CHEMILUMINESCENCE FROM OXYGEN ATOM-HYDRAZINE FLAMES
371
A Study of the Chemiluminescence from Oxygen Atom-Hydrazine Flames
by K. H. Becker' and K. D. Bayes Department of Chemistry, University of California, Los Angeles, California 900.24 (Receiued August 8,1966)
The behavior of the y bands of NO, observed in emission from low-pressure oxygen atomhydrazine flames, suggests that the electronically excited NO is formed by energy transfer rather than directly in a chemical reaction. The addition of NO to the flame increased the emission intensity of the y bands and decreased the emission from NH. The energy carrier appears to have a lifetime longer than 3 X sec, suggesting a metastable state. The most likely energy carrier is the lowest triplet state of nitrogen, Nz(A3Z). Two additional experiments support this assignment : the addition of mercury vapor to the flame results in emission of the 2537-A mercury line and addition of NO to a system known to contain N2(A3Z)resulted in the almost exclusive emission of the y bands of NO.
Introduction Mixtures of gaseous hydrazine and atomic oxygen undergo a rapid chemiluminescent reaction. The emitting species have been identified as the NH, NH2, OH, and NO molecule^.^^^ The excitation mechanism for any of these species is unknown. A variety of radicals and intermediates are known to be formed from hydrazine. The photolysis of N2H4 can form both the NH and the NH2 radical^.^ In addition, the OH and KO molecules were observed when N2H4 was photolyzed in the presence of 02.5Intermediates such as NHz, NzH3,and N2H2have been observed mass spectrometrically in the N2H4 phot~lysis,~ in a hydrazine decomposition flame,' and in the products from a high-frequency discharge in h y d r a ~ i n e . ~Recently, ~~ the NzH2 molecule has been observed by infrared absorption in :t matrix following the decomposition of S2H4 or HN3.10r11 The only observed emission from the NO molecule in the GNZH4 flame is the y bands, corresponding to the transition A21: + X 2 r . Excitation of the A2Z state requires 126 kcal/mole. There is no obvious reaction between any of the known intermediates and an oxygen atom that is sufficiently exothermic to form NO directly in this excited state. The current investigation concerns primarily this chemiluminescence of the NO molecule.
Experimental Section A schematic diagram of the apparatus is shown in
Figure 1. The oxygen atoms were generated by flowing a mixture of Ar and 0 2 through a microwave discharge in a cylindrical cavity (2450 MHz)12 operated at a power of 80 w. The oxygen atom concentration in the reaction chamber was controlled by varying the O2 content of the argon up to a maximum of 2%. Relative oxygen atom concentrations were measured by adding a known amount of NO through one of the inlets A or B and observing the intensity of the NOz emission with a photomultiplier (RCA 6199). A simple red filter in front of the photomultiplier removed wavelengths shorter than 6000 A. It has been shown that the NOn emission intensity is proportional to (0)(NO) (1) Requests for reprints should be directed to this author a t the Institut f ~Physikalische r Chemie, Universitat, Bonn, West Germany. (2) (a) G. E. Moore, K. E. Shuler, S. Silverman, and R. Herman, J . Phys. Chem., 60, 813 (1956); (b) A. R. Hall and H. G. Wolfhard, Trans. Faraday SOC., 52, 1520 (1956). (3) H. Guenebaut, Bull. SOC.Chim. France, 26, 962 (1959). (4) D.A. Ramsay, J . Phys. C h m . , 5 7 , 415 (1953). (5) D. Husain and R. G. W. Norrish, Proc. Roy. SOC. (London), A273, 145 (1963). (6) F. I. Vilesor, B. L. Kurbator, and A. N. Terenin, Dokl. Akad. Nauk SSSR, 122, 94 (1958). (7) K. H.Homann, D. I. MacLean, and H. G . Wagner, NUturVJk8enshafta9 523 l2 (1965)* (8) S. N. Foner and R. L. Hudson, J . Chem. Phys., 2 8 , 719 (1958). (9) S. N. Foner and R. L. Hudson, ibid., 29, 442 (1958). (10) E.J. Blau and B. F. Hochheimer, ibid., 41, 1174 (1964). (11) K.Rosengren and G. C. Pimentel, ibid., 43, 507 (1965). (12) Cavity 2A described by F. C. Fehsenfeld, K. M. Evenson, and H. P. Broida, Rev. sci. Instr., 36, 294 (1965).
Volume 71. Number I January 1967
372
K. H. BECKERAND K. D. BAYES
DISCHAME
n NEEDLE
SPEClROMElIR
I PUMI
Figure 1. Schematic diagram of the apparatus.
in this pressure range.13 However, for a given oxygen atom flow in the above apparatus, it was observed that the continuum intensity was less than linear in (NO) at the higher addition rates of nitric oxide due to the consumption of oxygen atoms within the reaction vessel. But for a given NO concentration the NO2 emission intensity was proportional to the inlet oxygen atom concentration, since the fraction of 0 atoms consumed within the vessel depended only on the NO pressure, the total pressure, and the residence time, all of which were constant. Thus the continuum intensity, at a constant NO partial pressure of 11.5 mtorr, was used as a measure of the relative oxygen atom concentration. This relative atom concentration was converted into an absolute Concentration a t the inlet by titrating with NO2, using the disappearance of the NO2 emission as the end point.14 It is this absolute concentration, in terms of partial pressure of atomic oxygen at the inlet, which is of significance, since the fast 0-NZH4 reaction should be confined to the vicinity of the inlet. Small partial pressures of the added gases (NO, NO,, and NzH4) were controlled in the following way. Known mixtures of the added gas or gases were made with argon in a 3-1. bulb up to a total pressure near 1 atmosphere. After thorough mixing a controlled flow of this mixture was admitted through inlets A or B sufficient to increase the total pressure in the reaction chamber from 5.0 to 6.0 torr as measured with the O-20-torr Wallace and Tiernan diaphragm gauge. The partial pressure of the added gas in the chamber mole fraction in the was then 1 torr times its original 3-1. bulb, since the volume flow rate was approximately constant. All quantitative experiments were carried out a t a total pressure of 6.0 torr. The reaction chamThe Journal of Physical Chemistry
ber volume was approximately 0.3 1. and the flow rate 0.7 l./sec a t 6 torr. The chemiluminescence of the 0-NzH4 reaction was investigated qualitatively with a l-m Ebert spectrometer (Jarrell-Ash Co.) having a dispersion of 1 mm/16 A and an effective aperture fl8.6. A search for emission below 2000 A was made with a l-m McPherson vacuum spectrometer. Quantitative intensity measurements of the NO emission were made with a 0.25-m monochromator (Bausch and Lomb) using a RCA 1P28 photomultiplier. A band pass of about 200 A centered on 2500 A allowed both the 0,2 and 0,3 y bands to be measured. Time constants of 0.5-2 sec were used. Intensity measurements of the NH and OH bands were taken from peak heights on the scanned spectrum using the Ebert spectrometer. Gaseous hydrazine was taken directly from the commercial liquid (Matheson Coleman and Bell, 95% anhydrous) and used after degassing a t Dry Ice temperature. Further purification was unnecessary since it was shown that small amounts of KH3 and H2O did not affect the flame. Both the NO and KO2 were purified by repeatedly distilling at low temperatures in the vacuum system. Commercial argon (Liquid Carbonic, 99.99%) and oxygen (Gordon Duff, 99.975%) were used without further purification.
Results Figure 2 shows a typical emission spectrum of the GN2H4 flame. The observed bands can be assigned to electronic transitions of the NH molecule, (A3a + X32), the OH molecule, (A22 X 2 r ) , and the y bands of NO, (A22 + X2r). The NO2 continuum, due to the reaction of 0 with NO, is also present at longer wavelengths. No great efforts were made to observe the NH2 emission since, at the resolution used, it would be easily lost in the NO2 continuum. The addition of argon, up to a total pressure of 150 torr, had little effect on the intensity of the NH, OH, or NO bands. The addition of NO, however, increased the NO and the NOz emission strongly and decreased the N H emission, as can be seen in Figure 3. The OH emission was relatively constant, decreasing only slightly with large additions of NO. For small addition rates of NO, the NO and NOz emission intensities were approximately linearly dependent on the NO partial pressure. These two intensities can be extrapolated to a common zero at about - 3 mtorr (Figure 3): this is interpreted simply as the amount of NO --f
(13)F. Kaufman, Proc. R O ~ . sot. (London), ~ 2 4 7 ,123 (1958). (14) F. Kaufman, J. C h m . Phys., 28, 352 (1958).
CHEMILUMINESCENCE FROM OXYGENATOM-HYDRAZINE FLAMES
3400
3200
3000
373
2800 Wavelength, A.
2400
2600
2200
Figure 2. Emission spectrum from the oxygen atom-hydrazine flame. Partial pressures before reaction: (0) = 14 mtorr; (N2H4)= 6.7 mtorr. No added (NO). Total pressure 6 torr. Ebert spectrometer, 0.2-mm slits, EM1 62568 detector. 1
I
.-d a
li
+= 150 . I
P
2
0
10 20 30 Partial pressure of added NO,mtorr.
40
Figure 3. Emission intensity as a function of the partial pressure of added nitric oxide: 0,NO(A22-t X%); 0, "(A*, + XYZ); A, NO2 continuum. Each emission in different arbitrary units. The original hydrazine pressure was 3.5 mtorr for the NO and NO2 measurements and 13 mtorr for the NH measurements. Total pressure 6 torr.
which is formed by the complete oxidation of the original 3.5 mtorr of hydrazine. The intensity of the NO y bands, INO,is plotted in Figure 4 and Figure 5 for a variety of conditions. The abscissa represents the absolute oxygen atom concentration a t the inlet with no hydrazine present. The curves show a dktinct maximum in the NO emission a t an oxygen atom partial pressure of about 14 mtorr. Measurements on the NOz emission during the hydrazine addition indicate that very few oxygen atoms survive the flame until there is an excess over that needed to completely oxidize the hydrazine. This
e
L*
100
60
0
-
0
d
20
10
30
( O ) , mtorr.
Figure 4. Emission intensity of the NO y bands as a function of the original oxygen atom pressure for different amounts of added NO: A, 3.5 mtorr; 0, 6.8 mtorr; 0, 13.6 mtorr; 0,21.9 mtorr; A, 38.6 mtorr. Original hydrazine . pressure was 3.5 mtorr for all data.
confirms the earlier observation2&that the O-NZH~ reaction is very fast. The reaction of hydrazine with hydrogen atoms, made in a manner similar to the oxygen atoms, resulted in no observable light emission. Very weak emission (
