVES
NEEDLE
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:
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HOLBEKEEPER
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SURGE
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ammonia-rich side of the burner. The spectrum of an oxy-hydrogen diffusion flame was recorded under similar slit conditions to establish the j?ositions and appearance of the emission lines due to water and the OH radical. The spectrum of this flame did not depend (apart from intensity) upon the region where the radiation originated. (b) Hydrazine decomposition Jlame. A schematic diagram of the apparatus for producing and regulating this flame is shown in Fig. 1. The flame, which was 607
R. C. LORD
and C. H. SEDEREOLM
stabilized at reduced pressures (75 mm and 40 mm) in a 5 1. flask on top of a chimney of 20 mm Pyrex tubing, was ignited by a spark between electrodes on either side of the chimney. The flask was evacuated with a pump isolated from the system by a The pumping rate was controlled by means of a stopcock in the 50 1. surge tank. pumping line. Hydrazine vapor from a reservoir of boiling hydrazine* travelled through two needle valves separated by a 2 1. surge bottle, and then up the chimney. The two needle valves and the surge bottle regulated the hydrazine flow rate and All parts reduced fluctuations in the flow caused by uneven boiling of the hydrazine. of the hydrazine supply system were heated and the temperatures of these various parts ‘were adjusted to stabilize the flame at the desired burning pressure. The temperatures were all near 100°C. Radiation from the flame was observed through a rock-salt window in the side of the flask. An unmagnified image of a horizontal section of the flame was focused on the vertical slit of the spectrometer by a spherical mirror and two appropriately oriented plane mirrors. One of the plane mirrors was mounted in such a way that segments at various heights in the flame could be reproducibly focused on the slit. All of this optical path as well as the instrument housing was purged with dry nitrogen to reduce atmospheric absorption. This technique was not as effective as it was with the ammonia-oxygen diffusion flame because of the bulk of the source optics.
Emission spectra in the range 4000-600 cm-l from the ammoniwxygen diffusion flame are presented in Fig. 2. The region 2420-2000 cm-l, which shows nothing but some feeble radiation from H,O and OH, is omitted. In the figure, curve A is a comparison spectrum of absorption by ammonia at room temperature, B is the emission spectrum from the dim, ammonia-rich part of the flame, C is that from the bright visible part near the interface between the two streams of gas, and D is the spectrum from the same part obtained when ammonia was replaced by hydrogen. Apart from the wider spectral slit widths mentioned above, the instrumental operating conditions were essentially those described in [5] and [6]. The emission spectra from the hydrazine decomposition flame are presented in Fig. 3. The spectral regions 2900-2000 and 820-600 cm-l are not presented because there is insufficient emission from the flame to be detected. Five emission spectra are presented in these figures. Curve a shows the emission of the flame at a point 18 mm above the top of the chimney with a pressure of 40 mm. Curves D, E, P and G were each recorded at a pressure of 75 mm. Curve D represents the emission 6 mm above the chimney ; curve E, 12 mm above the chimney ; curve P, 18 mm above the chimney; curve G, 24 mm above the chimney. Curve C is an atmospheric absorption spectrum taken with approximately the same slit widths as the corresponding curves D through a. By comparing the curve C with curves D though H, allowance can be made for apparent but spurious “emission bands” which result in fact from atmospheric absorption of the continuous emission from the flame. * For safety’s sake the reservoir of boiling hydrczine was placed outside the building adjacent to the room contein$g the remainder of the experimental set-up, and was screened by heavy stone slabs. One severe explosion of obscure origin occurred in the reservoir, but the damage was confined to the reservoir system.
608
Infrared
emission
spectra
of ammonia-oxygen
and hydrazine
flames
40OOcrr
Fig.
2 (i)
diffusion flame and oxy-hydrogen spectra of the ammoni*oyxgen Fig. 2. Emission spectrum from bright dame : curve A, absorption spectrum of ammonia; curve B , emission spectrum from ammonia-rich part of ammonia-oxygen diffusion flame ; curve C, emission spectrum from oxy-hydrogen part of ammonia-oxygen diffusion flame; curve D, emission diffusion flame.
1 I 37OOcm-’
I
.
*
I
.
I
t c 26OOcm-’
Fig.
L
I
2 (ii)
609
,
.
a
(
,
J 3500cm-1
3
’
*
’
R. C. LORD
md
C. H. SEDERHOLM
Curve B gives the absorption spectrum of gaseous hydrazine under high For a survey spectrum resolution and curve A, that of ammonia at room temperature. of gaseous hydrazine, the reader canrefer to GIGUI~E and Lm [7]. At one time, while the hydrazine decomposition flame was burning, a dry-ice trap was introduced in the pumping line at a distance of about 6 ft from the flame flask. After the flame had been running for several minutes, this trap was completely filled with white solid material which was mostly ammonia. The system was shut down and pressurized with/dry nitrogen, and the solid was allowed to warm up to room temperature. A good deal of ammonia gas was given off, but a small amount of liquid remained in the trap. The residual ‘liquid remained colorless for several days under nitrogen, but when the system was taken apart and the liquid exposed to air, it turned dark red in a period of several more days. This experiment was repeated several times. Each time, the liquid remained completely colorless as long as it was not exposed to the atmosphere, but after exposure, it turned color slowly. The amount of colored material seemed to be a function of the conditions under which the flame was stabilized. Its infrared absorption spectrum showed the presence of considerable hydrazine plus a general m&znge of other bands indicating traces of unknown components. No identification could be made. A separation of the active species was attempted by trap-to-trap distillation, but the amount involved was too small to isolate by this technique.
Discussion Spectrum
of the ammonia-oxygen
diffwion
jlame
By comparing
the emission spectrum of the oxy-hydrogen diffusion flame (curve that of the ammonia-oxygen diffusion flame (curves B and C), one sees that throughout most of the spectral region investigated the spectra of these two flames coincide closely except for overall intensity. Additional bands are present in the ammoni+oxygen flame in the regions around 1000 cm-l, 1600 cm-l and 2900 cm-l. Since the only species in the oxy-hydrogen flame which radiate in the infrared above 600 cm-i are H,O, which emits throughout the infrared, and OH radical, which emits from 2700 cm-l upward, we can say that the emission from the ammonis-oxygen diffusion flame in all regions investigated except the three mentioned above is due to H,O or OH radical. The almost exact coincidence of the emission due to H,O and OH in the two flames is somewhat surprising when one considers the difference in temperature between them. WOLFHARD and PARKER [l] give the temperature of the ammoniaoxygen diffusion flame as around 2000°C. According to GAYDON and WOLEMRD [8] the temperature of an oxy-hydrogen flame is about 2800°C. However, the similarity between the emission spectra of the two flames throughout most of the spectrum makes it reasonable to assume that any emission from the ammonia-oxygen flame which does not appear in the oxy-hydrogen flame is due to some species other than H,O or OH radical.
D, Fig. 2) with
[7] P. A. GICVX~E [8] A. G. GAYDON
end Hall,
and I. D. Lm, and
London
H.
J. Chem. Phys. Flumes,
G. WOLPBARD,
20, 136 (1952). Their Structure,
(1965).
610
Radiation
and Temperature.
Chapman
8.
.I
340ocm-
3300cm-’
.,..
Fig.
a:,,,,
3000cm-’
,,
,,,,,,,,.,,,,
32OOcm'
L
,,,
2 (iii)
,
I
2900cm
Fig.
3lOOCrn~
.,
eeoocm-
2 (iv)
611
,
.,
,.
,
, 27oom
R.C.
LORD
andC. H.SEDERHOLM
In the region from 850 cm-l to 1250 cm-1 there are numerous lines which under the above criterion cannot be accounted for by H,O or OH. Most of the lines can be assigned readily to emission by ammonia because of the close coincidence between them and lines in the absorption spectrum of ammonia at room temperature (curve A). Several differences can be noted, however, between the‘emission and absorption spectra of ammonia. In the emission spectrum the Q-branches at 930 and 960 cm-l have become broader on the jaw-frequency side, owing to the increased population of the upper rotational states, although the positions of the maxima appear unchanged. In the R-branch of this band the lines of maximum intensity have moved out toward .higher frequencies, again because of the increased population of higher rotational states. The P-branch of the band is almost entirely missing. If one looks at the absorption spectrum of ammonia in this region, the P-branch is found to be less intense than the R-branch, but is nevertheless readily observed. It is probable that the absence of the P-branch in emission and its diminished intensity’ in absorption are due to an effect which has previously been observed in diatomic molecules [9]. This effect increases the intensity of either the P-branch or the R-branch of a band at the expense of the other branch. It comes about as a result of the rotation-vibration interaction of the permanent dipole moment of the molecule with the vibrating dipole. Table
1. New emission lines from ammonia-oxygen
li’requency (cm-l)
Spacing (cm-‘)
3047.1*t: 3034.7t $ 3022.6t$ 3009*7t $ 2997.0*t1 297cott 2964.8 2950.6 2937.6 2921.3 2905.6 2591.4*
12.4 12.1 12.9 12.7 19.0 13.2 14.2 13.0 16.4 15.6 14.2
diffusion
Frequency (cm-l)
Spacing (cm-‘)
2875.2 2858.5 2842.9 2829.0 2813.2 2794.8 2780.7 2765.4 2748.6* 2736.4t 2718.6$
16.2 16.7 15.6 13.9 15.8 18.4 14.1 15.3 16.8 12.2 17.8
flame
* Measurement affected by emission lines of OH. t Measurement affected by emission lines of H,O. $ Line center uncertain because of unusual line breadth.
In the region from 1000 to 1100 cm-I there are some additional lines which do not coincide exactly with absorption peaks in room-temperature ammonia, but in view of the temperature difference it seemsimpossible to exclude ammonia as the origin of these lines. The variations between curves B, C and D in the region around 1600 cm-l are very small. These can probably be accounted for in terms of emission by the degenerate bending vibration of ammonia. In the present, investigation, the [9] R. HERMAX and R. F. WALLIS, J. Chem. Phya.
23, 637 (1955); see also earlier references there cited.
612
Infrared
I,,
emission
spectre
,
and
,
2700cm-’
70cm-
of ammonia-oxygen
I
2600cm-’
hydrazine
,
*
,
flames
,
,
,
,
25oocm-’
I,, ISOOcm-
19OOcm-’
Fig.
2 (vi)
613
,
,
,
,
,
,
R. C. LORD
and
C. H. SEDERHOLM
‘large amount of emission by ammonia in this region reported by DOWS et al. [2] was not observed. The region from 2700 to 3050 cti-l is of most interest. Table 1 lists a series of twenty-three lines which appear in this region of the spectrum.’ They are rather evenly spaced, as the second column of the table shows. If only the lines in the middle of this band, where identi&ation is more straightforward, are considered the average spacing is 15.5 cm-l. Some unassigned lines were observed by Dows et al. [2] in this region, which under their conditions were much weaker than the lines due to OH radical,. and they stated “These lines may be due to emission by water”. In the visible spectrum of the flame [l] emission from only a limited number of species was observed. These included OH, H,O, NH, NH, and NH,. Other species which might exist in the flame are HNO and any of the general formula N,H,. Comparison of the emission spectrum of the ammonia-oxygen flame with that of the oxyshydrogen flame shows that this series of lines does not appear in the latter. Two bands of emission lines due to OH radical were found in the oxy-hydrogen flame in this region and these agree well with’ the spectra of OH radical published by &EN et al. [lo]. In the spectrum from the bright yellow portion of the ammoniaoxygen diffusion flame, it is possible to see a superposition of the new lines on the bands due to water and OH radical. The only fundamentals of ammonia near this region are the two N-H stretching It will be noted that the Q-branch of v1 does not appear in the frequencies y1 and va. emission spectra at 3330 cm-l. If the Q-branch is too weak to be seen, it appears impossible for the P-branch of this band to be intense enough for observation in this region of the spectrum. As for v3, it should be still weaker than vl. Combination and overtone bands in the region have been investigated by BENEDICT et al. [l 11; The new lines in no way resemble their combination and overtone bands. Electronically excited ammonia might be considered as a source of this emission band, but such an assignment seems unlikely in view of the very short lifetime of electronic states. In addition, there is no evidence for vibrational transitions in electronically excited ammonia elsewhere in the spectrum. The energy of the tist known excited electronic state is too high (46167 + 30 cm-l) [ 121 to permit a large population at the temperature of the flame. Moreover, the spacing is too irregular for a symmetrical top, which is the structure of the ammonia molecule in both ground and first excited states [l3, 141. The rotation-vibration spectrum of the NH radical is a possibility, since its fundamental frequency should occur in this general region. However, its rotational constant [16] is 16.7 cm-l, which means that it should exhibit a rather regular rotational line spacing of 33.4 cm-l, in contrast to the observed spacings of Table 1. The remaining possibilities appear to be HNO, NH, and fragments of the formula N,H, with y = 1, 2, 3 or 4 (species containing more than two nitrogen atoms are [lo] H. c. k&EN, [ll] [12] [131 [14] 1161
W. A. A. W. G.
JR., L. R. BLAINE and E. K. PLYLER, Spctr~chim. Acta 9, 126 (1967). S. BENEDICT, E. K. PLYLER md E. D. !CIDWELL, J. Chem. Phya. 99, 829 (1958). B. F. DONCAN, Phya. Rev. 50, 700 (1936). B. F. Dmcm and 0;. R. -ISON, Phys. Rev. 49, 211 (1936). S. BENEDICT, Phys. Rev. 47, 641A (1936). ~RZBEW-3, i@ecfm of Dicchic Molecules (2nd Ed.). Van Nostrand, New York (1960).
614
Infrared
I 170ocm-
,
emission
,
,
,
spectra
,
,
I
of ammonia-oxygen
I
.
I 1600cm-~
I
Fig.
2 (vii)
Fig.
2 (viii)
616
I
and hydrezine
I
I
I
flames
.
I
I l500cm-1
I
R. C. LORD
and C.
Fig.
Fig.
H. SEDERHOLM
2 (ix)
2 (x)
Infrared
emission
spectra
I
of ammonia-oxygen
I
and hydrazine
flames
I
Fig.
2 (xi)
, VI a I
I
S.OOcm-1
I
,
I
L
,
,
,
I
,
,
70ocm-
Fig.
.
,
,
.
,
,
,
1
600cm-1
2 (xii)
617
R. C. LORD
snd C. H. SEDEREOLM
thought to be unlikely to give rise to a rotational spacing of 15 cm-l). Rotational constants A, B and C are known for HNO [16] (10.3, 1.29 and 1.15 cm-l, respectively) and for NH, [17] (23.62, 12.94 and 8.19 cm-l) and have been estimated [18] for hydrazine (4.52, 030 and 0.76 cm-l). Rough guesses can also be made for the others : 1 cm-l), N,H, (A < 9, B = C = 1 cm-l), N,H, (A < 6, N,H (A<17, B=C= B = C = 1 cm-l). All of the above species except NH, are approximately symmetrical-top molecules. If the direction of the vibrating electric moment is parallel to the A-axis, the rotational spacing of the corresponding vibrational band is B + C, and if the direction is per@endicular to the A-axis, the spacing is 2A - (B + C). Since there are probably no reasons of symmetry why the direction of the vibrating moments should be either parallel or perpendicular to the A-axis in most if not all of these molecules, the best that can be said is that the rotational spacing of their bands should lie between B+C and 2A-(B+C). .
Table
2. Series
of H,O
with
nearly
constant
spacing
Spacing (cm-l)
Frequency (cm-l)
Spacing (cm-l)
8.0 10.2 11.7 13.1 14.6 16.2
3734.8 3724.6 3712.6 3699.3 3684.6 3668.6
10.2 12.0 13.3 14.8 15.9
3722.6 3710.7 3697.2 3681.8
11.9 13.6 15.4
22.3 21.3 21.9
3688.6 3568.3 3547.0 3526.4 3506.0 3484.4 3462.5 3440.6 3417.7 33964 3372.6 3349.4 3325.8
20.3 21.3 20.6 20.4 21.6 21.9 21.9 22.9 22.2 22.9 23.3 23.6
3586.5 3566.9 3545.6 3524.8 3604.8 3482.6 3460.7 3438.5 34157 3393.3 3370.0 3347.3 3323.6
19.6 21.4 20.7 20.0 22.2 21.9 22.0 22.8 22.4 23.3 22.7 23.8
Spacing (cm-l)
4003.7 3991.1 3977.2 3962.7 3948.2 3932.8 3917.8 3902.7
12.6 13.9 14.6 14.5 15.4 16.0 15.1
3610.9 3488.6 3467.3 3445.4
lines
Frequency (cm-l)
Frequency (cm-l)
3744.5 3736.5 3726.3 3714.6 3701.6 3686.9 3670.0
of emission
[16] D. A. RAMSAY, Ann. N.Y. AC&. Sci. 67, 485 (1957). [17] D. A. RAMSAY, J. Chem. Phya. 26, 188 (1966). [18] D. W. Scorn,. G. D. OLIVER,‘ M. E. GROSS, W. N. HUEBARD rsoc. 71, 2293 (1949).
618
and
H. M. HICKMAN,
J. Am.
Chem.
I&am&
emission
spectra I
’
of wnmonia-oxygen 7
’
and hydrazine .
’
’
’
flemee
’
.
Fig. Fig.
3 (i)
3.
Emission spectra of the hydrszine decomposition flame: c~llre 4, absorption of ammonia; curve B, absorption spectrum of gaseous hydrazme; Cm~e 0, &m&pheric absorption spectrum under resolution comparable to CupVes D-H; Cm+~es D-H, emission spectra of the hydrazine decomposltlon flame: D, h= 6 -, P= 75 mm, E, h=l2 mm, ~~76 mm; ,P, h=lS mm, p=76 mm; Q, h=24 111l11,p=76 mm; H, h= 18 mm; P=40 nun.
spe&um
Fig.
3 (ii)
619
R. C. LORD
end
C. H. SEDERHOLM
It can thus be concluded that the values of B + C are too small in all cases, and only for HNO, N,H and NzHz are the values of 2A - (B + C) large enough to give a stretching spacing of 155 cm- 1. In these three molecules it appears that the N-H vibrations would give rise to hybrid bands of more nearly parallel than perpendicular character (except for one vibration of the G-form of N,H,), but this is not sufficient basis for eliminating them as possible sources of the observed lines. The rotational constants of the NH, radical fit the magnitude of the spacing somewhat better. To be sure, NH, is an asymmetrical top and a more complicated spectrum is expected than a series of lines with approximately constant spacing. However, study of the emission spectrum of H,O in the region 3000-4000 cm-l shows that an asymmetrical top can in fact give rise to series of evenly spaced lines. In Hz0 the bands in the above region arise from the OH stretching vibrations vi and +, and the series of even line spacings arise from transitions between the lowest K-, levels of adjacent values of J. The expected spacing is no larger than B +C and can be somewhat smaller than 2C. A number of these series of water lines and their spacings are listed in Table 2. Since for H,O [19], A = 27.79, B = 14.51, and C = 9.29 cm-l, it is seen that there are examples of the spacing running all the way from less than 2C up to B+C. Considering a similar effect in NH, with rotational constants as given above, one might expect it to give rise to an emission band with lines of roughly constant spacing at 155 cm-l. The spectroscopic data therefore suggest that the series of lines of Table 1 is probably due to NH,, N,H, N,H, or HNO, with some preference for NH, over the other species. This preference is strengthened when the kinetics of the flame are considered. The only way in which the species other than NH, can be formed is by three-body collisions between an NH group on the one hand and a second NH group, an N atom, or an 0 atom on the other. The probability of three-body collisions involving two free radicals is very low since the free radicals are in low concentration. On the other hand, NH, can be formed by a two-body collision involving one free radical and the ammonia molecule, which is present in high concentration. Therefore, one expects the concentration of NH, to be much higher. It is to be noted that the emission of the unknown species is stronger in the ammonia-rich region of the flame than it is in the brightest part of the flame. In the visible spectrum, on the other hand, the most intense emission from NH, comes from the brightest part of the flame [l]. These facts might appear to argue against the assignment of the infrared lines to NH,, but intensity of emission depends on important factors other than concentration (temperature, electronic excitation, etc.) and therefore it is not necessary that the maxima of visible and of infrared emission by the NH, radical occur at the same point in the flame. Recently TANNER and KING [4] reported observing NH, in infrared absorption They found a broad band centered under low resolution by means of flash photolysis. at 3200 cm-‘. This is not in contradiction to the assignment of lines in the 280~~ 3090 cm-l to NH,, and the large amount of emission by H,O above 3050 cm-1 would obscure the center of the NH, band in our spectra. [19] G.
HERZBERQ,
Infrared
and Raman
i3pctra.
Van
620
Nostrand,
New
York
(1945).
Infrared
emission
spectra
of ammonia-oxygen
Fig.
Fig.
3 (iii)
3 (iv)
621
and hydrezine
flames
R. C. LORD and C. H. SEDERHOLM
Spectrum of the hydrazine decomposition J’lame In this flame one expects to see emission by ammonia, hydrazine and fragments of hydraxine. If the ammonia-oxygen flame is a reliable guide, the emission spectra of ammonia and hydrazine should be similar to their absorption spectra at room temperature (curves A and B of Fig. 3). It is of interest that the relative intensities of the lines in the emission spectra (with a few exceptions to be discussed further) are independent of the p&t of the flame studied and of the two operating pressures. This shows that none of the species which are in sufficiently large concentration to be detected by their infrared emission is cormned to a small region in the flame. The clearly observable structure of the visible flame must therefore result from variation in concentration of species (for example, electronically excited NH,) whose infrared emission spectra are too weak to measure. In the region 600-850 cm-l there was no detectable emission. There is a rather sharp. band between 835 and 965 cm-l which does not seem to be due to ammonia because of its frequency and lack of inversion doubling. It may be slightly sharpened because of re-absorption by ammonia in the optical path within the flame flask. Since hydrazine itself has a broad absorption band in this region, it might be responsible for the observed emission. The hydrazine absorption appears very much broader than does the emission band, possibly because of the exponential relationship between transmission and molecular concentration, as compared to the linear dependence of emission on concentration. Table
3. Comparison
Designation
--
of emission lines in hvdrazine absorptik lines
extrapolation
of observed
-
Observed ammonia lines (cm-l)
Designation
--
Calcd.* ammonia lines (cm-‘)
Observed flame lines (cm-l)
1177.0 1194.9 1212.6 1230.1 1247.4 1264.5 1281.4 1298.1 1314.6 1330.9 1347.0
1195.6 1213.0 1230.7 1247.8 1265.1 1281.4 1298.3 1314.3 1330.6 1346.3
1007.5 1026.9 1046.3 1065.5 1084.6 1103.5 1122.1 1140.6 1158.9
* Extrapolation
flame with
Difference (cm-l) 0.7 0.4 0.6 0.4 0.6 8:; -0.3 -0.3 -0.7
of observed lines with second dBerences assumed constant.
There is considerable structure around 1100 cm-i which corresponds neither to hydrazine absorption nor to that of ammonia. This structure must be due to some other species but there is insufficient basis for assignment. In the spectrum taken 24 mm above the chimney (curve C of Fig. 3), there is de&&e evidence of re-absorption by ammonia at 1122.1, 1140.6 and 1168.9 cm-l. This does not show up in any of the other curves in this region. It should be noted that in this same spectrum, there is a broad maximum at about 1130 cm-i, absent from all the other spectra, 622
Infrared
emission
spectra
of emmonitGoxygen
Fig.
3 (v)
Fig.
3 (vi)
623
and hydra&e
&mea
R. C. LORD and C. H. SEDEREOLM which is also diflicult to assign. These unassigned bands continue in all of the spectra as far as about 1200 cm-l. At I 196 cm-i a series of regularly spaced lines begins in all spectra, which looks like a continuation of the SE-branch of ammonia. In Table 3 calculated ammonia lines are compared with the observed emission lines. The agreement furnishes an excellent basis for assigning the series to ammonia. In the hydrazine flame as in the ammoniaoxygen flame, it is found that the P-branch of the inversion vibration of ammonia is missing whereas the R-branch is quite intense. Toward higher tiequency, the next band of interest is centered around 1615 cm-l. Hydrazine has intense absorption at this point and this band can probably be attributed to hydrazine. Again, the difference in- width of the band in absorption and emission is probably due to the comparison of an exponential function with a linear appears in the emission spectrum which is not apparent one. A good deal of structure in absorption. This structure is difficult to assign to hydrazine, as one would not expect new fine structure to appear at a band center as the temperature is elevated. Other species with NH, groups are probably emitting in this region, producing the added structure. From 1615 to 1900 cm-i there seems to be a broad emission band with very little structure in it. Most of the apparent structure is due to the absorption of continuous flame radiation by water vapor in the optical path. From 2000 to 2950 cm-l the intensity of the emission was too small to be recorded. It was rather disappointing that no emission band was observed in the hydrazine flame at the position of the band which has been assigned to NH, in the ammoniaoxygen flame. NH, is certainly present, but must be in too low concentration to be detected. Indeed, the visible bands of NH, in this flame are much less intense than the same bands in the ammonia-oxygen flame. Above 2950 cm-l, there are again a large number of emission bands. In general these do not correspond to the absorption bands of either ammonia or hydrazine. The strong Q-branch of v1 of ammonia at 3335 cm-i is apparent in emission as a shoulder on the side of a much stronger band at 3325 cm-l. This strong band is at the same frequency as an absorption in hydrazine. Again, this band seems too sharp to be due to hydrazine, but the difference between absorption and emission spectra may account for the effect. There is another strong emission line at 3848 cm-1 with structure on either side of it. Neither ammonia nor hydrazine absorbs at this point. It is hard to imagine any species containing ody nitrogen and hydrogen which would have a fundamental in this region, SO this must be a combination or overtone band. It should be mentioned that the superior sensitivity of the photocell used at this point tends to exaggerate the apparent emission intensity compared with bands below 2000 cm-l, where thermal detectors must be employed. It is safe to assume that none of the observed emission lines in Fig. 3 are due to water vapor as an impurity in the hydrazine. At one time a small crack occurred in the bottom of the reservoir flask which allowed some water to contaminate the hydrazine. Water emission peaks corresponding exactly to the water emission from the oxy-hydrogen flame were immediately noted in the spectrum. None of the peaks shown in the spectra of the hydrazine flame corresponds to water peaks from the oxy-hydrogen flame. 624
Infrared
emission
spectra
of ammonia-oxygen
Fig.
3 (vii)
Fig.
3 (viii)
625
and hydrazine
flames
R. C. LORD
and C. H. SEDERHOLN
In conclusion, there are many lines and bands in the emission spectrum of the hydrazine decomposition flame which are not produced by ammonia or hydrazine. These are due to intermediate species in the flame, but with the data at hand, it is impossible to make any assignment of them since all the species in question have NH stretching and bending frequencies which lie in the regions of the unassigned bands. Further resolution of fine structure in these bands, or comparison of the spectra reported here with/spectra from other sources, may ultimately lead to their assignment and to more detailed understanding of the chemical processes of the flame., Acknou&xIgements-The ALLEN and one with
Dr.
writers wish to acknowledge H. G. WOLFHARD.
626
several
helpful
discussions
with
Dr.
H.
C.
Acta, 1959, pp. 606 to 026. Pergamon
Press Ltd.
Printed
In Qrest Britain
Infrared emission spectra of ammonia-oxygen and hydrazine flames* R. C. LORD and C. H. SEDERHoLMt Spectroscopy
Laboratory
and Department
of Chemistry, Massachusetts Cambridge 39, Mass.
(Received
27 B’ebrumy
Institute
of Technology,
1959)
Abstract-The infrared emission spectra of a hydrazine decomposition flame and of an ammonia-oxygen diffusion flame have been studied with high-resolution grating spectrometers spectra of the hydra&e flame were recorded at in the spectral range 600-4000 cm- 1. Emission various heights in the flame and at two burning pressures. High-resolution absorption spectra of ammonia and hydrazine vapor have been recorded on the same scale for comparison. Emission spectra have also been recorded from two areas of the ammonia-oxygen diffusion flame, one in the ammonia-rich part of the flame and the other in the region of intense yellow radiation due to the NH, radical. An oxy-hydrogen diffusion flame was examined to establish The high-resolution absorption the spectrum emitted by hot water vapor and by the OH radical. spectrum of ammonia was again recorded on the same scale. There are many emission lines from the hydrazine decomposition flame which can be assigned neither to ammonia nor to hydra&e. These lines are pointed out and discussed, but no definite assignment seems possible from the data because of the large number of short-lived species that might exist in this flame, all containing only nitrogen and hydrogen, and hence all having their infrared bands in similar spectral regions. Nearly all of the emission lines from the ammonia-oxygen flame have been assigned to water, ammonia or the OH radical. Several lines of uncertain assignment appear near 1100 cm-l. These may be due to ammonia, but no correlation exists between them and the ammonia absorption spectrum such as is found in other regions. The most striking new lines are found in the region 2700-3050 cm-l, where a series of twenty-three almost evenly spaced lines occur which cannot be assigned to known lines of water, ammonia or OH. The new lines have tentatively been ascribed to the NH, radical. In the emission of both flames, the R-branch of the band due to the inversion vibration at 950 cm-* in ammonia is easily visible, but the P-branch is not. This phenomenon is attributed to an effect, previously observed in diatomic molecules, which increases the intensity of either the P-branch or the R-branch of a band at the expense of the other branch. THE ready availability of infrared grating spectrometers of high aperture and moderate resolving power improves the prospect of successful study of the compositjon and kinetics of flame reactions by means of their infrared emission spectra. Prompted by an interest in detecting the NH, radical through its infrared emission, the writers have studied two flames in which this radical is known to be present from its visible emission spectrum. These are the ammonia-oxygen diffusion flame and the hydra&e decomposition flame. * This article is based on the Ph.D. thesis of C. H. SEDERHOLM, submitted to the Chemistry, Massachusetts Institute of Technology, January 1959. Acknowledgement financial support by the U.S. National Science Foundation (Grants G-1892 snd G-6230). t U.S. National Science Foundation Pre-doctoral Fellow, 1955-1959.
605
Department is made
of of
&. C. LORD
and
C. H. SEDERHOLM
The ammonia-oxygen diffusion flame has been studied in the visible region [lJ, but only recently have Dows et al. [2] investigated its infrared emission, using They found no emission to. be attributed to low-resolution prism equipment. NH, or NH. The emission spectrum of the hydrazine decomposition flame has been photographed by HALL and WOLPHARD [3], who found that the spectrum was produced by NH, and NH. No tiared’spectrum of this flame has been reported up to now, although it appears to offer an excellent possibility for observing NH, and NH because of the freedom of the flame from OH and water. It was hoped to study spectra originating at various heights in the flame and to gain information thereby about the course of the decomposition reaction. Study of the spectrum of the ammonia-oxygen flame was undertaken first. Despite the negative resuIts of DOWS et al. [2] it seemed possible that the considerably higher resolution achieved by the grating spectrometer might enable detection of NH, and perhaps NH. This study, which included the observation of some new bands and their tentative assignment to NH,, was completed before we learned of the recent work of TANNER and KING [4]. They report a low-resolution infrared absorption spectrum (ascribed to NH,) in the products of the flash photolysis of hydrazine.
Experimental Spectrometers Two small grating spectrometers of aperture f 4.0 were used [5, S]. One employed cooled lead-sulfide and lead-telluride photoconductive detectors and was used at frequencies above 2000 cm-l. The other covered the range 2000-600 cm-l with a Golay detector. In the absorption spectra reported below for comparison purposes, the spectral slit &idth employed was 0.5 cm-r. In the emission spectra, however, energy considerations forced the use of wider slits, which varied from a minimum of about 1 cm-r in the high-frequency portion of the ammonia-oxygen flame spectrum to as much as 4 cm-l for certain regions below 2000 cm-i in the hydrazine flame. Spectra were recorded on strip charts and the figures below were traced directly from the original records. At frequent intervals the mechanical slit width or amplifier gain was changed ; discontinuities in the curves make clear where these changes were made. Flume systems (a) Ammonia-oxygen diffusion Jame. The burner used was identical to the one described by Dows et al. [Z]. It was mounted with the ammonia-oxygen interface parallel to the optical path. The mounting permitted delicate horizontal motion of [I] [2] [3] [4] [5] [63
H. D. A. K. R. R.
G. A. R. N. C. C.
WOLFHARD and W. G. PARKER, Proc. Phyys. Sot. Dow, G. C. PIMENTEL and E. WHITTLE, J. Chem. HALL and H. G. WOLFHARD, Traw. Paraday Sot. TANNER and R. L. KINQ, Nuture 181, 963 (1958). LORD and T. K. MCCUBB~N, JR., J. Opt. Sot. Am. LORD and T. K. MCCUBEIN, JR., J. Opt. Sot. Am.
606
(A) 62,
(1949); Ibid. (1955). 52, 1520 (1956). Phys.
722 23,
499
45, 441 (1955). 689 (1957).
47,
65,
2 (1952).
Infrared
emission
spectra
of ammonia-oxygen
and hydrazine
flames
the burner perpendicular to the optical path, so that radiation from various sections of the flame beginning 1 cm above the top of the burner could be focused on the entrance slit by the conventional source optics. The source optics and spectrometer were enclosed in a housing which was purged with dry nitrogen to eliminate atmospheric absorption. The burner was placed about 1 cm from a rock-salt window through which the radiation entered the housing. Spectra were recorded both of the radiation from the bright yellow portion of the flame near the ammonia-oxygen interface and of that from the dim portion on the
.3-L,
nwrvrn
SPARK IGNITlON ELECTRODES 2Omm GLASS TUBING
ELECTRICALLY GLASS TUBING
,.. . . .
.
..
‘A. . .
.. -
.
.
.
ELECTRODES
HEATED
- “.“..‘--.. ---.
INTRODUCTION HYDRAZINE
OF
BOILING
WATER
I LITER
RESERVOIR
HEATING
COIL
Fig.
1. Apparatus
‘i
: i ; : : I 1
BELLOWS VALVES
NEEDLE
i :
:
‘/
HOLBEKEEPER
_
t ! LITER BOTTLE
SEALS
__
for production
of the hydrazine
SURGE
decompositionSflame.
ammonia-rich side of the burner. The spectrum of an oxy-hydrogen diffusion flame was recorded under similar slit conditions to establish the j?ositions and appearance of the emission lines due to water and the OH radical. The spectrum of this flame did not depend (apart from intensity) upon the region where the radiation originated. (b) Hydrazine decomposition Jlame. A schematic diagram of the apparatus for producing and regulating this flame is shown in Fig. 1. The flame, which was 607
R. C. LORD
and C. H. SEDEREOLM
stabilized at reduced pressures (75 mm and 40 mm) in a 5 1. flask on top of a chimney of 20 mm Pyrex tubing, was ignited by a spark between electrodes on either side of the chimney. The flask was evacuated with a pump isolated from the system by a The pumping rate was controlled by means of a stopcock in the 50 1. surge tank. pumping line. Hydrazine vapor from a reservoir of boiling hydrazine* travelled through two needle valves separated by a 2 1. surge bottle, and then up the chimney. The two needle valves and the surge bottle regulated the hydrazine flow rate and All parts reduced fluctuations in the flow caused by uneven boiling of the hydrazine. of the hydrazine supply system were heated and the temperatures of these various parts ‘were adjusted to stabilize the flame at the desired burning pressure. The temperatures were all near 100°C. Radiation from the flame was observed through a rock-salt window in the side of the flask. An unmagnified image of a horizontal section of the flame was focused on the vertical slit of the spectrometer by a spherical mirror and two appropriately oriented plane mirrors. One of the plane mirrors was mounted in such a way that segments at various heights in the flame could be reproducibly focused on the slit. All of this optical path as well as the instrument housing was purged with dry nitrogen to reduce atmospheric absorption. This technique was not as effective as it was with the ammonia-oxygen diffusion flame because of the bulk of the source optics.
Emission spectra in the range 4000-600 cm-l from the ammoniwxygen diffusion flame are presented in Fig. 2. The region 2420-2000 cm-l, which shows nothing but some feeble radiation from H,O and OH, is omitted. In the figure, curve A is a comparison spectrum of absorption by ammonia at room temperature, B is the emission spectrum from the dim, ammonia-rich part of the flame, C is that from the bright visible part near the interface between the two streams of gas, and D is the spectrum from the same part obtained when ammonia was replaced by hydrogen. Apart from the wider spectral slit widths mentioned above, the instrumental operating conditions were essentially those described in [5] and [6]. The emission spectra from the hydrazine decomposition flame are presented in Fig. 3. The spectral regions 2900-2000 and 820-600 cm-l are not presented because there is insufficient emission from the flame to be detected. Five emission spectra are presented in these figures. Curve a shows the emission of the flame at a point 18 mm above the top of the chimney with a pressure of 40 mm. Curves D, E, P and G were each recorded at a pressure of 75 mm. Curve D represents the emission 6 mm above the chimney ; curve E, 12 mm above the chimney ; curve P, 18 mm above the chimney; curve G, 24 mm above the chimney. Curve C is an atmospheric absorption spectrum taken with approximately the same slit widths as the corresponding curves D through a. By comparing the curve C with curves D though H, allowance can be made for apparent but spurious “emission bands” which result in fact from atmospheric absorption of the continuous emission from the flame. * For safety’s sake the reservoir of boiling hydrczine was placed outside the building adjacent to the room contein$g the remainder of the experimental set-up, and was screened by heavy stone slabs. One severe explosion of obscure origin occurred in the reservoir, but the damage was confined to the reservoir system.
608
Infrared
emission
spectra
of ammonia-oxygen
and hydrazine
flames
40OOcrr
Fig.
2 (i)
diffusion flame and oxy-hydrogen spectra of the ammoni*oyxgen Fig. 2. Emission spectrum from bright dame : curve A, absorption spectrum of ammonia; curve B , emission spectrum from ammonia-rich part of ammonia-oxygen diffusion flame ; curve C, emission spectrum from oxy-hydrogen part of ammonia-oxygen diffusion flame; curve D, emission diffusion flame.
1 I 37OOcm-’
I
.
*
I
.
I
t c 26OOcm-’
Fig.
L
I
2 (ii)
609
,
.
a
(
,
J 3500cm-1
3
’
*
’
R. C. LORD
md
C. H. SEDERHOLM
Curve B gives the absorption spectrum of gaseous hydrazine under high For a survey spectrum resolution and curve A, that of ammonia at room temperature. of gaseous hydrazine, the reader canrefer to GIGUI~E and Lm [7]. At one time, while the hydrazine decomposition flame was burning, a dry-ice trap was introduced in the pumping line at a distance of about 6 ft from the flame flask. After the flame had been running for several minutes, this trap was completely filled with white solid material which was mostly ammonia. The system was shut down and pressurized with/dry nitrogen, and the solid was allowed to warm up to room temperature. A good deal of ammonia gas was given off, but a small amount of liquid remained in the trap. The residual ‘liquid remained colorless for several days under nitrogen, but when the system was taken apart and the liquid exposed to air, it turned dark red in a period of several more days. This experiment was repeated several times. Each time, the liquid remained completely colorless as long as it was not exposed to the atmosphere, but after exposure, it turned color slowly. The amount of colored material seemed to be a function of the conditions under which the flame was stabilized. Its infrared absorption spectrum showed the presence of considerable hydrazine plus a general m&znge of other bands indicating traces of unknown components. No identification could be made. A separation of the active species was attempted by trap-to-trap distillation, but the amount involved was too small to isolate by this technique.
Discussion Spectrum
of the ammonia-oxygen
diffwion
jlame
By comparing
the emission spectrum of the oxy-hydrogen diffusion flame (curve that of the ammonia-oxygen diffusion flame (curves B and C), one sees that throughout most of the spectral region investigated the spectra of these two flames coincide closely except for overall intensity. Additional bands are present in the ammoni+oxygen flame in the regions around 1000 cm-l, 1600 cm-l and 2900 cm-l. Since the only species in the oxy-hydrogen flame which radiate in the infrared above 600 cm-i are H,O, which emits throughout the infrared, and OH radical, which emits from 2700 cm-l upward, we can say that the emission from the ammonis-oxygen diffusion flame in all regions investigated except the three mentioned above is due to H,O or OH radical. The almost exact coincidence of the emission due to H,O and OH in the two flames is somewhat surprising when one considers the difference in temperature between them. WOLFHARD and PARKER [l] give the temperature of the ammoniaoxygen diffusion flame as around 2000°C. According to GAYDON and WOLEMRD [8] the temperature of an oxy-hydrogen flame is about 2800°C. However, the similarity between the emission spectra of the two flames throughout most of the spectrum makes it reasonable to assume that any emission from the ammonia-oxygen flame which does not appear in the oxy-hydrogen flame is due to some species other than H,O or OH radical.
D, Fig. 2) with
[7] P. A. GICVX~E [8] A. G. GAYDON
end Hall,
and I. D. Lm, and
London
H.
J. Chem. Phys. Flumes,
G. WOLPBARD,
20, 136 (1952). Their Structure,
(1965).
610
Radiation
and Temperature.
Chapman
8.
.I
340ocm-
3300cm-’
.,..
Fig.
a:,,,,
3000cm-’
,,
,,,,,,,,.,,,,
32OOcm'
L
,,,
2 (iii)
,
I
2900cm
Fig.
3lOOCrn~
.,
eeoocm-
2 (iv)
611
,
.,
,.
,
, 27oom
R.C.
LORD
andC. H.SEDERHOLM
In the region from 850 cm-l to 1250 cm-1 there are numerous lines which under the above criterion cannot be accounted for by H,O or OH. Most of the lines can be assigned readily to emission by ammonia because of the close coincidence between them and lines in the absorption spectrum of ammonia at room temperature (curve A). Several differences can be noted, however, between the‘emission and absorption spectra of ammonia. In the emission spectrum the Q-branches at 930 and 960 cm-l have become broader on the jaw-frequency side, owing to the increased population of the upper rotational states, although the positions of the maxima appear unchanged. In the R-branch of this band the lines of maximum intensity have moved out toward .higher frequencies, again because of the increased population of higher rotational states. The P-branch of the band is almost entirely missing. If one looks at the absorption spectrum of ammonia in this region, the P-branch is found to be less intense than the R-branch, but is nevertheless readily observed. It is probable that the absence of the P-branch in emission and its diminished intensity’ in absorption are due to an effect which has previously been observed in diatomic molecules [9]. This effect increases the intensity of either the P-branch or the R-branch of a band at the expense of the other branch. It comes about as a result of the rotation-vibration interaction of the permanent dipole moment of the molecule with the vibrating dipole. Table
1. New emission lines from ammonia-oxygen
li’requency (cm-l)
Spacing (cm-‘)
3047.1*t: 3034.7t $ 3022.6t$ 3009*7t $ 2997.0*t1 297cott 2964.8 2950.6 2937.6 2921.3 2905.6 2591.4*
12.4 12.1 12.9 12.7 19.0 13.2 14.2 13.0 16.4 15.6 14.2
diffusion
Frequency (cm-l)
Spacing (cm-‘)
2875.2 2858.5 2842.9 2829.0 2813.2 2794.8 2780.7 2765.4 2748.6* 2736.4t 2718.6$
16.2 16.7 15.6 13.9 15.8 18.4 14.1 15.3 16.8 12.2 17.8
flame
* Measurement affected by emission lines of OH. t Measurement affected by emission lines of H,O. $ Line center uncertain because of unusual line breadth.
In the region from 1000 to 1100 cm-I there are some additional lines which do not coincide exactly with absorption peaks in room-temperature ammonia, but in view of the temperature difference it seemsimpossible to exclude ammonia as the origin of these lines. The variations between curves B, C and D in the region around 1600 cm-l are very small. These can probably be accounted for in terms of emission by the degenerate bending vibration of ammonia. In the present, investigation, the [9] R. HERMAX and R. F. WALLIS, J. Chem. Phya.
23, 637 (1955); see also earlier references there cited.
612
Infrared
I,,
emission
spectre
,
and
,
2700cm-’
70cm-
of ammonia-oxygen
I
2600cm-’
hydrazine
,
*
,
flames
,
,
,
,
25oocm-’
I,, ISOOcm-
19OOcm-’
Fig.
2 (vi)
613
,
,
,
,
,
,
R. C. LORD
and
C. H. SEDERHOLM
‘large amount of emission by ammonia in this region reported by DOWS et al. [2] was not observed. The region from 2700 to 3050 cti-l is of most interest. Table 1 lists a series of twenty-three lines which appear in this region of the spectrum.’ They are rather evenly spaced, as the second column of the table shows. If only the lines in the middle of this band, where identi&ation is more straightforward, are considered the average spacing is 15.5 cm-l. Some unassigned lines were observed by Dows et al. [2] in this region, which under their conditions were much weaker than the lines due to OH radical,. and they stated “These lines may be due to emission by water”. In the visible spectrum of the flame [l] emission from only a limited number of species was observed. These included OH, H,O, NH, NH, and NH,. Other species which might exist in the flame are HNO and any of the general formula N,H,. Comparison of the emission spectrum of the ammonia-oxygen flame with that of the oxyshydrogen flame shows that this series of lines does not appear in the latter. Two bands of emission lines due to OH radical were found in the oxy-hydrogen flame in this region and these agree well with’ the spectra of OH radical published by &EN et al. [lo]. In the spectrum from the bright yellow portion of the ammoniaoxygen diffusion flame, it is possible to see a superposition of the new lines on the bands due to water and OH radical. The only fundamentals of ammonia near this region are the two N-H stretching It will be noted that the Q-branch of v1 does not appear in the frequencies y1 and va. emission spectra at 3330 cm-l. If the Q-branch is too weak to be seen, it appears impossible for the P-branch of this band to be intense enough for observation in this region of the spectrum. As for v3, it should be still weaker than vl. Combination and overtone bands in the region have been investigated by BENEDICT et al. [l 11; The new lines in no way resemble their combination and overtone bands. Electronically excited ammonia might be considered as a source of this emission band, but such an assignment seems unlikely in view of the very short lifetime of electronic states. In addition, there is no evidence for vibrational transitions in electronically excited ammonia elsewhere in the spectrum. The energy of the tist known excited electronic state is too high (46167 + 30 cm-l) [ 121 to permit a large population at the temperature of the flame. Moreover, the spacing is too irregular for a symmetrical top, which is the structure of the ammonia molecule in both ground and first excited states [l3, 141. The rotation-vibration spectrum of the NH radical is a possibility, since its fundamental frequency should occur in this general region. However, its rotational constant [16] is 16.7 cm-l, which means that it should exhibit a rather regular rotational line spacing of 33.4 cm-l, in contrast to the observed spacings of Table 1. The remaining possibilities appear to be HNO, NH, and fragments of the formula N,H, with y = 1, 2, 3 or 4 (species containing more than two nitrogen atoms are [lo] H. c. k&EN, [ll] [12] [131 [14] 1161
W. A. A. W. G.
JR., L. R. BLAINE and E. K. PLYLER, Spctr~chim. Acta 9, 126 (1967). S. BENEDICT, E. K. PLYLER md E. D. !CIDWELL, J. Chem. Phya. 99, 829 (1958). B. F. DONCAN, Phya. Rev. 50, 700 (1936). B. F. Dmcm and 0;. R. -ISON, Phys. Rev. 49, 211 (1936). S. BENEDICT, Phys. Rev. 47, 641A (1936). ~RZBEW-3, i@ecfm of Dicchic Molecules (2nd Ed.). Van Nostrand, New York (1960).
614
Infrared
I 170ocm-
,
emission
,
,
,
spectra
,
,
I
of ammonia-oxygen
I
.
I 1600cm-~
I
Fig.
2 (vii)
Fig.
2 (viii)
616
I
and hydrezine
I
I
I
flames
.
I
I l500cm-1
I
R. C. LORD
and C.
Fig.
Fig.
H. SEDERHOLM
2 (ix)
2 (x)
Infrared
emission
spectra
I
of ammonia-oxygen
I
and hydrazine
flames
I
Fig.
2 (xi)
, VI a I
I
S.OOcm-1
I
,
I
L
,
,
,
I
,
,
70ocm-
Fig.
.
,
,
.
,
,
,
1
600cm-1
2 (xii)
617
R. C. LORD
snd C. H. SEDEREOLM
thought to be unlikely to give rise to a rotational spacing of 15 cm-l). Rotational constants A, B and C are known for HNO [16] (10.3, 1.29 and 1.15 cm-l, respectively) and for NH, [17] (23.62, 12.94 and 8.19 cm-l) and have been estimated [18] for hydrazine (4.52, 030 and 0.76 cm-l). Rough guesses can also be made for the others : 1 cm-l), N,H, (A < 9, B = C = 1 cm-l), N,H, (A < 6, N,H (A<17, B=C= B = C = 1 cm-l). All of the above species except NH, are approximately symmetrical-top molecules. If the direction of the vibrating electric moment is parallel to the A-axis, the rotational spacing of the corresponding vibrational band is B + C, and if the direction is per@endicular to the A-axis, the spacing is 2A - (B + C). Since there are probably no reasons of symmetry why the direction of the vibrating moments should be either parallel or perpendicular to the A-axis in most if not all of these molecules, the best that can be said is that the rotational spacing of their bands should lie between B+C and 2A-(B+C). .
Table
2. Series
of H,O
with
nearly
constant
spacing
Spacing (cm-l)
Frequency (cm-l)
Spacing (cm-l)
8.0 10.2 11.7 13.1 14.6 16.2
3734.8 3724.6 3712.6 3699.3 3684.6 3668.6
10.2 12.0 13.3 14.8 15.9
3722.6 3710.7 3697.2 3681.8
11.9 13.6 15.4
22.3 21.3 21.9
3688.6 3568.3 3547.0 3526.4 3506.0 3484.4 3462.5 3440.6 3417.7 33964 3372.6 3349.4 3325.8
20.3 21.3 20.6 20.4 21.6 21.9 21.9 22.9 22.2 22.9 23.3 23.6
3586.5 3566.9 3545.6 3524.8 3604.8 3482.6 3460.7 3438.5 34157 3393.3 3370.0 3347.3 3323.6
19.6 21.4 20.7 20.0 22.2 21.9 22.0 22.8 22.4 23.3 22.7 23.8
Spacing (cm-l)
4003.7 3991.1 3977.2 3962.7 3948.2 3932.8 3917.8 3902.7
12.6 13.9 14.6 14.5 15.4 16.0 15.1
3610.9 3488.6 3467.3 3445.4
lines
Frequency (cm-l)
Frequency (cm-l)
3744.5 3736.5 3726.3 3714.6 3701.6 3686.9 3670.0
of emission
[16] D. A. RAMSAY, Ann. N.Y. AC&. Sci. 67, 485 (1957). [17] D. A. RAMSAY, J. Chem. Phya. 26, 188 (1966). [18] D. W. Scorn,. G. D. OLIVER,‘ M. E. GROSS, W. N. HUEBARD rsoc. 71, 2293 (1949).
618
and
H. M. HICKMAN,
J. Am.
Chem.
I&am&
emission
spectra I
’
of wnmonia-oxygen 7
’
and hydrazine .
’
’
’
flemee
’
.
Fig. Fig.
3 (i)
3.
Emission spectra of the hydrszine decomposition flame: c~llre 4, absorption of ammonia; curve B, absorption spectrum of gaseous hydrazme; Cm~e 0, &m&pheric absorption spectrum under resolution comparable to CupVes D-H; Cm+~es D-H, emission spectra of the hydrazine decomposltlon flame: D, h= 6 -, P= 75 mm, E, h=l2 mm, ~~76 mm; ,P, h=lS mm, p=76 mm; Q, h=24 111l11,p=76 mm; H, h= 18 mm; P=40 nun.
spe&um
Fig.
3 (ii)
619
R. C. LORD
end
C. H. SEDERHOLM
It can thus be concluded that the values of B + C are too small in all cases, and only for HNO, N,H and NzHz are the values of 2A - (B + C) large enough to give a stretching spacing of 155 cm- 1. In these three molecules it appears that the N-H vibrations would give rise to hybrid bands of more nearly parallel than perpendicular character (except for one vibration of the G-form of N,H,), but this is not sufficient basis for eliminating them as possible sources of the observed lines. The rotational constants of the NH, radical fit the magnitude of the spacing somewhat better. To be sure, NH, is an asymmetrical top and a more complicated spectrum is expected than a series of lines with approximately constant spacing. However, study of the emission spectrum of H,O in the region 3000-4000 cm-l shows that an asymmetrical top can in fact give rise to series of evenly spaced lines. In Hz0 the bands in the above region arise from the OH stretching vibrations vi and +, and the series of even line spacings arise from transitions between the lowest K-, levels of adjacent values of J. The expected spacing is no larger than B +C and can be somewhat smaller than 2C. A number of these series of water lines and their spacings are listed in Table 2. Since for H,O [19], A = 27.79, B = 14.51, and C = 9.29 cm-l, it is seen that there are examples of the spacing running all the way from less than 2C up to B+C. Considering a similar effect in NH, with rotational constants as given above, one might expect it to give rise to an emission band with lines of roughly constant spacing at 155 cm-l. The spectroscopic data therefore suggest that the series of lines of Table 1 is probably due to NH,, N,H, N,H, or HNO, with some preference for NH, over the other species. This preference is strengthened when the kinetics of the flame are considered. The only way in which the species other than NH, can be formed is by three-body collisions between an NH group on the one hand and a second NH group, an N atom, or an 0 atom on the other. The probability of three-body collisions involving two free radicals is very low since the free radicals are in low concentration. On the other hand, NH, can be formed by a two-body collision involving one free radical and the ammonia molecule, which is present in high concentration. Therefore, one expects the concentration of NH, to be much higher. It is to be noted that the emission of the unknown species is stronger in the ammonia-rich region of the flame than it is in the brightest part of the flame. In the visible spectrum, on the other hand, the most intense emission from NH, comes from the brightest part of the flame [l]. These facts might appear to argue against the assignment of the infrared lines to NH,, but intensity of emission depends on important factors other than concentration (temperature, electronic excitation, etc.) and therefore it is not necessary that the maxima of visible and of infrared emission by the NH, radical occur at the same point in the flame. Recently TANNER and KING [4] reported observing NH, in infrared absorption They found a broad band centered under low resolution by means of flash photolysis. at 3200 cm-‘. This is not in contradiction to the assignment of lines in the 280~~ 3090 cm-l to NH,, and the large amount of emission by H,O above 3050 cm-1 would obscure the center of the NH, band in our spectra. [19] G.
HERZBERQ,
Infrared
and Raman
i3pctra.
Van
620
Nostrand,
New
York
(1945).
Infrared
emission
spectra
of ammonia-oxygen
Fig.
Fig.
3 (iii)
3 (iv)
621
and hydrezine
flames
R. C. LORD and C. H. SEDERHOLM
Spectrum of the hydrazine decomposition J’lame In this flame one expects to see emission by ammonia, hydrazine and fragments of hydraxine. If the ammonia-oxygen flame is a reliable guide, the emission spectra of ammonia and hydrazine should be similar to their absorption spectra at room temperature (curves A and B of Fig. 3). It is of interest that the relative intensities of the lines in the emission spectra (with a few exceptions to be discussed further) are independent of the p&t of the flame studied and of the two operating pressures. This shows that none of the species which are in sufficiently large concentration to be detected by their infrared emission is cormned to a small region in the flame. The clearly observable structure of the visible flame must therefore result from variation in concentration of species (for example, electronically excited NH,) whose infrared emission spectra are too weak to measure. In the region 600-850 cm-l there was no detectable emission. There is a rather sharp. band between 835 and 965 cm-l which does not seem to be due to ammonia because of its frequency and lack of inversion doubling. It may be slightly sharpened because of re-absorption by ammonia in the optical path within the flame flask. Since hydrazine itself has a broad absorption band in this region, it might be responsible for the observed emission. The hydrazine absorption appears very much broader than does the emission band, possibly because of the exponential relationship between transmission and molecular concentration, as compared to the linear dependence of emission on concentration. Table
3. Comparison
Designation
--
of emission lines in hvdrazine absorptik lines
extrapolation
of observed
-
Observed ammonia lines (cm-l)
Designation
--
Calcd.* ammonia lines (cm-‘)
Observed flame lines (cm-l)
1177.0 1194.9 1212.6 1230.1 1247.4 1264.5 1281.4 1298.1 1314.6 1330.9 1347.0
1195.6 1213.0 1230.7 1247.8 1265.1 1281.4 1298.3 1314.3 1330.6 1346.3
1007.5 1026.9 1046.3 1065.5 1084.6 1103.5 1122.1 1140.6 1158.9
* Extrapolation
flame with
Difference (cm-l) 0.7 0.4 0.6 0.4 0.6 8:; -0.3 -0.3 -0.7
of observed lines with second dBerences assumed constant.
There is considerable structure around 1100 cm-i which corresponds neither to hydrazine absorption nor to that of ammonia. This structure must be due to some other species but there is insufficient basis for assignment. In the spectrum taken 24 mm above the chimney (curve C of Fig. 3), there is de&&e evidence of re-absorption by ammonia at 1122.1, 1140.6 and 1168.9 cm-l. This does not show up in any of the other curves in this region. It should be noted that in this same spectrum, there is a broad maximum at about 1130 cm-i, absent from all the other spectra, 622
Infrared
emission
spectra
of emmonitGoxygen
Fig.
3 (v)
Fig.
3 (vi)
623
and hydra&e
&mea
R. C. LORD and C. H. SEDEREOLM which is also diflicult to assign. These unassigned bands continue in all of the spectra as far as about 1200 cm-l. At I 196 cm-i a series of regularly spaced lines begins in all spectra, which looks like a continuation of the SE-branch of ammonia. In Table 3 calculated ammonia lines are compared with the observed emission lines. The agreement furnishes an excellent basis for assigning the series to ammonia. In the hydrazine flame as in the ammoniaoxygen flame, it is found that the P-branch of the inversion vibration of ammonia is missing whereas the R-branch is quite intense. Toward higher tiequency, the next band of interest is centered around 1615 cm-l. Hydrazine has intense absorption at this point and this band can probably be attributed to hydrazine. Again, the difference in- width of the band in absorption and emission is probably due to the comparison of an exponential function with a linear appears in the emission spectrum which is not apparent one. A good deal of structure in absorption. This structure is difficult to assign to hydrazine, as one would not expect new fine structure to appear at a band center as the temperature is elevated. Other species with NH, groups are probably emitting in this region, producing the added structure. From 1615 to 1900 cm-i there seems to be a broad emission band with very little structure in it. Most of the apparent structure is due to the absorption of continuous flame radiation by water vapor in the optical path. From 2000 to 2950 cm-l the intensity of the emission was too small to be recorded. It was rather disappointing that no emission band was observed in the hydrazine flame at the position of the band which has been assigned to NH, in the ammoniaoxygen flame. NH, is certainly present, but must be in too low concentration to be detected. Indeed, the visible bands of NH, in this flame are much less intense than the same bands in the ammonia-oxygen flame. Above 2950 cm-l, there are again a large number of emission bands. In general these do not correspond to the absorption bands of either ammonia or hydrazine. The strong Q-branch of v1 of ammonia at 3335 cm-i is apparent in emission as a shoulder on the side of a much stronger band at 3325 cm-l. This strong band is at the same frequency as an absorption in hydrazine. Again, this band seems too sharp to be due to hydrazine, but the difference between absorption and emission spectra may account for the effect. There is another strong emission line at 3848 cm-1 with structure on either side of it. Neither ammonia nor hydrazine absorbs at this point. It is hard to imagine any species containing ody nitrogen and hydrogen which would have a fundamental in this region, SO this must be a combination or overtone band. It should be mentioned that the superior sensitivity of the photocell used at this point tends to exaggerate the apparent emission intensity compared with bands below 2000 cm-l, where thermal detectors must be employed. It is safe to assume that none of the observed emission lines in Fig. 3 are due to water vapor as an impurity in the hydrazine. At one time a small crack occurred in the bottom of the reservoir flask which allowed some water to contaminate the hydrazine. Water emission peaks corresponding exactly to the water emission from the oxy-hydrogen flame were immediately noted in the spectrum. None of the peaks shown in the spectra of the hydrazine flame corresponds to water peaks from the oxy-hydrogen flame. 624
Infrared
emission
spectra
of ammonia-oxygen
Fig.
3 (vii)
Fig.
3 (viii)
625
and hydrazine
flames
R. C. LORD
and C. H. SEDERHOLN
In conclusion, there are many lines and bands in the emission spectrum of the hydrazine decomposition flame which are not produced by ammonia or hydrazine. These are due to intermediate species in the flame, but with the data at hand, it is impossible to make any assignment of them since all the species in question have NH stretching and bending frequencies which lie in the regions of the unassigned bands. Further resolution of fine structure in these bands, or comparison of the spectra reported here with/spectra from other sources, may ultimately lead to their assignment and to more detailed understanding of the chemical processes of the flame., Acknou&xIgements-The ALLEN and one with
Dr.
writers wish to acknowledge H. G. WOLFHARD.
626
several
helpful
discussions
with
Dr.
H.
C.