T H E
J O U R N A L
OF
PHYSICAL CHEMISTRY Registered in 77. S . Patent Ofice
@ Copyright, 1978,by the American Chemical Society
VOLUME 76, NUMBER 11 MAY 25, 1972
Rates of Interaction of Vibrationally Excited Hydroxyl.(v = 9) with
Diatomic and Small Polyatomic Moleculee by S. D. Worley, R. N. Coltharp, and A. E. Potter, Jr.* Earth Observations Division, N A S A Manned Spacecraft Center, Houston, Texas 77068 (Received December 6 , 1971) Publication costs assisted by N A S A
The present investigationfollows the loss of OH? (v = 9) produced by the reaction of H and 0 3 at the entrance of a fast-flow system in the presence of various diatomic and polyatomic molecules of aeronomic interest by observing the decay in intensity of the 9-7 emission band of OH?. Several loss processes for OH? have been found to be important. These are (1)radiative decay, (2) quenching by the walls of the flow-tube, (3) chemical reaction with ozone, and (4) interaction with other molecules present. In this study the first three loss processes were held essentially constant, while process 4 was varied, thus enabling the determination of the rate constants for quenching of OH? (v = 9) by the various molecules of interest. The rate constants (k X lo1* cm* molecule-1 sec-1) measured were ( k , M): 0.36, Nz; 15, NO; 4.8, NzO; 1.4, CHa; 2.4, COZ; 2.5, COS; 2.4, SO$; 25, HzS; 20, HzO.
Introduction Very few reaction rate data have been reported for vibrationally excited molecules in their upper vibrational states. Until recently, experiments deriving vibrational deactivation parameters have been essentially limited to electronically excited states.' Work in these l a b o r a t o r i e ~ ~and - ~ in those of Hancock and Smith6r6 concerning reactions of vibrationally excited OH and CO, respectively, has begun basic kinetic studies of vibrationally excited species in their ground electronic states. Such studies should be continued, for upon refinement they should prove to be useful in testing current theoretical treatments of vibrational energy transfer. Previous papers have reported the rate constants for the reaction of OH? (v = 9) with ozone2 and with o ~ y g e nthe , ~ mean radiative lifetime of OHt (v = 9),2 and the rates of reaction of OHt (v = 2-9) with ozone.a The reason for concentrating on the v = 9 state of OH? in preference t o the other vibrational states is that from the reaction between H and 0 3 , OH? (v = 9) is the
highest level produced. Consequently, no complications in interpretation of the data arise from cascading from higher vibrational states, as would be the case if lower levels were studied.
Experimental Section A complete description of the fast-flow reactor and the experimental techniques used in the current study has been given elsewhere,2 and thus only the main points will be summarized here. The procedure followed was to monitor the intensity of the OH? decay at viewing ports spaced down a metal flow tube. The (1) For example, see J. I. Steinfeld and W. Klemperer, J. Chem. Phys., 42, 3476 (1965). (2) A. E. Potter, Jr., R. N. Coltharp, and S. D. Worley, ibid., 54, 992 (1971). (3) R . N. Coltharp, S. D. Worley, and A. E. Potter, Jr., A p p l . O p t . , 10, 1786 (1971). (4) S. D. Worley, R. N. Coltharp, and A. E. Potter, Jr., J. Chem. Phys., 55, 2608 (1971). (5) G. Hancock and I. W. M. Smith, Chem. Phys. Lett., 8 , 4 1 (1971). (6) G. Hancock and I. W. M. Smith, A p p l . Opt., 10, 1827 (1971).
1511
1612
S.D. WORLEY,R. N. COLTHARP, AND A. E. POTTER, JR.
detector used in this study was a lead sulfide photoconductor cooled to 195°K. A 2.1-2.5-11 interference filter, which passed predominantly the 9-7 emission band of OH?, completely covered the detector.' A mechanical chopper was used to modulate the light intensity viewed by the detector, and the output signal was synchronously detected with a Princeton lock-in amplifier. As in the earlier ~ t u d i e s , ~the - ~ concentrations of H (ca. 4.5 X mol la-') and 0 3 (ca. 1.7 X 10-8 mol 1.-') and the flow rate (ca. 1.5 X lo6 cm3 sec-l) were 0 3 reaction zone adjusted to compress the initial H into the first few centimeters of the tube so that only loss processes for OHt (v = 9) were followed a t the viewing ports. A "blank" run, in which only H, 5% 03/96% 0 2 , and argon were mixed in the tube, was made before each run in which a species of aeronomic interest, M, was added. All gases used in this work were obtained from Matheson and were thought to be of sufficient purity to use directly from the cylinders. To obtain an adequate vapor pressure of HzO in the flow tube, it was necessary to bubble high-purity argon through a porous fritted disk immersed in distilled water. The argon was then saturated with H20, and appropriate corrections were made in deriving the concentration of H2O present in the tube. Since argon is a very poor quencher relative to the other species A 1 in this study, its contribution to the reaction system can be neglected. Indeed, the earlier studies2 showed that argon present in the flow tube merely acts to minimize wall deactivation.
+
:i 2
1
0
10
20
40
30
50
60
70
DISTANCE. cm
Figure 1. Decay of OHt (9-7) radiation intensity with and without M present in the flow tube.
Results
reaction tube pressure in Torr, was determined before. The reactions which have proved to be i m p ~ r t a n t ~ - ~Equation 4 represents the reaction of OHt (v = 9) in the current experiments in the flow tube are with ozone, the previously measured value of k49 being 7.7 f 0.3 X 10-l2 cma molecule-' sec-'; the contribuH 03 OH" (V 5 9) 0 2 (1) tion of this reaction was quite significant in the current experiments because ozone was always present OHt (V = 9) -% OHt (V 5 8) hv (2) in large excess relative to atomic hydrogen. The fifth equation illustrates the reaction of OH? (v = 9) with OH? (v = 9) -%- deactivation by wall (3) various molecules, M; the rate constants k 2 are the parameters which were measured in this work. The OHt (v = 9) O3 k4B products, OH? (V 5 8) (4) rate equation for the loss processes described above OH? (v = 9) It -% products, OH? (v 5 8) (5) (neglecting reaction 1) is
+
+
+
+ +
Equation 1 represents the initial chemical reaction which produces OHt; the rate constant kl has been accurately measured by Phillips and Schiffs to be 2.6 i= 0.5 X 10-l' cm3 molecule-' sec-'. As stated previously, reaction 1 is essentially complete before the first viewing port in the flow tube, and thus its contribution may be neglected in the final rate equation. Equation 2 represents radiative decay, 7 9 being the mean radiative lifetime of OH? (V = 9); a value of 6.4 f 1.4 X sec was found for T~ in one of the earlier works. The third equation represents collisional deactivation by the walls of the flow tube. The kW9used in this study, 6.2 sec-' Torr/P, where P is the The Journal of Physical Chemistry, Vo. 76, No. 11, 19722
[d[OHt]/dt],,s
=
- ~g[OHt]9
(6)
where (7) The transmission of the interference filter employed in this study varied smoothly between 4 and 48% over the OH? (9-7) emission band (2.10-2.27 a) with a mean transmission of 22.6% while the transmission over the OHt (8-6) emission band (1.95-2.11 a) was only 0 to 4,5% with a mean transmission of 1.4%. The transmission was 0.2% a t the 8-6 band origin. The ratio of the mean spectral response of the cooled PbS detector in the range 2.10-2.27 p to that in the range 1.95-2.11 p was 1.11. From these data, the response of the detector-filter system to the 8-6 band was estimated t o be 1/20 of the response to the 9-7 band. It was considered that the rejection of the 8-6 band was sufficient that its contribution to the measured signal could be neglected. (8) L.F. Phillips and H. I. Schiff, J. Chem. Phys., 37, 1233 (1962).
INTERACTION OF HYDROXYL WITH DIATOMIC MOLECULES ~g
=
~ 9 - l
+ k9+ L9[0al+ h 9 [ M l
(7)
Expression 6 is a simple first-order rate equation, and since the intensity of the OH? 9-7 emission band is proportional to [OHt]g, a semilog plot of the intensity 1 us. time or distance of flow must be linear, as shown in Figure 1. Values of kg for a variety of M concentrations were determined from the linear plots in the manner described in the earlier work.2 To minimize errors for kb9 arising from errors in the parameters 7 9 , kV9, and h9,the “blank” runs (see Experimental Section) were compared directly with the runs in which $1 was introduced. Since the ozone concentration varied only slightly between the “blank Yun” and the typical runs with M present, the errors in these parameters essentially cancel, and the main source of error in kb9is the measured value of the concentration of M. The error in [MI was estimated to be less than 5% for most of the i\!t studied. Table I lists the molecules studied, the concentration ranges employed, and the average values of I C ~ ~ .The indicated errors in Table I represent the standard deviation from the mean of between 8 and 20 different experimental measurements, but the absolute error inherent in the measured values of kj9 is at least 5%) and more for some XI (see Discussion section).
Table I: Rate Constants for Interactions of O H + (v = 9 ) with M [MI range
x
M
mol 1.-1
Oaa OZb N9 NO NaO
0.1-1.9 67.0-332 466-1078 2.8-6.8 9.3-47.5 34.5-104 19.2-43.5 12.4-49.2 8.7-26.3 1.1-6.8 1.4-4.0 19.6-66.0 2.0-3.2
CHI
coz cos SO96 HzSc
HzOo
co “3
ka9,
oms molecule -1
108,
sec-1
7 . 7 =t 0 . 3 x 1.0 f: 0.1x 3.6 f 0.5 x 1.5 f 0 . 3 X 4 . 8 =t 2 . 2 x 1.4 0.2 x 2 . 4 =t 1.0 x 2 . 5 f: 1 . 5 x 2 . 4 =t 0.4 X 2 . 5 f 0.9 x 2 . 0 i: 1 . 6 x
10-l2 10-14 10-l6 10-13 10-14 10-14
10-14 10-14 10-14 10-13 10-13
*
c
a Previously reported in ref 2. Previously reported in ref 4. Minimum rate constants. See text.
Discussion The recent paper of Hancock and Smith6 reports the measurement of rates of de-excitation of COT (v = 4-13) by a variety of small molecules, several of which have been included in this work. With the exception of N20, the ordering of the rate constants for M interacting with OH+ (v = 9), ie., KO > X20 > COS COZ > 02 (Table I) is qualitatively the same as that for
-
1513
M interacting with COT in its upper vibrational levels; e.g.,6 for COt COS > CO2 > NzO > 02, for those M studied in the two works. Hancock and Smith6 assume only vibrational deactivation mechanisms (V-V and V-T energy transfer) in interpreting their data for COT. However, such an interpretation is untenable for OH? (v = 9). Since the efficiency of V-T and V-V energy transfer processes depends on the reduced mass and vibrational frequencies of the collision partner^,^ one would expect trends to appear in the data in Table I. For example, the vibrational frequency of IYz (2331 cm-’)’O is in nearer resonance with the 9-8 emission frequency (2236 cm-l)ll for OH1 (v = 9) than is that (1555 crn-’)lo for 0, of similar reduced mass; yet 0 2 interacts more efficiently with OHt (v = 9) than does Nz (see Table I). It seems clear that chemical reaction must be considered in addition to vibrational deactivation as a mode of interaction for most of the M and OH? (v = 9). Thus, the values of kb9in Table I represent a sum of vibrational deactivation processes and chemical reaction; this experiment could not distinguish between the various processes. Methane is the only M for which reaction data involving OH has been studied p r e v i o ~ s l y . ‘ ~Our ~~~ cm3 rate constant for OH? (v = 9) was 1.4 X molecule-’ sec-’. The previous work^'^,'^ reported values of 1.08 X and 8.8 X cm3 molecule-’ sec-l, respectively; these rate constants represent chemical reaction entirely because the OH generated in the experiments was in its ground vibrational state. Thus, the present reaction rate constant for CH, and OH? (v = 9) is reasonable, considering that vibrational excitation could increase chemical reaction rates. Earlier work3 in these laboratories has shown that k4O for interaction of O3 and OH? (v = 2-9) decreases as v becomes smaller; however, preliminary seems to indicate that kju for 02 increases, or remains constant, as v becomes smaller. Furthermore, the recent study6 of interactions of COT (v = 4-13) has shown that when M = K2, NzO, CO, or COS, lower vibrational states of CO are deactivated more efficiently than upper ones, but the reverse is true when 13 = NO, He, and COZ. It seems clear that many more kinetic studies are needed for reactions involving vibrationally excited reactants before one can begin to understand (9) A. B. Callear, “Photochemistry and Reaction Kinetics,” Cambridge University Press, New York, N . Y., 1967, Chapter 7. (10) G. Hersberg, “Molecular Spectra and Molecular Structure I. Spectra of Diatomic Molecules,” Van Nostrand-Reinhold, Princeton, N. J., 1950, p 62. (11) J. W. Chamberlain, “Physics of the Aurora and Airglow,” Academic Press, New York, N. Y., 1961. (12) W. E. Wilson and A. A. Westenberg, “11th International Symposium on Combustion,” University of California, Berkeley, 1966. (13) N. R. Greiner, J . Chem. Phys., 46, 2795 (1967). (14) A preliminary report of this work was presented at the 162nd National Meeting of the American Chemical Society under the title, “Quenching of Vibrationally Excited Hydroxyl by Oxygen,” Washington, D. C., 1971.
The Journal of Physical Chemistry, Vol. 76, No. 11, 1972
LOUISM. ARIN AND PETER WARNECK
1514 the partitioning of the various deactivation and reaction processes. The kS9 for the molecules denoted by footnote c in Table I may represent minimum rate constants. Small concentrations of M caused large decreases in intensity of the OHt 9-7 emission band, but the slope of the log I vs. distance plot did not change as markedly (see Figure 1). This probably indicates that M is reacting with one of the initial reactants, probably 0 3 since it is always in excess relative to H. This would in effect cause the values of [MI and [ O x ] which , were used in deriving hQ, to be too large, and the resulting IcS9 to be too small. The k5" reported in Table I for NO is questionable. There was ea. 0.7% NO2 present in the NO as an impurity, and reaction 8 is of compar-
+ NO
+ NO2*
(8) able rate (2.1 X cm3molecule-' sec-')15 to those reported in Table I. Small amounts of NO2 cause large intensity decreases in the OH? 9-7 band because reaction 9 is very eEcient (4.8 f 0.5 X lo-" cm3 mole0 3
H
---+ 0 2
+ NOz-NO
+ OH (V
=
0)
(9)
cule-' sec-1)8 in competing with reaction 1. No ICb9 have been reported in Table I for CO and NHs. The reason for this omission is that the plots of log I vs. distance when M = CO or NHB had smaller slopes than
did the plots for the "blank" runs, and thus no value of kG9 could be determined. It is quite clear why CO is not suitable for study by this technique. The reaction of CO and OH (v = 0) as in eq 10 has been fol-
CO
+ OH +COz* + H
(10)
lowed by a fast-flow esr method a t 300°K by DixonLewis, et aZ.,16their rate constant being 1.9 X cm3 molecule-' sec-I. The H is thus being regenerated down the tube and can then further react with 0 3 t o produce additional OH? (v = 9), causing a lesser slope in the plot. The same process is probably occurring for "3, i.e. NH3
+ OH +NHzOH + H
(11)
It is concluded that the present technique can derive only minimum values for when R/i reacts significantly with OB,and it cannot be used for studying those M which react with OH? to regenerate H. However, the method does seem to be worthwhile for studying reactions of M and OHt which do not suffer from these limitations. (16) L. F. Phillips and H. I. Schiff, J. Chem. Phys., 36, 1609 (1962). (16) G. Dixon-Lewis, W. E. Wilson, and A. A. Westenberg, ibid., 44, 2877 (1966).
Reaction of Ozone with Carbon Monoxide by Louis M. Arin I o n k s , Inc., Watertoum, Massachusetts 02172
and Peter Warneck" Max-Planck-Institut f a r Chemie (Otto-Hahn-Institut), 66 Mainz, West Germany (Received September 27,1971) Publication costs assisted bu Max-Planck-Institut f u r Chemie
Ozone and carbon monoxide were found to react rapidly due to catalysis by a volatile impurity in the CO, but when the impurity was removed, the reaction was too slow for its rate to be measurable. Recent observations by Seiler and Jungel have demonstrated that carbon monoxide, present in tropospheric air a t a level of 10-7 parts per volume, is rapidly consumed in the stratosphere. The stratospheric oxida~ ~account ~ tion of GO has been predicted t h e ~ r e t i c a l l yon of the reaction with OH radicals, but the observed decay of co concentration with altitude above the tropopause' is much faster than the calculated photoThe Journal
of
Physical Chemistry, Vol. 76,No. 11, 1972
chemical equilibrium concentrations of OH would allow. Hence, additional CO oxidation processes must be considered. One possibility is the reaction with ozone 0 3
+ co +coz +
w. Seiler and c, Junge,
0 2
Tellus, 21, 447 (196Q).
(2) J. Pressman and P. Warneck, J . A t m . Sci., 27, 155 (1970). (3) E.Hesstvedt, Nature (London), 225, 50 (1970).
(1)
