THE JOURNAL, O F
PHYSICAL CHEMISTRY Registered i n U.S. Patent Office 0 Copyright 1979, by the American Chemical Society
VOLUME 83, NUMBER 6
MARCH 22,1979
Absolute Rate Constant and Temperature Dependence of the Reaction between Hydrogen (*S) Atoms and Ozone L. F. Keyser Jet Propulsion Laboratory, California Instifute of Technology, Pasadena, California 9 1 103 (Received October 12, 1978) Publicafion costs asslsted by the National Aeronautics and Space Administration
-
The absolute rate constant of the reaction H(2S)+ O3 OH(X*n,u 5 9) + O2 was determined between 196 and 424 K using the discharge flow resonance fluorescence technique. The experiments were carried out under pseudo-first-order conditions with ozone in large excess. Ozone concentrations were determined by UV photometry both up- and downstream of the reaction zone. The rate constant results are best fitted by the following Arrhenius expression: h(cm3 rnolecule-'s-') = (1.50 f 0.18) x exp(-499 f 3 2 / T ) , 196 IT I 4 2 4 K. Addition of vibrational quenchers to the reaction mixture showed that secondary reactions of vibrationally excited OH are not important under the conditions used. The present results are compared with earlier measurements of this rate constant.
Introduction The reaction between atomic hydrogen and ozone
H(*S) + 03 OH(X211,u I 9) + 02 (1) AH" = -76.8 kcal mol-l is of importance in the HO, chemistry of the mesosphere and upper stratosphere. Reaction 1 is a major !source of the OH vibrational overtone (Meinel) bands observed in the airglow ~pectrum.l-~ Observed Meinel band intensities, along with values for the rate constant hl, have been used to calculate hydrogen atom concentrations in the meso~ p h e r e . The ~ H O3 reaction has also been used in laboratlory studies of the Meinel bands5 and in kinetic studies of vibrationally excited OH.G8 Studies of the initial vibration-rotation distribution in OH produced by reaction 1at low pressures indicate that 90% of the available energy enters vibrational excitation and that the upper vilbrational levels ( I J = 7-9) are predominantly e x ~ i t e d . ~ J ~ Three previous measurements of hl have been carried out using three different experimental techniques. Phillips and Schiff'' used discharge flow mass spectrometry to study the reaction at room temperature and obtained (2.6 f 0.5) 2: lo-'' cm3 molecule-l s-'. Clyne and Monkhouse12 studied the reaction over the temperature range 298-638
+
0022-3654/79/2083-06~5$01.OO/O
K using discharge flow resonance fluorescence; their result cm3 molecule-l s-l. The a t 298 K is (1.76 f 0.21) X most recent study is that of Lee e t al.13using flash photolysis resonance fluorescence over the temperature range 219-360 K. Their result a t 300 K is (2.85 f 0.22) X cm3 molecule-1 s-l, in good agreement with the early measurement by Phillips and Schiff but some 60% higher than the recent measurement by Clyne and Monkhouse. If the results are extrapolated to 200 K, which is characteristic of the upper mesosphere, the value obtained by Lee et al. is about a factor of 1.9 higher than that of Clyne and Monkhouse. Because of the large discrepancies in kl observed in the recent studies, additional accurate measurements over a wide temperature range are needed. In the present study, hl was determined over the temperature range 196-424 K using discharge flow resonance fluorescence. Experimental Section The experimental apparatus used in the study has been described previ0us1y.l~ For the present study, an ozone handling and measurement system has been added upstream of the movable inlet tube. Ozone was produced by discharging oxygen in a Welsbach Model T-816 ozonator 0 1979 American Chemical
Society
646
The Journal of Physical Chemistry, Vol. 83,
L.
No. 6, 1979
and trapped a t 195 K on silica gel. Excess oxygen was then removed by pumping. Ozone, eluted in a stream of helium, was determined photometrically a t 253.7 nm in a flowthrough 5-cm quartz cell just upstream of the movable inlet. An absorption cross section of 1.14 X cm2 (base e ) was used;15transmittances ranged between 16 and 65%. Ozone concentrations in the reaction zone were controlled by varying the helium flow rate and cell pressure. Typical flow rates were 120-300 atm cm3 min-l (at 293 K); pressures were 200 to 600 torr. Downstream of the measurement cell, ozone was in contact with quartz, Pyrex, or teflon surfaces only, except for one stainless steel needle valve. To check for possible ozone decomposition, samples were taken from the flow tube a t a point downstream of the reaction zone and analyzed on a Cary 15 spectrophotometer. During these sampling experiments, tempertures in the reaction zone were 298, 363, and 424 K. Ozone concentrations obtained from the flow-tube analyses agree within 5% with those obtained from the flowthrough photometer a t all the temperatures studied. Hydrogen atoms a t concentrations less than 10l1 cm-3 were generated in a microwave discharge of a dilute (0.1%) mixture of hydrogen in helium. To minimize production of impurity atoms, the main helium flow bypassed the discharge. Fluorescence of atomic hydrogen a t 121.6 nm was excited by a resonance lamp operated a t 30 W microwave power. Maximum signals were obtained by flowing approximately hydrogen in helium at a total pressure of 1.3 torr. Fluorescence signals were calibrated by titration of hydrogen atoms with N02.12J6 Plots of fluorescence intensity vs. [HI were linear up to a concentration of 1 X 1Ol1 cm-3 a t which point a signal of 2150 counts s-l was observed. Background signals generally were below 30 counts s-l; thus, for a 100 s counting time S / N > 20 a t [HI = lo9 ~ m - ~Concentrations . used in the present study were between l o 9 and lo1’ cm-3 with 10 to 100 s counting times. Gases used were chromatographic grade helium (99.9999%), research grade hydrogen (99.9995%), ultrahigh-purity oxygen (99.99%), nitrogen dioxide (99.5% ), and sulfur hexafluoride (99.8%). All were used without further purification except the helium carrier gas which was passed through a molecular sieve trap a t 77 K.
Results and Discussion The experiments were carried out under pseudo-firstorder conditions with ozone in large excess in order to minimize secondary reactions. Hydrogen atom concentrations were between lo9 and 10l1 cm-3 with molecular hydrogen typically 0.5 to 1 X 1011 ~ m - ~Ozone . concen. total trations were between 3 and 40 X 1OI2 ~ m - ~The pressure ranged between 1.5 and 4 torr with flow velocities of 2000-3000 cm sc1. Under these conditions -d d In [H]/dl = k1[O3] = kl’ or In (Io/I‘)= kl‘l/o where D is the average flow velocity, I and Io are respectively the hydrogen atom fluorescence intensity with and without added ozone, 1 is the reaction length, and k ; is the pseudo-first-order rate constant. T o determine Io, the ozone-containing silica gel trap was bypassed while maintaining essentially constant cell pressure and helium flow rate. Plots of In (loll) vs. 1 are shown in Figure 1 for several ozone concentrations a t 298 K. The lines through the points are linear least-squares fits of the data. Some of the plots show small nonzero intercepts with a slight trend
F. Keyser
6,01 5. 0
z
4.
\
0
t!
5
3. 2. 1.
30
20
10
0
50
40
P ICM FI ure 1. In ( I o / I Jvs. reaction length, I , at 298 K. [O3]/IO’* and (8/cm s-’) as follows: 0 ,15.0 (2470); H, 9.66 (1980); 0, 7.95 (2480); 0 , 6.22 (2480); 0 ,4.48 (1990).
5oc
400
3oc
Yw l
x
20c
1W
C
1
1
2
[OJ
1
1
L
3
4
1013 CM-3
Figure 2. Pseudo-first-order rate vs. [O,] at four temperatures.
to more positive values as the ozone concentration decreases. However, the intercepts are always small compared to the total range of reaction length used and should have no significant effect on the observed rate constants, hl’, which are obtained from the slope of the In ( I o / I )vs. 1 plots. The bimolecular rate constant, kl, was determined from the linear least-squares slope of k,’ vs. [O,]plots and from an average of individual h 1 ’ / [ 0 3 ]values. Both methods agreed within two standard deviations. Values obtained from the average are used in the remainder of this discussion. Typical kl’ vs. [Os] plots are shown in Figure 2 and the results are summarized in Table I. Within experimental error, the bimolecular rate constants are independent of the ozone concentration used. This is evidence that the slight trend in nonzero intercept observed in the In (loll)vs. 1 plots is not significant in the rate constant determination. Corrections were made for the viscous pressure drop between the reaction zone and the pressure measurement
The Journal of Physical Chemistty, Vol. 83, No. 6, 1979 647
Reaction between Hydrogen (*S) Atoms and Ozone
TABLE I: Rate Constants for the H .t 0, Reaction temp, K press., torr 196 223 242 2 65 2,98 363 4 24
hl(lO-'* cm3 molecule-' s-'I)
runs
8 15 8 8 28 10 7
2 2 2 2 1.5-4 2 2
1.21 t. 1.63 ?: 1.86 i2.22 t. 2.79 f 3.67 k 4.89 t.
0.04" 0.16 0.08 0.12 0.18 0.22 0.38
a Erram are 1 2 0 .
port, located downstream of the fluorescence celll, and for axial diff~sion.'~Pressure corrections to kl were less than 5% a t the highest flow velocities used. Axial diffusion corrections were generally 3-5% except a t the two highest temperatures studied where they averaged 6-'7 5%. No corrections were made for radial diffusion since estimates18J9of the magnitude show it to be less than 3% a t all temperatures studied. The present results along with earlier measurements of kl are plotted in Arrhenius form in Figure 3. The line through the data is a linear least-squares fit of the present results alone. The resulting Arrhenius expression is compared with other studies of the temperature dependence of k , in Table 11. The agreement between the results of this study and the flash photolysis results of Lee e t al.13 is excellent over the entire temperature range studied. As discussed p r e v i ~ u s l y , ' ~the J ~ agreement with the earlier study of Phillips and Schiff1' may be alccidental since these authors worked a t high concentrations of atomic hydrogen and ozone and used short reaction times where ]problems due to incomplete mixing and nonisothermal conditions could have occurred. The results of Clyne and Monkhouse are approximately 60% lower than the present results and those of Lee et al. in the temperature range where the three studies overlap. This is somewhat outside the combined experimental errors, estimated a t &15-20% for each study. The reason for the difference is not clear since the experimental conditions used were very similar to those of the present study. Estimates indicate that secondary reactions of unexcited species should not interfere under the conditions used in the present studies. However, reactions of vibrationally excited OH, which is known to be formed in high yield by reaction 1, could possibly be important. Reactions such as the following must be considered:
-+ + + - +
OH* + O3
HOz f O2 H
OH*
(2b)
+0+02+OH
(2c)
HzO -t H
(34
Hz
OH*
202
(24
-OH+Hz H 0 Hz
(3b) (44
-0Ht-H (4b) The totid rate constant for reaction 2 has been measured
131T, K - l
+
Figure 3. Arrhenius plot of H O3 rate data. The symbols are as follows: (A) Phillips and Schiff, ref 11; (0)Clyne and Monkhouse, ref 12; (0) Lee et al., ref 13; ( 0 )present results.
in several studies;Gb-8values range from 0.8 X lo-'' to a lower limit of 4 X IO-'' cm3 molecule-' s-'. Some evidence exists that reaction channel 2b may be a relatively important pathway.20 Under typical experimental conditiions used in the present study ([HI = 5 X 1O1O and taking [OH*] 5 5 X 1O1O cm-3 and k2,, x 1 X lO-l'), the rate of hydrogen atom formation from reaction 2b could be as high as 36% of hydrogen atom loss by reaction 1. In the case of reaction 3, the sum of quenching and reaction rate ~ depend constants (k3a + k3J has been m e a ~ u r e d .Values on the vibrational level and range from 5.8 to 8 X cm3 molecule-' s-] for u' 2 6. In this same study evidence was presented that quenching channel 3b predominates over chemical reaction 3a. However, model calculations of a chemically pumped water vapor laser imply that chemical reaction is important with kSa about 7 X cm3 niolecule-I s-'.'' Estimates indicate that with the high stoichiometric ratios used ([03]/[H] = 30-400), reaction 3 should not interfere as long as k3, is not considerably greater than cm3 molecule-' s-'. Measured values of k 4 indicate that the reaction is very rapid. For u' = 9, a cm3 molecule-' s-l has been value greater than 7 X obtained.Gb For u' = 1 and 2 , the results are 2.7 x and 3.3 X cm3 molecule-' s-l, respectively,22with nonreactive quenching predominating. However, because of the high stoichiometric ratios used, reaction 4a should not interfere with the present measurements even if it were as rapid as 1 X cm3 molecule-I s-'. To test for possible interference from reactions of vibrationally excited OH, vibrational quenchers were added
TABLE 11: Temperature Dependence of k , Arrhenius expressiona for kl(cm3molecule-' s - ' ) (1.50 i 0.18) x
(1.33 * 0.32) x (9.89 i 2!.61) x a
lo-"
Errors are f 20.
exp(-499 exp(-449 exp(-516
f
i 2
32/T) 5811') 60/T)
-
temp range, K
k,(2!38 K) (lo-'! cm3 molecule-' s-l)
k,(200 K ) (IO-" cm3 molecule-' s-l ) b
ref
196-424 219-360 298-638
2.79 2.85 1.'76
1.24 1.41 0.75
this work 13 12
Calculated from the Arrhenius expression.
-
648
The Journal of Physical Chemistry, Vol. 83,
TABLE 111: Effect of Added quenching rate constant
quencher (cm3molecule-' s-') O,a
SFeC
(0.1-7.8) x
No. 6, 1979
V i b r a t i o n a l Quenchers
concn d
range, (3-11)
x 10l4
(4-9) X 10"
1.0 ?: 0.1 1.0 i 0.05
a [HI,< 1 x lo1' ~ m -6~x ; l o i 2 G [ O , ] < 1 6 x 10l2 ~ m - ~b References . 6, 7, and 8. [HI, < 1 x 10" ~ m - ~ ; 7.6 x 10l2 < [ O , ] < 9.6 x 10l2 ~ m - d~ k,(298 . K ) with quencher/k,( 298 K ) without quencher.
to the reaction mixture. Four runs were carried out with each added quencher at several ozone concentrations. The results are summarized in Table 111. In the presence of high concentrations of vibrational quenchers, hl observed agrees with the values obtained without added quenchers to within f10% which is well within expected experimental error. In addition, no trends with increasing quencher concentration were observed. These results show that reactions of vibrationally excited OH are not important under the experimental conditions used in the present study. The absence of any significant effect with added quencher implies that k2b[OH*]is considerably less than 0.5 s-l, otherwise interference from reaction 2b should have been observed. As an additional check for possible interference from secondary reactions, the initial hydrogen atom concentration was varied from 0.5 to 8 X 10" cm-3 with the ozone . obconcentration held constant a t 1.3 X 1013~ r n - ~The served value for kl was found to be independent of [HI, at concentrations below about 2 X 10l1~ m - However, ~, the observed hl decreased at higher [HI,; for example, at [HI, = 8 X lo1], h l = 2.1 X 10-l' cm3 molecule-I which is approximately 30% lower than the value observed a t [HI, 5 2 X lo1' ~ m - Small ~ . corrections were made for the loss of ozone a t the higher hydrogen atom concentrations. To avoid operating in the nonlinear region of the fluorescent intensity vs. [HI curve, the runs were carried out a t sufficiently long reaction times to allow [HI to fall below 1 X 10l1 ~ m - The ~ . reason for the decrease in hl a t [HI,
P. Bortolus and
S.Monti
> 2 X 1011 cm-3 is not clear at this time. Initial model calculations of the system suggest that reactions of vibrationally excited OH may be responsible. Additional model calculations and experiments with added vibrational quenchers at high [HI, are planned in the future to check this point. However, the results should have no bearing on the present rate constant measurements which were carried out a t [HI, 5 1.2 X lo1' where hl was found to be independent of [HI, and of added vibrational quenchers. Acknowledgment. The research described in this paper was carried out a t the Jet Propulsion Laboratory, California Institute of Technology, under NASA Contract NAS7-100.
References and Notes (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (171 (l8j (19) (20) (21) (22)
A. B. Meinel, Astrophys. J., 111, 555 (1950). D. R. Bates and M. Nicolet, J . Geophys. Res., 55, 301 (1950). G. Herzberg, J . Roy. Astr. SOC.Can., 45, 100 (1951). W. F. J. Evans and E. J. Llewellyn, J. Geophys. Res., 78, 323 (1973). (a) A. M. Bass and D. Garvin, J . Mol. Spectrosc., 9, 114 (1962); (b) D. Garvin, H. P. Broida, and H.J. Kostkowski, J . Chem. Phys., 32, 880 (1960), and earlier references cited there. (a) T. E. Kleindienst and B. J. Finlayson-Pitts, Abstracts of the 175th National Meeting of the American Chemical Society, Anaheim, Calif., March, 1978. (b) B. J. Finlayson-Pitts, to be published. G. E. Striet and H. S.Johnston, J . Chem. Phys., 64, 95 (1976). S. D. Worley, R. N. Coltharp, and A. E. Potter, Jr., J . fhys. Chem., 76, 1511 (1972), and earlier references cited there. J. C. Polanyi and J. J. Sloan, Int. J. Chem. Kinet. Symp., 1, 51 (1975). P. E. Charters, R. G. Macdonald, and J. C. Polanyi, Appl. Opt., 10: 1747 (1971). L. F. Phillips and H. I. Schiff, J . Chem. Phys., 37, 1233 (1962). M. A. A. Clyne and P. B. Monkhouse, J. Chem. SOC.,Faraday Trans. 2, 73, 298 (1977). J. H. Lee, J. V. Michael, W. A. Payne, and L. J. Stief, J. Chem. mys., 69, 350 (1978). L. F. Keyser, J . Chem. Phys , 69, 214 (1978). Reviewed by R. D. Hudson, Can. J . Chem., 52, 1465 (1974). H. Gg.Wagner, U. Welzbacher, and R. Zellner, Ber. Bunsenges. Phys. Chem., 80, 902 (1976). F. Kaufman. Proa. React. Klnet.. 1. 1 119611. R. V. Pokier and 6 . W. Carr, Jr., J: fhys: Cheh., 75, 1593 (1971). P. J. Ogren, J . Phys. Chem., 79, 1749 (1975). W. B. DeMore, J . Chem. fhys., 47, 2777 (1967). G. D. Downey and D. W. Robinson, J. Chem. fhys., 64, 2858 (1976). J. E. Spencer and G. P. Glass, Chem. fhys., 15, 35 (1976).
Cis-Trans Photoisomerization of Azobenzene. Solvent and Triplet Donor Effects' Pietro Bortolus" and Sandra Monti Laboratorio di Fotochimica e Radiazioni di Alia Energia of the Consiglio Nazionale delle Ricerche, 40 126 Bologna, Italy (Received August 14, 1978: Revised Manuscript Received December 11, 1978) Publication costs assisted by Consiglio Nazionale delle Ricerche (Rome)
-
-
The quantum yields for trans cis (&) and cis trans (&) photoisomerization processes for azobenzene have been determined at a A,, of 317 and 439 nm in solvents of different polarity. At both irradiation wavelengths, & increases and 4tdecreases with increasing polarity of the solvent; the sum (& + &), however, remains fairly constant. For the triplet sensitized reaction, +t 'v 1 and the ratio &/& 65. The data suggest that upon direct photolysis isomerization proceeds in the singlet manifold. lntroduction Azobenzene, in solutions of nonpolar solvents, undergoes photochemical cis G trans isomerization with different quantum yields following excitation in the n,r* or in the T,T* The photoprocess mechanism is still a subject of debate in spite of the investigations carried out on substituent, temperature, and viscosity effects5 on the photoisomerization yields. The reasons are manifold. In 0022-3654/79/2083-0648$01 .OO/O
photosensitization experiments by triplet energy donors, it is very difficult to avoid direct adsorption of light from azobenzenes owing to the absorption spectrum of both isomeric compounds which have high molar extinction coefficients up to -500 nm. Therefore, results obtained in different laboratories are in sharp Moreover, the excited state properties of azobenzene are difficult to investigate because both isomers fail to exhibit detectable 0 1979 American Chemical Society
