NOTES
2238 anion radical of dibenzo [b,f]thie~in,~ the spin density in the sulfoxide radical at the aromatic ring position para to the sulfur atom (position 2) is small. Thus, conjugation through the sulfoxide bridge appears to be much weaker than conjugation through the vinyl resi'd ue.
Acknowledgments. We gratefully acknowledge the support of the National Science Foundation.
observations of the AT2(A32,+-X'B, +) and N(2P-4S) transitions were made along the 10-cm path length. Several reaction vessels were used in attempts to enhance mixing and increase emission intensities; the same general behavior reported below was found for all vessels. The N2(A3Z,+) molecules were generated in one discharge-flow system using collisions of argon metastable atoms (3P~,Jand molecular nitrogen;'-lo the concentration was 109-10'0 molecules/cm3 which was sufficient for direct monitoring of the forbidden Nz(A3Zu++ X'B,+) Vegard-Kaplan emission by viewing the flow tube end-on. The Ar" Xz system is a l ( clean" s o u r ~ e , of ~ JN2(A3Z,+); ~ i e . , it is free from other reactive species such as nitrogen atoms. This source gives almost equal concentrations of the 0 and 1 vibrational 1evels'O which permits study of the role of vibrational excitation in the reactions of Nz(A32,+). The second discharge-flow system was a flow of pure nitrogen or a flow of nitrogen in an argon carrier through a microwave discharge. The concentration of nitrogen atoms was measured using the nitric oxide titration technique and by monitoring the nitrogen first positive bands of the Lewis-Rayleigh afterglow" with a bare photomultiplier tube which viewed across the reaction vessel. The concent,rationof nitrogen atoms was varied by altering the discharge power in pure nitrogen or by altering the flow of molecular nitrogen into the argon carrier a t constant discharge power. Spectroscopic observations were made with a 0.75 m Jarrell-Ash CzernyTurner scanning spectrometer equipped with an EM1 9558Q photomultiplier and a PAR phase sensitive amplification system. Use of maximum sensitivity and largest slit widths was necessary to measure the very weak ?J(2P -+ 4S), T = 12 sec,12 and N z ( ~ ~ , + XIZ,+) emissions, T = 2.1 ~ e c . ~ ~ , l *
+
Energy Transfer Reactions of N2(A3Z,+).
11. Quenching and Emission by Oxygen a n d Nitrogen Atoms by J. A. Meyerl D. W. Setser, and D. H. Stedman Chemistry Department, Kansas State University, Manhattan, Kansas 66602 (Received January 26, 1970)
One of the major processes for removal of N2(A3Z,+) in active nitrogen involves reaction with ground state nitrogen atoms.'-3 Young2 obtained a rate of 3 X 1013 cm3 mol-' sec-I from measurements of the decay time of Y2(A3Z,+)in the presence of N(4S), and Wray4 obtained a similar value from shock tube studies. Thrush6 has suggested a value which is a factor of 10 less and, most recently, Weinreb and Mannella6 deduced an upper limit 3 X 10" cm3 mol-' sec-' for N2(A3Z,+) in high vibrational levels. None of these studies, however, has been concerned with the disposition of the high exothermicity (6.17 eV from N2(A3Z,+, v = 0) to N2(X1Z,+,v = 0) of this process. This energy may go into excitation of the N(T(3.57 eV), 2D(2.38 eV)) states and/or vibrational excitation of N2(X1B,+). We have found definite evidence that some of the released energy goes into excitation of the N(2P)state by directly observing N(2P-+ 4S) emission ( T = 12 sec) at 3466 A. This reaction may be an excitation source for 3466 A emission in the aurora. We previously showed that N2(A3Bu+)reacts with O(3P) to give excitation of O(%) emission (T = 0.8 sec) a t 5577 A. Experimental Section Our system employed a double-discharge flow apparatus in which a flow of K2(A32,+)molecules and a flow of nitrogen atoms were brought together just upstream of a quartz window. This window formed the front end of a 10 cm long reaction cell (22-mm i.d. quartz). For our pumping speeds the time for the gas to flow through this 10-cm reaction vessel was 4 msec. Spectroscopic The Journal of Physical Chemistry
(1) R. A. Young, Can. J . Chem., 44, 1171 (1966).
(2) R. A. Young and G. St. John, J . Chem. Phys., 48, 896 (1968). (3) C. H. Dugan, J. Chem. Phys., 47, 1512 (1967). (4) K. L. Wray, ibid., 44, 623 (1966). (5) B. A. Thrush, ibid., 47, 3691 (1967). (6) M. Weinreb and G. G. Mannella, ibid., 50, 3129 (1969), (7) J. A. Meyer, D. H. Stedman, and D. W. Setser, Astrophys. J., 157, 1023 (1969). A spectrum of the Vegard-Kaplan emission obtained from our apparatus is shown in this reference. See text for improved values of rate constants for 0 and Oa with Nz(A) previously reported by this reference. (8) D. H. Stedman, J. A. Meyer, and D. W. Setser, J. Chem. Phys., 48, 4320 (1968). (9) D. H. Stedman and D. W. Setser, Chem. Phys. Lett., 2, 542 (1968); J. Chem. Phys., 50, 2256 (1969). (10) D. W. Setser, D. H. Stedman, and J. A. Coxon, ibid., in press. (11) A. Wright and C. A. Winkler, "Active Nitrogen," Academic Press, Inc., New York, N. Y., 1968. (12) R. H. Garstang, "Aurora and Airglow," Pergamon Press, New York, N. Y., 1956, p 324. (13) D. E. Sheniansky and N. P. Carleton, J . Chem. Phys., 5 1 , 682 (1969). (14) D. E. Shemansky, ibid., 51, 689 (1969).
NOTES
2239
Results and Discussion No nitrogen atom emission was detected in the mixing region when either discharge was turned off. However, with both discharge-flo? systems operating, N(2P + emission a t 3466 A was observed. This emission intensity for constant [N] was studied as a function of Nz(A32,+)concentration by simply varying the discharge current in the Nz(A32,+)source over a small current range. The intensity of the (0, 6) and (1,9) bands from the NZVegard-Kaplan emission served as a measure of the concentration of N2(A32,+). Typical results of such an experiment are shown in Figure 1, and a linear correlation is evident. The N(2P+ emission a t 3466 A was also studied as a function of [N] a t constant [N2(A32,+)] and a linear correlation existed as shown in Figure 2. The linearity of the
cline in intensity of the (0, 6) and (1, 9) Vegard-Kaplan bands as a function of [N]; both levels behaved approximately in the same way, although the v = 1 level may have a 10% higher rate constant. No emission from the N(2D) state (N(2D-4S),7 = 26 hr)12 was observed; our detection system is not !dequate for detecting the N(2P --+ ZD) line a t 10,400 A. This transition also is weak and would be obscured by Nz first positive bands." The rate constant for quenching of N2(A3Z,f) by N atoms was estimated in two ways.
30
I
2ol 16
0"
2
4 f4 A
(N]
IO'^
1'0
:'2
1'4
atoms/cm3
Figure 2. Variation of t h e intensity of the N(2P-PS) transition with [N] a t a constant initial concentration of N2(ASZ,+), total pressure = 4.3 Torr. The emission was viewed along the reaction vessel; t h e deviation from linearity probably arises from significant quenching of Nz(AaZ.+), thereby causing [Nz(AQ,)+] t o decline.
I
,
1
203
I
I
40
N,(A c - X Z: )
I
60
Intensity
I
The quenching of nitrogen atoms was compared to molecular oxygen by increasing [N] until 30% quenching of Nz(A) emission was obtained; this degree of quenching then was matched by adding Oz, and kl was found from h [ N ] = k 0 , [ 0 2 ]a t the same degree of quenching. Using a value16 for ko2 of 3.6 X 1OI2 cm3 mol-' sec-' gives kl = 3.1 X 1013cma mol-' sec-l. The second method employed Hg as a monitorg of Nz(AaZ,+) concentration. Mercury was added to the
I
BO
Figure 1. Variation of t h e intensity of the N(2P-4SS) transition with Nz(A3Z,+)concentration a t a constant [N]; t h e total pressure is 3.2 Torr. T h e (1, 9), 0, and (0, 6), 0, bands were used to monitor t h e concentration of Nz(A*&+). T h e intensities from both emissions are in arbitrary units.
figure extends to [N] = 1 X 10laatoms/cm3 in an argon carrier gas. Experiments with molecular nitrogen as the N atom carrier showed deviation from linearity at approximately the same [N]. These data provide conclusive evidence for reaction 1. N2(A3&+, v = 0, 1)
+ N(4S) + Nz(X'&+)
+ N('P)
(1)
The relative quenching behavior of the v' = 0 and 1 levels of Nz(AaZU+)was studied by observing the de-
(15) (a) J. A . Meyer and D. W. Setser, unpublished work. Steadystate measurements of Nz(A82, +) reactions in active nitrogen (see ref 9) have been done; the quenching rate for OZ was 4.6 X 10'2 cm3 mol-' sec-1. (b) R. A . Young, G. Black, and T . G. Slanger, J. Chent. Phys., 50, 303 (1969). This investigation used the vacuum uv photolysis of Nz0 as a source of Nz(A32,+) and found koa = 2.3 X 1012 em3 mol-' sec-1. (c) We also measured koz directly by the mercury technique in the present double discharge study and found ko2 = 3.6 X 1012 cms mol-' sec-1. Quenching measurements indicate the Nz(ASZ,+) v = 1 level reacts 1.3 times more rapidly than the v = 0 level with Oz. This ko2 is for an equal mixture of 'L' = 0 and 1; the values listed in (a) and (b) are for an unknown mixture of vibrational states. Adding NO downstream to the products from NdA) OZgave strong air afterglow (0 NO) emission. Therefore, a significant fraction of OZ quenching leads to formation of oxygen atoms.
+
+
Volume 7 4 , Number 10 M a y 14, 1970
2240
NOTES
flowing gas until the intensity of the Hg(3P1-1SO) transition a t 2537 was sufficient to be observed (perpendicular to the reaction vessel) 6 cm downstream from the point of mixing. This small quantity of Hg did not cause quenching of gegard-Kaplan emission. The variation of the 2537-A mercury emission with [N] was monitored 6 cm downstream from the point of mixing. The intensity fitted a first-order decay plot; combining the slope with the known flow velocity gave a rate constant of 2.6 X 1013 cm3 mol-’ sec-’. Our measurements thus confirm Young’s and Wray’s value of 3 X 1013cc mol-’ sec-’ for the quenching of N2(A) by N(4S) rather than the lower values proposed by otherse5 Results similar to those depicted in Figures 1 and 2 previously have been reporLed7 for excitation of the O(’S-’D) emission at 5577 A by N2(A3Z,+, v = 0,l). That work was repeated in the present apparatus and reaction 2 was confirmed. The N2(A3Z,+)v = 0 and 1 levels reacted at nearly the same rate, as was found m6
iYz(A3Zu+,v
=
0, 1)
+ O(3P) +
first term of the denominator be 0.20 of the second. The role of wall removal is unknown but it may be the dominant step; then our experiments only set a limit to IC3 based upon the competition between k3[N] and k(wal1). The N(2P-4S) line is absent from spectra of the airglow as is Nz Vegard-Kaplan emission; however, both of these emissions ar,e present in the spectra of the aurora,lg as is the 5577-A oxygen line. It is evident, therefore, that N(2P-4S)emission at 3466 8 is present in systems where both N(4S) and N2(A3Z,+) occur and our data show that reaction 1 may be one source of this emission. Very recently, Weinreb and Mannella20 suggested that the products from a microwave discharge in O2 (probably excited O2 molecules) induce a collisional crossing from high vibrational levels of N2(A3Z,+), formed by N atom recombination at metallic surfaces, into N2(B3a,). I n view of the relatively rapid vibrational relaxationlo of N2(A32,+) molecules by piz to levels v = 0 and 1 and of the fast removal of N2(AaZU+) by 0 2 , 1 5 c N, and 0 directly demonstrated by our work, an alternative explanation of this phenomenon should be considered. The question arises whether reaction 1 is an energy transfer step or an atom exchange reaction and whether reaction 1 is the only reaction channel for N atom quenching of N2(A3Zu+). A direct answer will require 15K labeling experiments. A linear correlation diagram shows that N3 states formed from K(4S) NZ(A3&+) do not correlate with states from X(2P) 4 K2(X1Z,+). Nonlinear N3 states (at present unknown) 21 would make such a correlation possible. The N(4S) N2(A3Zu+)linear states may correlate with K(4S) S2(X1Z,+) or with pi(”) Ne(X’Z,+); however, unknown Z - states of N3 are needed. At the present time an energy transfer collision is the most attractive mechanism for reaction 1.
for reaction 1. Absolute intensity measurements previously were niadeIBfor this case and 4 rt: 2 AT2(A38,f) molecules gave one 5577-B proton. l6 We suggested that collisions with the wall and reactive impurities remove O ( % ) and that reaction 2 is a significant and perhaps the main channel in the overall quenching of i\-z(A3Zu+) by oxygen atoms. The rate constant for overall quenching by oxygen atoms was remeasured by comparison with 02,and kz/koz = 3-4 was confirmed. Using a better estimate150for ko2 gives k2 = 1.3 X 1013 cm3 mol-’ sec-’. The N(2P-4S) line is only very weakly present at low pressures in the Lewis-Rayleigh” afterglow, but a t higher pressures (20-760 Torr) N0x0nI7 observed this emission with an “ozonizer type” discharge through Acknowledgments. This work was supported by the pure nitrogen, He also observed N2 Vegard-Kaplan Pu’ational Air Pollution Control Administration Conemission but discarded the possibility of S2(A3Zu +) exciting N(2P) with the argument that the [N2(A3ZU+) sumer Protection and Environmental Health Service, was too small to account for the r\;(2P-4S) intensity. Public Health Service, Grant AP-00391. Campbell and Thrush, l8 however, reconsidered Noxon’s data and suggested reaction 1 followed by fast quenching of N(2P) by K(4S),reaction 3. If this is the case, (16) Use of the new radiative lifetime1at14 (2.1 sec) for N2(A3ZU+)
+
+ +
IY(2P)
+ N(4S)
---f
N(lS)
+ N(4S)or N(zD)
(3)
at sufficiently high [AT] the A-(2P-4S) emission should be independent of [N]: I(3466) = (kl[Nz(A)][N])/ k g [ N ] T-’[X(~P)] k (wall)). The data of Figure 2, however, show a linear increase of I(3466) with [N] at constant [r\j2(A32,+)]. For the maximum [W] of 1.6 X lo-“ mol for which the plot is linear, k3 must be less than 1 x lo9 om3 mol-’ sec-’ in order that the
+
+
The Journal of Physical Chemistry
+
rather than the old 12 sec-1 value changes our previous measurement of 20 10 Nz(A) molecules per O(1S) photon to that quoted in the text. (The O(1S) lifetime is -1 sec so the number of measured O(1S) photons is equivalent to the number of O(1S) atoms). (17) J. F. Noxon, J. Chem. Phys., 36,926 (1962). (18) L. M. Campbell and B. A. Thrush, Trans. Faraday Soc., 6 5 , 32 (1969). (19) J. W. Chamberlain, “Physics of the Aurora and Airglow,” Academic Press, Inc., New York, N. Y., 1961. (20) M. P. Weinreb and G. G. Mannella, J . Chem. Phys., 51, 4973 (1969). (21) G. Herrberg, “Electronic Spectra of Polyatomic Molecules,” Van Nostrand-Reinhold Co., Inc., Princeton, N. J., 1966, p 593.
*
