110
J. Phys. Chem. 1980, 84, 119-122
Chemiluniinescence of 02(1Xg+-+3Eg-)and SO('Xt-+3E-) in the 0-COS-02('Ag)/021 System T. Ishiwata and I. Tanaka" Department of Chemistry, Tokyo Institute of Technology, Ohokayama, Meguro-ku, Tokyo, 152, Japan (Received June 6, 1979) Publication costs assisted by the Tokyo Institute of Technology
Chemiluminescenceof 02(1Zg++3Z;) and SO(18+-+3Z-) was studied in the O-COS-02(1Ag)/02system. These emission intensities were strongly dependent on the SO and 02(lAg) concentrations. Results show that Oz(lZ,+) was produced by the following mechanism: O2(lAg)+ SO(3Z-) -* 0&2:g) + SO(1A); 02(1$)+ SO('A) OZ(lZg+) + SO(3Z-), in addition to the energy pooling reaction between 02(lAg). The following mechanism for the formation of SO(lZ+) in this system is proposed: SO(lA) + SO('A) SO('Z+) + SO(3Z-).
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Introductiain Gas phase quenching of singlet molecular oxygen (lA and l Z +) in the presence of deactivating atoms and molecufes is one of the most extensively studied areas of research on energy transfer. It has been suggested that singlet molecular oxygen provides a means of energy storage and plays an important role in combustion processes and in polluted atm~spheres.l-~ Singlet molecular oxygen is now believed to be responsible for HNO chemiluminescence in the system active oxygen-N0-olefins through electronic-electronic transfer proce~ses.~ The SO, like 02,molecule also possesses lower lying singlet states. Colin5 observed the second singlet state (l2+)by the transition SO(12+-+3E-). Carrington, Levy, and Miller6 observed the ESR spectrum of SO(lA) when active oxygein was mixed with carbonyl sulfide. They postulated that the formation of SO(lA) in this case could be attributed to the direct energy transfer reaction OZ(lAg) + SO(3E-) SO(lA) + 02(3E,-). This process was confirmed by Brleckenridge and Millers7 However, little has been reported on the kinetics involving these two excited states of SO, l2? and lb. In the present work, the energy transfer reaction to yield the electronically excited 02(lEg+) and SO(l2+)is studied in the O-COS-Oz(1AJ/02 system. The SO(lA) molecule is shown to a c t as an energy carrier and to give rise to chemiluminescence in the infrared region by a disproportionation reaction between SO(lA) molecules.
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Experimental Section Infrared emission was studied in the conventional discharge flow system when O2(lAg)was added to the reaction system of oxy,genatoms with carbonyl sulfide. The basic flow apparatus has been described previ~usly.~ Atomic oxygen was generated by passing oxygen diluted in argon through a microwave discharge or by NO titration of active nitrogen. COS was then introduced from another inlet downstream. Singlet oxygen was added to the flow through a cold trap to remove mercury vapor. The metastable oxygen molecules, free from oxygen atoms, were formed by subjecting oxygen diluted in argon to a microwave discharge over HgO film. The emissions were observed from a window 4 cm downstream of the mixing point. A flow speed of about 650 L/min was used, corresponding to a typical flow speed of 2 m/s. The flow rates of gases were controlled by needle valves, and measured by calibrated flowmeters or by timing a pressure drop in a known volume. 0022-3654/80/2084-0 119$0 1.OO/O
The infrared emission was dispersed by a Nikon monochromator (G-500, f = 8.6) with a grating of 1200 grooves/mm at a blazed wavelength of 500 nm and was detected by a Hamamatsu TV R-316 photomultiplier (S-1 type) cooled at -20 "C. The output was fed to an electrometer and recorded by a strip chart recorder. In order to eliminate second-order light, a long wavelength pass filter with cutoff at 560 nm was placed in front of' the entrance slit. To monitor NOz and SO2afterglow, a Nikon monochromator (P-250, f = 4.8) with a grating of L200 grooves/mm blazed at 300 nm was combined with a Hamamatsu TV R-376 photomultiplier cooled at -20 "C. A PbS cell (Hamamatsu TV R-819), equipped with an interference filter (Arnm = 1270 nm, AA1j2 = 23 nm), was placed upstream of the observation vessel to monitor the emission of OZ('A -32.9-). The infrared em-ission was and the signal was measured with a modulated at 19 lock-in amplifier (NF LI 512). Argon, nitrogen, oxygen, and nitric oxide from commercial cylinders (Takacliiho research grade), and carbonyl sulfide from a Matheson lecture bottle were used without further purification.
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Results and Discussion When carbonyl sulfide reacted with active oxygen containing both oxygen atoms and metastable 0 2 ( l A ) molecules, near-infrared emission bands were observec! at '760, 860, and 955 nm. The emission bands at 760 and 860 nm correspond to the (0,O) and (0,l) transitions of the atmospheric band, Oz(1Eg++32g-). The chemiluminescence appearing at 955 nm could be assigned to the strong (0,O) and weak (1,l)bands of the (1F+32.9-)transition of the SO molecule. It should be noted that no infrared spectrum could be detected when atomic oxygen generated by NO titration of active nitrogen reacted with carbonyl sulfide. However, if singlet molecular oxygen is added to the stream containing 0 atom and COS, infrared spectra which were identical with those described above could be observed. ,In the O-COS-02(1Ag)/02 system, strong afterglow appeared by the reaction so + 0 so,* (1) in which SO was formed from the reaction of COS with 0 atoms. The presence of SO resulted in a remarkable emission intensity and in the increase of the 0,(12g+-*3&-) appearance of additional bands of SO(1F+38-). The intensities of these infrared emission bands were strongly dependent on the SO and 02(lAg)concentrations. In orlder to examine the dependence of emission intensity on the SO concentration, the infrared emission intensities wiere
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0 1980 American Chemical Society
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The Journal of Physical Chemistry, Vol. 84, No. 1, 1980
Ishiwata and Tanaka
(3.2) I
t
0
2
1
3
C o n c e n t r a t i o n of 0
5
SO
10
15
(arb,)
Figure 3. Emission intensity of S0(1Z+-+3Z-)vs. the SO concentration. Parentheses indicate the emission intensity of 02(' -+3ZJ in arbitrary units. Flow rates of NP,02,Ar, and COS are 2.0, 2. , 3.0,and 0-0.031 X mol min-', respectively. The titrating point is at a flow rate of 31 X lo-' mol min-' for NO. Total pressure is 0.9 torr. The SO concentration is plotted on a quadratic scale.
?l
C o n c e n t r a t i o n (arb,)
Flgure 1. Emission intensity of O&Z ' :-+3Z;) vs. the SO concentration concentrations. Parentheses indicate the emission for various OZ('Ag) intensity of 02('Ag+38[) in arbitrary units. Flow rates of N p , Opr Ar, and COS are 3.9,3.1,8.9, and 0-0.11 X IO3 mol min-', respectively. The titrating point is at a flow rate of 70 X lo4 mol min-' for NO. Total pressure is 1.1 torr.
ergy pooling reactionlo from the observation of the square dependence of the emission intensity of Oz(l8;) on that of OZ(' A,).
Oz('Ag)
6
-w
+ Oz('Ag)
+
Oz('2pf)
-
+ OzPZg-)
(2)
In the presence of SO, the energy transfer r e a ~ t i o n ~ ~ ~ ~
I
SO(%)
m
t
.0
+ 02(lAg)
SO('A)
+ 02(38g-)
(3)
is known to occur in systems which contain both SO(3Zc-) and O2(lAg) molecules. Then the following reaction (4a) would be possible for the formation of O2(lZ;).
Y
v
4
II)
O2(lAg) + SO(lA)
r
0
=-,
+.-
VI
C
aJ 1
.-C
.-: .-VI
02('%)
-
02(lZg+) + SO(32-)
-
02(38,-)+ SO(lZ+)
(4a) (4b)
The quenching of SO(lA) occurs rapidly by the walls with an efficiency of approximately 0.L7 There would be sufficient time for the steady state concentration of SO('A) to be established between the mixing point and the observation window. Applying the steady state treatment, the relation
2
E w
0
1
2
3
SO
4
5
C o n c e n t r a t i o n (arb,)
Figure 2. Gradient of emission intensity of 02('Zg+-+3Z;) in Figure 1 vs. the O,('A,) concentration. The 02('Ag) concentration is plotted on a quadratic scale.
measured at constant OZ(lAg)concentration with various COS flow rates in the presence of an excess of 0 atoms. The SO concentration was estimated from the afterglow intensity? Iso2 = IOso,[SO][O], where the oxygen atom concentration was measured from INoZ= IONO,[NO][OI by adding a known amount of nitric oxide? The emission was monitored at 300 nm for SOz and at 650 nm for NOz. The NO2 emission intensity was corrected by subtracting the contribution of the SOz afterglow at 650 nm. Figures 1 and 2 show plots of the OZ(l8 + 4 Z ; ) and s0(12+-3z-) emission intensities against do concentrations at various O2(lAg) concentrations. The results in Figure 1show that the intensity of the 02(12,+) emission, which could be observed without introducing SO into the reaction vessel, increased linearly with SO concentration. In the absence of SO, it is clear that the Oz('Xg+) molecules are mainly formed by an en-
+
is obtained, where k4 = k4a klb and k , indicates the rate of first-order removal of SO(lA) presumably by the walls and the electronic quenching by collisions with a third body. The emission intensity of O2(lZg+) is then written by
The first term represents the formation of 02(lZ,+) by the energy pooling process between two OZ(lAg)molecules and the second is due to energy transfer reaction 3 followed by reaction 4a. When the slopes of the lines in Figure 1 are plotted against the square of the 02(lAg) concentrations, a linear relation is obtained, as shown in Figure 3. This relation is reasonable due to the assumption that k, > k4[O2(l$)]. The results in Figures 1and 3 are consistent with relation 6. The emission intensity of OZ(lZgf)was proportional to the square of the O2(lAg) concentrations in the absence of SO. The presence of SO resulted in a linear increase of intensities with the SO concentrations
Chemiluminescence of
The Journal of Physical Chemistry, Voi. 84, No. 1, 1980 121
O2and SO
E (cm-l) Excitation
15000
Exc i t a t i on
'f
10000
1.I
0
20
10
30
I
40
50
0
t I
$=If02
COP
partial
SO
pressure ( i n t o r r )
Figure 4. Variations of emission intenslties of 02(1A,+38J, 02(12 ++32 ), alnd SO(12+-+32-) vs. COP partial pressure. Total pressure is 0.9 torr. Atomic oxygen was produced by a microwave discharge of 0,(:30%)/Ar at a flow rate of 6.9 X lo3 mol min-l. Singlet molecular oxygein was formed by a microwave discharge of 0,(20%)/Ar over HgO film alt a flow rate of 7.4 X mol min-I. Flow rate of mol min-I. COS is 0.04 X
and proportional to the square of the O2(lAg)concentrations. Davidson and Abrahamson'l observed an emission from 02(lZ.,$) when SOzwas irradiated with light (X > 300 nm) in the presence of O2 They explained it by energy transfer from the triplet SOz*, formed by photoabsorption, to the ground state O2 molecule. Reaction 1 yields the triplet SO2*,which sleems to act as a energy carrier in our system. However, the 02(lZg+)molecule would not be formed by this energy transfer reaction in our system because the presence of O,(lA,) was essential to observe OZ(l21' . Accordingly, it is concluded that the SO(lA) mofecule could be used as an intermediate energy carrier and the 02(lZg+)moleicule would be formed by reaction 3 followed by reaction 4a in addition to being formed in energy pooling reaction 2. The emission of S0(1Z++3Z-) could be observed when both SO and OZ(lA,)were present. The results in Figure 2 show that the emission intensity of SO(1Z+43Z-)in the 0-C0S--02(1Ag)/0zsystem is dependent on the square of the SO concentration. It is also found from the slope in Figure 2 that the emission intensity is dependent on the concentration to the power of 1.6. Reactions 7,8, and 4b are possible as an explanation of the formation of SO('Z,+) in the system. SO(~AJ+ S O ( ~ A ) SO(~Z+)+ s0(3~-) (7) Reaction 7 is the energy disproportionation reaction between two SOPA) molecules and is similar to reaction 2. S30(32)1 + 02(12,+) SO(l2') + 02(3Z,-) (8)
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Oz(12g+)is formed both in reactions 2 and 4a and then energy is tranisferred to the SO molecule in the ground state. SO(lA) + O2(lA,) S O ( l P ) + 02(3&-)(4b) In this case, SO(lA) formed in reaction 3 reacts with an excess of 02(lA,)to yield SO(l2+). The 02(12g+) concentration was dependent on the SO concentration ais shown in Figure 1. In order to determine whether (32('Zg.') was involved in the formation of SO(lZ+), the variation of emission intensities from OZ(lAg),O2(lzgt), and SO('Z+) was studied as a function of added C 0 2partial pressure. C02 is known to be an effective quencherl2 of
-
3z-
0
Flgure 5. Schematic diagram of excitation mechanism to produce O,('2:) and SO('Z+) by SO('A).
Oz(lE:), and the 02(1E2)concentration is reduced by an order of magnitude, as shown in Figure 4. The emission intensity of SO(l2+)was also decreased by adding COPto the reaction cell, while the intensity ratios of O2(lZit)to SO(lZ') were varied at each COz partial pressure. If SO(lZ+) is formed by reaction 8 these intensity ratios should be constant, which disagrees with our results. For reaction 4b, the emission intensity of SO(1Z+432-) should show the same behavior as that of 02(lZ +-+3Z;),because reactions 4a and 4b are competitive. T i i s also disagrees with our results. If SO(lZ) is formed by reaction 7, the emission intensity is represented by
Relation 9 is consistent with the results in Figure 3 and predicts the near second-order relation of the emission intensity of SO(%t43Z-) to the 0 2 ( 1 ) concentration. We can suggest that SO(lZ+) is formed?J y reaction 7. Spectroscopic information about the electronic energy of SO(lA) has never been obtained. However, Colin6has proposed that the energy difference between SO('A) and SO(%-) should be approximately 6350 cm-l, judging from the position of the low lying states of isoelectronic species like O2 and S2. Assuming this energy, one finds that reactions 4a and 6 are exothermic and are expected to proceed at room temperature. The emission spectra showed that the rotational temperatures were 330 and 350 K for 02(lZg+)and SO(lZ+),respectively. These molecules are clearly thermalized by collision with a third body or wall, which is probably due to their longer radiative lifetimes. We have shown that SO(lA) is responsible for the infrared chemiluminescence of Oz(12g++32g-) and SO(1E+-+3Z-) in the O-COS-02(1Ag)/Oz system. In reactions 4a and 7, this species acts as an effective energy carrier to the metastable species, 02(lAg) and SO('A), to form O2(lZg+)and SO(lZ+), as schematically shown in Figure 5. Considering the much lower quenching rates of O#h ) by atmospheric gases12than that by the ground state SO(lA) seems to be formed in highly polluted atmospheres with SO. Investigation on the reactivity of SO(lA) should be initiated because of the possibility of SO(lA) being involved in the chemistry of polluted atmospheres.
Sd,
References and Notes (1) "Proceedings of the Symposium on Singlet Molecular Oxygen", Feb 10-13, 1975, Bombay, 1976.
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(2) S. Madroich, J. R. Wiesenfeld, and G. J. Wolga, Cbem. Pbys. Lett., 46,267 (1977). (3) K. H. Becker, E. H. Fink, P.Langen, and U. Schurath, J. Chem. Phys., 60,4623 (1974). (4) T. Ishiwata, I. Tanaka, and H. Akimoto, J. Pbys. Cbem., 82,1336 (1978). (5) R. Colin, Can. J. Phys., 46, 1359 (1968). (6) A. Carrlngton, D. H. Levy, and T. A. Miller, Trans. Faraday Soc., 62, 2994 (1966). (7) (a) W. H. Breckenridge and T. A. Miller, J. Chem. Pbys., 56, 456 (1972).(b)The decay time of SO(’A) can be estlmatedto be a few
Ishiwata and Tanaka milliseconds under our experimental conditions by uslng the data in ref 7a. It is at least 4 times shorter than the reaction time (20ms) of our measurements calculated from the flow velocity. (8) M. A. A. Clyne, C. J. Halstead, and 8. A. Thrush, Proc. R. SOC. London, Ser. A , 295, 355 (1966). (9) M. A. A. Clyne and B. A. Thrush, Proc. R . Soc. London, Ser. A , 295, 404 (1962). (10) R. P. Wayne, Advan. Photochem., 7,311 (1969). (11) J. A. Davldson and E. W. Abrahamson, Photochem. Photobiol., 15,
403 (1972). (12) D.R. Kearns, Chem. Rev., 71, 395 (1971).
