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Mechanism of Singlet Molecular Oxygen Formation from Photolysis of Ozone at2537 ÅM. Gauthier and D. R. Snelling Citation: The Journal of Chemical Physics 54, 4317 (1971); doi: 10.1063/1.1674679 View online: http://dx.doi.org/10.1063/1.1674679 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/54/10?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Collisional deactivation of laser excited singlet molecular oxygen by ozone J. Chem. Phys. 67, 5352 (1977); 10.1063/1.434715 Quantum Yield of Ozone Formation from Photolysis of the Oxygen Molecule at 1849 and 1931 Å J. Chem. Phys. 54, 1119 (1971); 10.1063/1.1674946 Ozone Ultraviolet Photolysis. I. The Effect of Molecular Oxygen J. Chem. Phys. 52, 6119 (1970); 10.1063/1.1672914 Reactions of 1 D Oxygen Atoms. III. Ozone Formation in the 1470Å Photolysis of O2 J. Chem. Phys. 46, 953 (1967); 10.1063/1.1840832 Mechanism of Propane Formation by Ethane Photolysis at 1470 Å J. Chem. Phys. 44, 424 (1966); 10.1063/1.1726493
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THE JOURNAL OF CHEMICAL PHYSICS VOLUME 54, NUMBER 10 15 MAY 1971
Mechanism of Singlet Molecular Oxygen Formation from Photolysis of Ozone at 2537 A M. GAUTHIER AND D. R. SNELLING
Defense Research Establishment, Valcartier, and Centre de Recherches sur les atomes et les molecules, P.O. Box 880, Courcelette, Quebec, Canada
(Received 10 November 1970)
Emission from 02(1Ll..) and O2(12:.) is observed from photolysis of ozone-oxygen mixtures at 2537 A. The O2(12:.) is formed by energy transfer from OeD) and the relative rates of removal of O(ID) by O2, N2, and 0 3 arefound to be 1 :0.7: 9.7, respectively. The O2 (lLl.o) is formed directly by photolysis. Photolysis and the reaction of 0(3P) and OeD) with 0 3 are discussed as sources of both O2 singlet states.
I. INTRODUCTION
The atmospheric 02(1~g+---->3~o-) (0,0) band at 7620 A and the infrared atmospheric 02(1~o---->3~o-) (0,0) band at 1.27 )J. are two of the most intense dayglow emissions,! with radiative lifetimes of 12 sec2 and 60 min,3 respectively. In this paper the results of a continuing study4,5 aimed at investigating the mechanism of these airglow features are reported. [The abbreviations 02(~) and 02(~) will be used for the I~g+ and l~g states of O2 and the symbols O2 and 0 will be used for the ground states of molecular and atomic oxygen, respectively.]
Recent rocket profiles of the atmospheric 1.27-)J. dayglow emission6 have confirmed earlier conclusions7,8
latter process can produce vibrationally excited N2, and it has been proposedl7 that this excited N2 may be responsible for heating the electrons in the E region. The relative rates of OeD) deactivation by N2 and O2
would then govern the amounts of O2 (~) and vibrationally excited nitrogen formed in the atmosphere.
In this paper we report results which also allow us to measure the relative rates of 0 (ID) removal by O2, N 2 ,
and 0 3 and to put limits on the efficiency of formation of 02(~) and 02(~) by Reactions (1), (2), (4), and (11).
II. EXPERIMENTAL
A. Apparatus
that the solar ultraviolet photolysis of ozone in the A horizontal cross section of the reaction vessel is Hartley continuum is responsible for the bulk of the shown in Fig. 1. The outer quartz tube was surrounded 02(~) production. However, although DeMore and by nine 88A45 U-shaped Hanovia Hg lamps. The NiS04
Rapet 9 have shown that for wavelengths less than solution (675 g NiS046H20/1000 ml H20) was circu-3000 A the oxygen atom is formed in the first excited lated between the two tubes to both filter out the un(ID) state, the state of the molecular-oxygen product wanted Hg lines in the vicinity of the 7620-A and 1.27-)J. and its possible variation with wavelength remain un- O2 bands and to provide for temperature control of the certain. Of the energetically possible reactions [( 1a)- cell. A portion of the cell was covered with aluminum (1c)], the formation of ground-state (3~o-) oxygen 'is foil to reduce the illuminated region to 6 in. With this spin forbidden, and the requirement of ozone photolysis arrangement and an axial view of the cell, the cone of to explain the 1.27 -)J. airglow requires, at least at wave- sight of the Czerny-Turner spectrometer was contained lengths between 2660 and 3100 A, that part of the within the reaction vessel. molecular-oxygen product be 02(~)' Preliminary The spectrometer of 48-in. focal length was equipped results5 of our study of the flow photolysis of ozone at with a 6-in.X8-in. grating with 600 lines/mm blazed for 2537 A have shown that 02(~) is formed predominantly 1.6)J. in the first order. An RCA liquid-nitrogen-cooled by photolysis and that 02(~) is formed by energy germanium photodiode and an RCA 7265 (S20 retransfer from 0 (ID), in distinction to the implications sponse) photomultiplier were used to detect the 1.27-)J. of the earlier work of Izod and Wayne. tO However, Jones and 7620-A radiations, respectively. The radiation and Wayne ll have now shown that the quantum yield entering the spectrometer was chopped at 23 Hz, and for 02(~) formation is nearounity in the primary step the output of the detector was coupled to a P.A.R. of ozone photolysis at 2537 A. HR8 lock-in amplifier and a recorder.
Energy transfer from O(1D) to O2 has been shown to The ozone concentration in the tube was monitored produce 02(~).12,13 The efficiency of this process is in continuously by measuring the absorption of 2537-A question13 ,14 although it appears to account for part of radiation from a small mercury lamp. The optical the 02(~) dayglow,!5 the O(1D) being produced by arrangement is shown in Fig. 1, and the response of the photodissociation of oxygen in the Schumann-Runge Dumond K1688 Solar Blind photomultiplier effectively continuum at high altitudes and by photodissociation isolated the 2537-A line which comprised 95% of the of ozone at lower altitudes. Since the radiative lifetime output of the lamp. of O(ID) is long the predominant fate of O(1D) below All flows were monitored using Fischer Porter 125 km is collisional deactivation by O2 and N2.13,16 The rotameter-type flow meters which were calibrated for
4317
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4318 M. GAUTHIER AND D. R. SNELLING
PEN RAY PM
PRESSURE MEASURING
- ALUMINUM FOIL
PUMPING
________ ~Yi~;;;;;;;;;;;;~~~~~~~1_1_11JL~O~UTLET
GAS IN---..
-QUARTZ TUBES - - HANOVIA Hg LAMPS
NiS04
SOLUTION
Ba S04 COATED AL REFLECTORS
each gas used. Pressures in the cell and at the flow meters were measured using a quartz Bourdon gauge (supplied and calibrated by Texas Instruments) and calibrated dial gauges, respectively.
B. Reagents
For production and purification of ozone we have generally followed the recommendation of Clough and Thrush.18 Matheson" Gold Label"-grade oxygen was fed through a Welsback "Ozonator" and the ozone was adsorbed on silica gel cooled to -68°C. At the end of the production period the trap was pumped for a few minutes to remove the oxygen and the ozone was desorbed into a 22-liter bulb by allowing the silica gel to warm up to room temperature. Ozone pressures of up to 60 torr in the bulb were attained. Diluent gases were then added to the ozone and the mixtures were stirred using a glass-enclosed magnetic stirrer equipped with a "Teflon" paddle. The oxygen and nitrogen used were ~atheson Gold Label and prepurified grades, respectively, and they were used without further purification. The SF6 was purified by passing it through a trap cooled to - 68°C to remove water and then condensing it at liquid-nitrogen temperatures and pumping to remove any air present.
III. RESULTS AND DISCUSSION
Spectra of the radiations observed at 7620 1 and 1.27 J.I. and their assignment to the O2 (1~g+-+3~g-) (0,0) band and 02eAo----t3~o-) (0,0) band, respectively, have been presented in the earlier publication.s The 7620-1 emission was observed superimposed on a continuum produced by fluorescence from the quartz cell. This fluorescence was observed with the cell under vacuum,
FIG. 1. Horizontal cros~ section of reaction vessel.
and the intensity of the 76201 was taken as the area of the band above the continuum.
In this paper we are only concerned with relative intensity measurements under varying conditions of added gases. In all the experiments the slow drift in intensity due to changes in the lamp output has been taken into account by repeating the measurement of the band intensities under a standard condition throughout a series of measurements. There was always some oxygen present in our system both from the decomposition which occurred when the ozone inert-gas mixtures were prepared and also from the further ozone decomposition that occurred during photolysis. (The fraction of ozone decomposed was kept as low as possible consistent with having a measurable singlet oxygen intensity.) Both these sources of oxygen were taken into account in plotting the data, and for each series of experiments the average pressures of ozone, oxygen, and inert gas as well as the fraction of ozone decomposed are shown for the standard condition (no added gas). In general, addition of oxygen and nitrogen reduced the amount of ozone decomposed somewhat, the latter more so than the former. For a given experiment the total volume flow rate was kept constant with addition of increasing amounts of added gas. This had the effect of maintaining the average ozon~ pressure in the cell constant to ±1O% for a given experiment. The reaction scheme which will be used in the following discussion is shown in Table 1.
A. Production and Removal of 02(~)
The rate constant for the reaction of O2 (~) with ozone19 is k5 = (7 ± 1.4) X 10-12 (all rate constants in this paper are given in units of cubic centimeters mole-
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PHOTOLYSIS OF OZONE AT 2537 A 4319
10.0. ADDED 0.2 RESULTS (BI B
PSF6 0..85 To.RR; Po.3 0..0.55 To.RR 90.
80. V> I-Z
ADDED N2 RESULTS (AI % 0 3 ::>
EXPTL AVERAGE PRESSURE To.RR PHo.To.LYZED >-a: 60. « 0.2 0.3 a: I- X 0..40. 0..0.12 30. ro 50. a: 0 0..72 0..022 22 «
+ 0..70. 0..0.0.5 18 +11'10040.
'V 0..46 0..0.17 25
ON 30. 0 0..5-0..9 0..0.1-0..0.2 13
>- 'V C, 0..0.1-0..0.2 0.0.0.15 - 0..0.0.4 13
l- c, V> Z W I-Z
l!. ........ --- c,
---------::=-:::::::::::::~====-~~~-----_____ A c, ---------------------
0. 15 20. 25 30. 35
B : [°2 ] / [°3 ]
FIG. 2. Variation of O2 (12:.+) intensity; A with addition of N, to mixtures of Os and O2 and B with addition of 0, to mixtures of Os and SF6•
TABLE 1. Reaction scheme.
OS+hV->O(lD) +0, >.~4100 A ->0 (lD) +0, (t.) >.~3100 A ->O(lD) +O,(~) >'9660 A
0(1D) +Os->20,
O(lD)+N,->O+N,
O(lD) +0,->0+0, ->o+o,(c.) ->O+o,(~)
k4=k4a+k4b+k4c
0,(2:) +03->20,+0
0,(2:) +N,->o,+N, wall
0, (2:) -->0,
0, (C.) +Os->20,+0
0, (c.) +0,->20, wall
O2 (C.) --->02
0+03->20,
O+o,+M->Oa+M wall
O-->!O,
( 1a) (lb) (lc)
(2)
(3)
(4a) (4b) (4c)
(5)
(6)
(7)
(8)
(9)
(10)
(11)
(12)
(13)
cules-I'second-I unless stated otherwise) whereas the rate constants for deactivation of 02(~) by N2 and O2 are (2.1±0.3)X10-15 and lXlO-Is, respectively (see Ref. 20 for a summary of quenching measurements). The bulk of our results have been obtained at [N2J/[OSJ ratios less than 300 so that Reaction (6) adds at most a small correction term to the deactivation rate by (5). The wall deactivation rate k7 is given by2l 'Yc/2R where 'Y is the probability of wall deactivation per collision, c is the mean molecular velocity, and R is the radius of the cylindrical reaction vessel. Values of 'Y in the range 2X 1O-L 2X 10-2 have been reportedl9 and even for the largest value, k7 will be 1.5X 102 secl in our system. For comparison, at the lowest Os pressure we have used, the first-order rate of disappearance of 02(~) by Reaction (5) is lX103 secl • Radiative loss of 02(~) is completely negligible and for our experimental conditions the predominant removal process for 02(~) is the reaction with ozone.
The processes energetically capable of forming O2 (~) are (lc), (2), (4c), and (11). The discussion of the possible effects of Reaction (11) will be deferred until later. Figure 2 shows the variation of 02(~) intensity with addition of O2 to mixtures of 0 3 and SFe and with addition of N2 to mixtures of 0 3 and O2 • SFs was chosen as an inert diluent since it is known not to deactivate 0(ID).22 If the 02(~) were formed by photolysis (1c) its intensity should be independent of added N2 or O2
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4320 M. GAUTHIER AND D. R. SNELLING
.09
.07
en + I-Z :J .06
>-a:: <t a:: I- .05 OJ a:: <t
- .04 I .--,
+ 01
W .03
OJ 0
>- .02 k2 SLOPE I-
~ = (jj INTERCEPT z w I-Z
I----J
o .2 .4 .6 .8
which is inconsistent with the results in Fig. 2. There will be a competition for 0 (1D) between O2 and 0 3
[Reactions (4) and (2)]. If the 02(~) were formed by the reaction of O(ID) with 0 0 (2), the 02(~) intensity should decrease with addition of O2 to mixtures of 0 0 and SF6, since the 0 (ID) will then be removed by O2 rather than by 0 3 • This is again contrary to our observations shown in Fig. 2. Our results are consistent with a mechanism involving 02(~) formation by energy transfer from O(1D) (4c), since in this case, the intensity should increase with addition of O2 to 00/SF6
mixtures and decrease with addition of N2 to 0 0/02 mixtures.
B. Formation of 02(~) by Reactions (Ic) and (2)
Although our results are entirely consistent with (4c) as the sole reaction forming 02(~) we can set limits on
+
+ +
FIG. 3. Reciprocal intensity of the O2(11:.+) 7619- A emission versus [03J/[02J for a mixture of 0 3, O2, and SF6•
= 9.7 ± 1.5
1.0 1.2
the possible contributions from Reactions (Ic) and (2). An OeD) atom is formed for each 0 3 molecule photolyzed,9 and if we assume for the moment that k4b = k4a =0 then at high [02]/([N2]+[00]) ratios each O(1D) atom will produce 02(~) by (4c). Formation of 02(~) by photolysis (Ic) will be independent of added N2 and O2 while the contribution from (4c) will decrease with addition of N2 to 0 3/02 mixtures. The results (Fig. 2 curve A) show that at higher N2 pressures, where most of the O(1D) atoms are deactivated by N2, the 02(~) intensity is reduced to 5% of its value in the absence of N2 where almost all the 0 (1D) atoms are deactivated by O2 • Thus no more than 5% of the photolysis produces O2 (~). It will be shown in a later section that the decrease in O2 (~) intensity cannot be attributed to quenching of O2 (~) by N2.
A similar argument and the results for 00/0z/SF6
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PHOTOLYSIS OF OZONE AT 2537 .'\ 4321
mixtures (Fig. 2 curve B) can be used to obtain an upper limit for the contribution from Reaction (2). Since all the 0 (ID) atoms must react with Oa in the absence of O2, production of 02(~) in (2) should lead to a positive intercept for the 02(~) intensity for curve B at zero O2 pressure. The data show that this intercept can hardly be more than 5% ofthe max O2 (~) intensity at higher O2 pressures where nearly all the 0 (ID) atoms are deactivated by O2 • Thus less than 5% of the O2
formed in Reaction (2) can be in the I~g+ state. If 02(~) is produced with less than unit efficiency in
Reaction (4) (i.e., k4b or k4a~0) then both the limits given above must be reduced correspondingly. This point is discussed further in a later section on production of 02(~) in the atmosphere, where it is concluded that k4c/ (k4a+k4b+k4c)?0.6.
C. Relative 0 (1 D) Quenching Rates
Assuming a formation mechanism by Reaction (4c) and deactivation by (5) the steady-state concentration of O2 (~) will be given by
[02(~) J= fJdk5(1 + ( (ka[N2J+k{OaJ)/k4[02JI), (A)
where 11[03J is the rate of ozone photolysis, k4= k4a+ k4b+k4c , and j (= k4c/k4) is the fraction of 0 (ID) molecules quenched by Oz which produce Oz(~). If we then multiply the expression (A) by the Einstein A value for the 7619-A transition we have an expression for the observed intensity of 7619-A emission. Since we are only concerned with relative intensities, the A factor is omitted in the equations that follow and we implicitly assume that the intensities observed are proportional to the steady-state concentration. II is approximately constant over the range of ozone pressures used since the absorption of light is small and hence approximately linear in oZOne. For example for an ozone pressure of 0.03 torr and a maximum path length of 7.0 cm only 7.5% of the incident light is absorbed. (Because of light losses in the NiS04 solution and in the quartz and also at the various surfaces, radiation is effectively limited to one transit through the celi.) It should be emphasized that for nearly all experiments the 0 3 pressure was maintained constant to ± 10% and over this range of pressures the linear absorption approximation will be excellent.
For the results in Fig. 2, where Oz was added to mixtures of Oa and SF6, the steady-state expression (A) can be rearranged to give
where K is a constant. Thus a plot of the reciprocal of the observed 7619-A intensity against [OaJ/[02J should be a straight line and the value of slope divided by intercept will give the ratio kdk4• Such a plot is shown in Fig. 3 and it gives a good straight-line fit. The data were fitted by a least mean squares routine and the value of kdk4 so obtained is 9.7±1.5. The stated error
110 " ~ 0
100
~ B INTENSITY
O. (.,,~)
90
>-t: en 80 <C z UJ
A INTENSITY
I-~+ ?" 0, ~
Z 70 0
UJ <!)
>-x 0
60 r-UJ --' FOR CURVES A AND B '" z 50 iii
0, PRESSURE .72 TORR UJ
DECOMPOSED .21 > FRACTION 0,
i:i 40 FLOW RATE 2.0 LITER SEC-I --' w a:
30
20
OZONE PRESSURE TORR
FIG. 4. Variation of relative singlet oxygen emissi~n w!t~ 0 3
pressure for a mixture of 0 3 and OJ. A, 02(1~0+) 7619-A emiSSIOn; B, O2(16.0) 1.27-.u emission; C, see text.
was obtained by taking twice the standard deviation and adding a further 10% to account for possible systematic errors in measuring flow rates and Oa concentrations.
With this value of kdk4 we can use the data in Fig. 4 to check the earlier conclusions that II is constant and that Reaction (5) is the predominant removal process for 02(~). Figure 4 (curve A) shows the variation of 02(~) intensity with varying Oa pressures and constant O2 pressure and flow rate. From (A) this intensity multiplied by 11+ (k2[OaJ/k4[OzJ)} (this quantity is plotted as curve C in Fig. 4 with kdk1=9.7) is proportional to jft/k5• The plot shows that this quantity is constant over the range of ozone pressures of 0.004-0.028 torr. If wall deactivation were important then k5 would have to be replaced by k5+hd[03J in the equations above and a falloff would be expected at lower ozone pressures. If we take an upper limit of 5% in the possible falloff for 0.004 torr of 0 3 and use a value of k5 of 7X 10-12 this leads to the conclusion that h7~ 50 sec1 and therefore that l' ~ 8 X 10-3• There is evidence to be discussed later that h5 is about a factor of 4 greater than the value used above and if this is true the limits for 117 and l' given above should be increased by this factor. Since the plot shows no pronounced falloff at higher ozone pressures we also conclude that II is constant and hence that the linear absorption approximation is a good one.
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4322 M. GAUTHIER AND D. R. SNELLING
5.5
5.0
4.5
4.0
?'1?3.5
.:I#!N .x" 3.0
+ ....=....- 2.5 .---...
-I
~ .. 2.0
"---'" 1.5
1.0
0.5
0
'1
'1 0
'V '1
0 0
w
x 0
0,~~~-L-72~~3~L-~4~~5~L-~6~~7
[N2] 1[02]
FIG. 5. Variation of the ratio of 0., (l2;g +) intensity with and without addition of N,(I/Io) versus [N,J/[O,].
For the N2 results, in order to plot several experiments on a common scale, we have divided the observed intensity with addition of N2 (I) by the observed intensity in the absence of N 2 (Io) and this quantity, expressed as a percentage, is plotted in Fig. 2. From the steady-state expression (A), it can be shown that
10 1+ {(k{N2J+k2[03J)/k4[02JI I 1+ (~[OaJ/k4[02J)
and this expression can be rearranged to give
(~ -1)(1+ k{03J) = k3[N2J . (B) I k4[02J k4[02]
A plot of the left-hand side of (B) (with kdk4=9.7) versus [N2J/[02J is shown in Fig. 5 for all the data for which [N2J/[02J:::; 7. The data at higher values of [N2J/[02J were excluded to minimize a possible correction for quenching of 02(~) by N2 to be discussed below. The data show a good straight line fit and the least mean squares slope of the line is 0.72 which, frolI} (B) above, can be equated to ka!k4 • If deactivation of Oa (~) by nitrogen is included then k5 in the expression (A) above must be replaced by k 5{1+ (k6[N2]! k5[Oa])}. Thus the term in braces appears as a correction factor and if we multiply the observed intensities in the presence of nitrogen (I) by this factor to obtain a corrected in tensi ty (Ie) then the expression (B) still holds if we replace I by Ie. With the values of k6 and k5 given above, the maximum value of k6/k5 is 4.3X 10-4
•
Using this ratio to obtain Ie the data in Fig. 5 have been recalculated and the slope obtained is 0.68. However work in progress in the authors' laboratory indicates that the value of k5 of 7X 10-12 is about a factor of 4 too low and this conclusion is supported by a recent direct measurement23 of k5 of 2.5X 10-11 • Using this value and a recent direct measurement24 of k6 of 1.8X 10-15, a more probable value for k6/k5 of 6.4X 10-5 is obtained and for this value quenching of O2 (~) by N2 is negligible for the data used in Fig. 5 and produces a maximum correction of 10% to the intensities at higher [N2J/[02J ratios shown in Fig. 2. We therefore adopt the uncorrected ratio of 0.72 as our best estimate of k3/k4 •
Using an average value of [02J/[03J the expression (B) has also been used to calculate the curves shown in Fig. 2 where the solid and dotted lines represent the uncorrected and corrected calculations, respectively. The solid line is a better fit to the data at high [N2J/ [02J ratios which is further confirmation that the maximum value of k6/k5 assumed is too high.
The error in our estimate of k3/k4 (0.72±0.08) was obtained by taking twice the standard deviation of the slope obtained from Fig. 5 and adding an 8% term to account for possible systematic errors in flow calibrations and in the ratio kdk4•
D. Comparison of Relative O(1D) Deactivation Rates
It has been established that deactivation of OeD) by O2 and N2 is almost collision limitedl3 but in this section we will discuss the relative deactivation rates only. From studies of the flash photolysis of 0 3 a value of kdk3 of 14 was obtained25 in excellent agreement with our value of 13. However, from more recent experiments and a re-evaluation of the earlier work26 the values of k2 :k4 :k3 obtained were 0.27: 1:0.13, respectively, in poor agreement with our own values of 9.7:1:0.72. In these experiments, and in similar work in our laboratory, a large O2 (~) production at relatively low [OaJ/[02J is required in order to explain the results. However, one possibility is that the reaction
may be very fast and the rates attributed to Reaction (4) in the flash photolysis work should really apply to the above reaction. The analysis used to obtain k2, ka, and k4 in Refs. 25 and 26 involves fitting experimental data to a numerical integration of all the rate processes involved. While including the reaction of OeD) with 02(ll) would primarily reduce the value of k4 required to fit the experimental data it could conceivably change the values of k2 and k3 obtained as well. This reaction can be important in the flashphotolysis experiments where 25%-50% of the 0 3
is photolyzed in a short time period ('"'-'20 jLsec) so that the instantaneous O2 (ll) concentration is very high. By contrast in our present experiments <5% of the 0 3 is
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PHOTOLYSIS OF OZONE AT 2537 X 4323
photolyzed over a much longer period ("-'1 sec) so the steady-state 02(~) concentration is much lower. We can readily estimate the 02(~) concentration in our experiments since it is produced by photolysis of OJ and removed by reaction with Oa (8). For our maximum production rate (S% photolysis in a period of 1 sec) the steady-state O2 (~) concentration is then given by 0.OS/k8 or SX 10-4 torr. Thus the reaction of O(!D) with Oa, O2, or N2 will predominate over the reaction with 02(~) in our experiments.
Young et al.27 photolyzed O2 at 1470 A to produce an OeD) and an O(3P) atom and determined relative quenching efficiencies indirectly by measuring the O(3P) concentration by titration with NO in mixtures containing either N20 or H2 with the quenching gas. o (lD) reacts with both H2 and N20 and addition of sufficient quenching gas to compete with these reactions leads to an increase in O(3P) production. With this method values of ka/k4 of 0.S7 and 1.2 were obtained for H2 and N20, respectively. The authors give arguments in favor of adopting the latter value but since their method is rather complicated we feel that the agreement with our results is satisfactory. Noxon!a also photolyzed O2 at 1470 A and deduced relative quenching efficiencies more directly by following the decrease in O2 (~) with addition of N2 and obtained a value of ka/k4 of 1.3±0.2. However, this ratio was calculated from only two points and perhaps with somewhat wider error limits this result would be consistent with our measurement. DeMore28 has recently obtained a value of ks/k4 of 1.0.
Our results and those of Young, Noxon, and DeMore indicate that the ratio of ka/k4 is approximately unity
1.03
1.02 a 0
1.01 Cl
§ 1.00 0 60 • •• ,.. .99 6 06 !1i 0 a:: .98
j 6 .97
--~'"
p;. ~. PSF6
FRACTION O. 0"
TORR TORR TORR DECOr.f'OSED ,.. 0 ... .030 .043 .54 .18 u;
z w 6 .031 .061 .82 .23 ... ~
a .014 .073 .78 .20
.2 .3 .4 .5 .6 .7 .8 .9
9.7 1'>0./( 9.7 po. + 1'>0. + .72PN• )
FIG. 6. Variation of 02('A.) intensity with addition of N2 to mixtures of 0 3 and SF6• The solid points are for no added N2 and the abscissa is the fraction of O('D) atoms reacting with 0 3 , The O. is present as an impurity in the 0 3•
1.02
1.01 <;J <;J
0 <;J
<;J 1.00 °0 0 0 •• • <;J ., .99 0 ...
~ .98 <;J
! <;J
.97 0 0 0 .. . 96 .<l
0"
,.. po. po. FRACTION O.
i TORR TORR DECOMPOSED
... 0 .042 1.16 .20 ... 0 .027 .68 .19 ~
'<J .032 1.10 .20
.2 .3 .4 .5 .6 .7 .8
FIG. 7. Variation of O.(IAg) intensity with addition of N2 to mixtures of 0 3 and O2• The solid points are for no added N. and the abscissa is the fraction of O('D) atoms reacting with 0,.
and the lower value obtained from flash-photolysis studies could perhaps be attributed to the reaction of OeD) with 02(~) discussed above.
E. Production and Removal of 02(1~g)
The principal process removing 02(~) is the reaction with Oa with a rate constant (k8) of """3.SX 10-15 at 300o K.29,80 We have also measured kg,a! and the roomtemperature rate is in agreement with the above value although the temperature dependence is about a factor of 2 greater than has been found previously.SQ Wall deactivation becomes important at lower Os pressures and we have measured this rate constant in our system (k!O=0.2 sec!) using mixtures of benzene and O2•4
Collisional deactivation by N24,32 and SF63! is negligible while deactivation by 0 2 4,32 contributes a constant term of <3% to the total deactivation rate for the experiments at high O2 pressures (Fig. 7). Thus the rate of removal of O2 (4) is given by R= ks[Oa]+klO+k9[02]. At very low Os pressures «0.01 torr) the 02(~) lifetime becomes sufficiently long that there is some loss of 02(~) by pumping from the reaction vessel.
Since we have shown previously5 that the bulk of the O2(4) is formed by photolysis then the steady.state concentration is given by
The variation of 02(~) intensity with Os pressure is shown as curve B in Fig. 4, and, as expected from the above expression, the intensity falls off at low Os pressure and tends to a constant value at higher pressures when the term ka[Oa] in R predominates. For the 02(~) experiments in Figs. 6 and 7 we have kept the
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4324 M. GAUTHIER AND D. R. SNELLING
average Os pressure constant to ±5% and the intensities have been corrected for the small variations which remain by using the data in Fig. 4 and other similar curves.
Previously5 we found that the 02(Ll) intensity showed no variation with addition of O2 to mixtures of 0 3 and SF6 but that it did decrease by 10% with addition of sufficient nitrogen. However, in more recent experiments we have not observed this decrease and we now find that the 02(Ll) intensity is independent of added N2 or O2• The variation of 02(Ll) intensity with addition of Nz to mixtures of Os/SF6 and OS/02 is shown in Figs. 6 and 7, respectively. The abscissa in Fig. 6 is the fraction of OeD) reacting with Os (i.e., k2[03]/ (kz[Os]+k4[02]+ks[N2]) I and in Fig. 7 the abscissa is the fraction of 0 (1 D) reacting with O2 (i.e., k4[02]/ (l?4[02]+k2[Os]+k£N2]) ). For the data in Figs. 6 and 7, in the absence of added N 2 ,
the bulk of OeD) reacts with 0 3 and 02, respectively, but with addition of sufficient N2 the predominant fate of O(1D) becomes deactivation by N2. Since the 02(Ll) intensity is constant over this range of experimental conditions then Reactions (2) and (4) cannot be significant sources of 02(Ll). We can deduce upper limits for the production of 02(Ll) in Reactions (2) and (4) from the data in Figs. 6 and 7, respectively. If we assume that O2 (Ll) is produced with unit quantum yield by photolysis then an OeD) atom is produced for each O2 (Ll) molecule. Since Fig. 6 shows that the increase in O2 (Ll) intensity can hardly be more than 2% for a change from 0.1 to O.S in the abscissa then less than 3% of the O2 formed in Reaction (2) can be in the Illy state. Similarly from Fig. 7 the variation of O2 (Ll) intensity is less than 3% over a range of 0.2-O.S in the abscissa therefore < 5% of Reaction (4) produces an 02(Ll) molecule (i.e., 1?4b/k4:S;0.05). If the quantum yield of 02(Ll) production in (1) is less than unity then the limits given above for 02(Ll) formation in (2) and (4) must be reduced correspondingly.
We have been unable to satisfactorily explain the earlier experiments which showed a 10% reduction in 02(Ll) intensity with addition of N2. In the earlier experiments spectral slitwidths of 60-120 A were used and in general the intensity was monitored by measuring the peak of the 1.27 -J.L emission. In the present experiments narrower spectral slitwidths (36 A) were used and the O2 (Ll) intensity was also monitored by scanning the 1.27-J.L band and measuring the area. It is possible that the observations in the earlier experiments were contaminated by emission other than the O2 band and indeed we have observed very weak emission from the 8-5 Meinel band of OH in this region. However, in our present experiments, this emission is less than 10% of the O2 (Ll) intensity and observations of the much stronger 2-0 and 3-1 bands of OH at longer wavelengths indicate that this emission decreases more rapidly with addition of O2 than with N2. The source of this radiation appears to be due to traces of water
vapor and the reaction sequence
o (ID) + HzO---+20H,
OH + 0---+02+ H,
H+03---+OH+02,
(14)
(15 )
(16 )
can lead to production of vibrationally excited OH in levels v:S;9 by Reaction (16). DeMoress has suggested that a reaction between vibrationally excited OH and 0 3 can occur,
(17 )
and this reaction could replace Reaction (15) in regenerating hydrogen atoms.
The intensity of the LlV= 2 sequence of OH was approximately equal to the O2 (Ll) intensity but a reasonable estimate of the transition probability for OR is 10 sec l while for 02(Ll)S it is 2.7X 10-4 secl
• Thus the OH concentration is about 3 X 10-5 that of O2 (Ll) and could be produced by traces of water vapor.
The reaction of 0 atoms with OJ is sufficiently exothermic that one or both of the product O2 molecules can be in the Illy or I~y+ states. The reactions leading to loss of 0 arc (11)- (13) and from the available data21 ,s4 it can be shown that, for our conditions, Reaction (11) will predominate. The O-atom production rate will be highest for excess O2 when, for each 0 3
molecule photolyzed, three 0 atoms arc produced in Reactions (4), (5), and (8). For excess N2 Reactions (3) and (S) produce two 0 atoms while for the condition of low N2 and O2 the 0 (lD) atom is lost in Reaction (2) and only Reaction (S) now produces an 0 atom. Thus if Reaction (11) were a source of either singlet molecule the intensity would be expected to increase with addition of both N2 and O2 but more so in the case of the latter. This is contrary to our observations for both 02(Ll) and 02(~)'
A stronger argument can be made by analyzing the results for the red photolysis of Os [which can only produce an Oep) atom] which showS. that a maximum quantum yield of 2 is observed under conditions where any 02(Ll) or 02(~) formed in (11) would react with Os to produce an 0 atom. This would then constitute a chain reaction. We can estimate the limits this puts on singlet O2 production by noting that for the red photolysis experiments in almost pure 0 3,35 essentially all (>96%) the 0 atoms formed react with 0 3 and that any singlet oxygen formed in this reaction would also react with 0 3 • Under these conditions the quantum yield is given by 2+ x/ (1- x) where x is the fraction of 0+03 reactions which produce a singlet O2 molecule. The absolute accuracy of the quantum yields is given as 5% so we can conclude that the quantum yield is less than 2.1 and therefore that x:S;0.05. A more detailed analysis of all the results by one of the authors shows that a more realistic limit is x:S;O.03. Thus less than 3%
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PHOTOLYSIS OF OZONE AT 2537 X 4325
of the oxygen formed in (11) is in either singlet state and the reaction will not be important under our conditions where the maximum quantum yield of O-atom production is 3.
G. Production of 02(~) in the Atmosphere
Dayglow profiles of the 7619-A emission indicate that there is a source of O2 (2;) production below 60 km which was attributed to solar ultraviolet photodissociation of 0 3 at wavelengths less than 2660 A (lc).15 A quantum yield of 0.25 or greater for solar radiation below this threshold was required to explain the profile. Our observation that O2 (2;) is not produced by photolysis at 2537 A, well below the energy threshold, suggests that this process is not important at shorter wavelengths either. Energy transfer from 0 (ID) was also consideredl5 as a source of O2 (2;) with k3/ 4c =5. Thus, in the atmosphere, about 5% of the O(ID) from the photodissociation of 0 3 was assumed to form O2(2;). With our value of k3/k4' up to 27% of the O(ID) can form O2(2;) if k4a =k4b =0, and this source can explain the production below 60 km without recourse to process (lc). Of course for this alternative explanation of O2 (2;) production the contribution of (4a) and (4b) must be small compared to (4c). Noxonl3 has measured the O2 (2;) yield from 1470-A photolysis of O2 and concluded that k4c/k4 was probably unity but certainly greater than 0.3 but Wayne and co-workers, using the same source, found originally that k4c/k4= 1/50019 and after some improvements in the O2 (2;) detection system that !?4c/!?4= 1/180.14 However, Clark36 has shown that with the flow system used for these latter measurements there is considerable loss of O2 (2;) due to vortex formation around the photolysis window and that the estimate of !?4c1!?4 could be a factor of 50 too small.
Flash-photolysis experiments in the authors laboratory37 with 0 3/02 and 03/N2 mixtures show that the fraction of 0 3 decomposed is 60% highet for the 0 3/02
mixtures in the time period 200-1000 J.Lsec after the flash. Ozone and added gas pressures of 0.01 and 10 torr, respectively, were used and the fraction of 0 3 photolyzed was varied from 0.25 to 0.50. The reaction rate of OeD) with O2 is almost collision limited [!?4= (6±3) X lO-11J and since, in the O2 experiments, there is at least a 1000-fold excess of O2 over any other species then essentially all of the 0 eD) must react with O2. Thus the experimental results require that the reaction of OeD) with O2 forms a species capable of decomposing an amount of 0 3 which is 60% of the initial amount photolyzed, with a rate constant of 10-11 or faster. These results can be easily understood if O2 (~) is the species formed and if !?4c/!?4?:.0.6. A complete analysis of these experiments may indicate that !?4c/!?4 is somewhat greater than 0.6 since we have not allowed for consumption of 0 3 by O2 (2;) during the
flash. When we make allowance for this effect the fraction of 0 3 photolyzed will be less in the experiments with Oa/02 than with 03/N2.
Thus we feel these results and Noxon's show that k4c/!?4?:.0.6 and hence that production of O2(2;) by (4c) is sufficient to explain the low altitude airglow measuremen ts.
1 J. F. Noxon, Space Sci. Rev. 8, 92 (1968). 2 J. H. Miller, R. W. Bocse, and L. P. Giver, J. Quant. Spectry.
Radiative Transfer 9, 1507 (1969). 3 R. M. Badger, A. C. Wright, and R. F. Whitlock, J. Chern.
Phys. 43, 4345 (1965). 4 F. D. Findlay, C. J. Fortin, and D. R. Snelling, Chern. Phys.
Letters 3, 204 (1969). 5 M. Gauthier and D. R. Snelling, Chern. Phys. Letters 5, 93
(1970) . 6 W. F. J. Evans, D. M. Hunten, E. J. Llewellyn, and A.
Vallance Jones, J. Geophys. Res. 73, 2885 (1968). 7 A. Vallance Jones and R. L. Gattinger, Planetary Space Sci.
11,961 (1963). 8 R. L. Gattinger and A. Vallance Jones, Planetary Space Sci.
14,1, (1966). 'W. B. DeMore and O. F. Raper, J. Chern. Phys. 44,1780
(1966). 10 T. P. J. Izod and R. P. Wayne, Nature 217,947 (1968). See
also R. P. Wayne, Advan. Photochern. 7,337 (1969), for further discussion.
11 I. T. N. Jones and R. P. Wayne, "The Photolysis of Ozone by Ultraviolet Radiation. V. Photochernical Excitation of O2 (1Ll.) ," Proc. Roy. Soc. (London) (to be published).
12 R. A. Young and G. Black, J. Chern. Phys. 47,2311 (1967). 13 J. F. Noxon, J. Chern. Phys. 52,1852 (1970). 14 T. P. J. Izod and R. P. Wayne, Chern. Phys. Letters 4, 208
(1969) . 15 L. Wallace and D. M. Hunten, J. Geophys. Res. 73, 4813
(1968) . 16 L. Wallace and M. B. McElroy, Planetary Space Sci. 14, 677
(1966) . 17 J. C. G. Walker, Planetary Spacc Sci. 16, 321 (1968). 18 P. N. Clough and B. A. Thrush, Chern. Ind. 1966, 1971. 19T. P. J. Izod and R. P. Wayne, Proc. Roy. Soc. (London)
A308, 81 (1968). 20 E. C. Zipf, Can. J. Chern. 47,1863 (1969). 21 F. Kaufrnan, Progr. Reaction Kinetics 1, 3 (1961). 22 K. F. Preston and R. J. Cvetanovic, J. Chern. Phys. 45,
2888 (1966). 23 R. Gilpin, H. I. Schiff, and K. Welge (unpublished). 24 S. V. Filseth, A. Zia, and K. H. Welge, J. Chern. Phys. 52,
5502 (1970). 25 D. R. Snelling and E. J. Bair, J. Chern. Phys. 47,228 (1967). 26 D. Biedenkapp and E. J. Bair, J. Chern. Phys. 52, 6119
(1970) . 27 R. A. Young, G. Black, and T. G. Slanger, J. Chern. Phys.
49,4758 (1968). 28W. B. DeMore, J. Chern. Phys. 52, 4309 (1970). 29 R. P. Wayne and J. N. Pitts, Jr., J. Chern. Phys. 50, 3644
(1969), and R. J. McNeal and G. R. Cook, ibid. 47, 5385 (1967). 30 I. D. Clark, I. T. N. Jones, and R. P. Wayne, Proc. Roy.
Soc. (London) A317,407 (1970). 31 F. D. Findlay and D. R. Snelling (unpublished). 32 I. D. Clark and R. P. Wayne, Proc. Roy. Soc. (London)
A314, 111 (1969), and R. P. Steer, R. A. Ackerrnan, and J. N. Pitts, Jr., J. Cheql. Phys. 51, 843 (1969).
33 W. B. DeMore, J. Chern. Phys. 46, 813 (1967); 47, 2777 (1967).
34 H. 1. Schiff, Can. J. Chem. 47, 1903 (1969). 35 E. Castellano and H. I. Schumaker, Z. Physik. Chem.
(Frankfurt) 34, 198 (1962). 361. D. Clark, Chem. Phys. Letters 5, 317 (1970). 37 See also D. R. Snelling and E. J. Bair, J. Chem. Phys. 48,
5737 (1968) and Ref. 26 for a description of similar experirnents and also Ref. 25 for a description of the technique.
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