478
J. Phys. Chem. 1980, 8 4 , 478-483
Photooxidation of o-Xylene in the NO-H,O-Air
System
H. Takagi, N. Washida," H. Akimoto, K. Nagasawa,+ Y. Usui,+ and M. Okuda The National Institute for Environmental Studies, P.O. Yatabe, Tsukuba, Ibaraki, 300-2 1 Japan (Received August 1, 1979) Publication costs assisted by the National Institute for Environmental Studies
The reaction products in the photooxidation of o-xylene were analyzed by a new gas chromatograph/photoionization mass spectrometer. There were two types of reaction products, i.e., high- and low-boiling point products. Tolualdehyde, dimethylphenols, and dimethylnitrobenzene were the main high-boiling point products and formaldehyde, acetaldehyde, glyoxal, methylglyoxal, and biacetyl were identified as low-boiling point products. The ratio of the total amount of high-boiling point products formed to the amount of o-xylene consumed was about 17%. On the other hand, the low-boiling point products collected amounted to about 45% of the o-xylene consumed. The reaction mechanisms and the importance of the cleavage reaction of the aromatic ring were discussed.
Introduction Aromatic hydrocarbons constitute a fairly high percentage of the hydrocarbons found in the urban atmosphere according to reports from the USA1 and also from Japan.2 After toluene xylenes especially are the largest component and their reactivities3are 2-4 times larger than toluene. Despite this fact, the reaction products and mechanisms of the gas phase oxidation and photooxidation of xylenes have not been clarified4although the importance of such studies for the photooxidation of aromatic hydrocarbons has been pointed out from the standpoints of aerosol formation5 and eye irritation6 A few studies on the photooxidation of xylenes in the NO,-air system reported previously show that there are two types of reaction products (high- and low-boiling point products). Kopczynski7 studied the photooxidation of various alkylbenzenes in the NO2+ system, and he found aldehydes, COz, CO, formic acid, and PAN from an analysis by IR absorption. Katou and Hanai8 performed a GC/MS analysis of the photooxidation products in the xylenes (1000 ppm)-NO (1000 ppm)-02 (1atm) system, and they reported the production of high boiling point products such as tolualdehydes, dimethylbenzoquinones, nitrodimethylphenols, nitrocresols, and dimethylnitrobenzenes. Nojima et ala9found low-boiling point products such as glyoxal, methylglyoxal, and dimethylglyoxal (biacetyl) in the photooxidation of the xylenes (1000 ppm)-NO (50-200 ppm)-air (1 atm) system. They measured these products by the combination of the wet method and ECD-gas chromatogram analysis. Among the low-boiling point products, biacetyl was found only in the case of o-xylene. These low-boiling point products should be formed by the cleavage of the aromatic ring. The occurrence of this cleavage was also supported by our previous studylo in which 2-3.5 NO molecules were converted to NO2 when one xylene molecule was consumed in the initial photooxidation period (before the accumulation of ozone). Such cleavage of the aromatic ring is a very interesting subject not only for the study of the formation of the photochemical smog but also for the pure photochemistry of aromatic compounds. In this study, we intend to analyze the reaction products of the photooxidation of o-xylene and to estimate the absolute amount of the high- and low-boiling point products. A new GC/PIMS (gas chromatograph/photoionization Department of Chemistry, Ibaraki University, 2-1-1 Bunkyo, Mito, 310 Japan. 0022-3654/80/2084-0478$01 .OO/O
mass spectrometer)" was used t o analyze the products. This GC/PIMS is a useful tool especially for the analysis of the low-boiling point products. This study was done under a lower concentration of xylene and NO than was used in previous ~ t u d i e s . ~ , ~
Experimental Section The reaction chamber, pumping system, and light source were as described previously.1°J2J3 Briefly, a Pyrex cylinder (240 mm i.d., 1660 mm length, and 67 dm3 volume) was used as the reaction chamber. Each end of the reaction chamber was sealed with a Pyrex window. The oil-free pumping system consists of an ion pump, a titanium getter pump, a cryo-sorption pump, and an oil rotary pump with a liquid nitrogen-cooled foreline trap. The light source was a parallel light beam of 200 mm ($1 created by a 500-W xenon short arc lamp. Experimental conditions were o-xylene (3-20 ppm), NO (2-10 ppm), HzO (about 60% relative humidity), and air (1atm) for the analysis of the high-boiling point products, and o-xylene (about 100 ppm), NO (about 20 ppm), H20 (60% relative humidity), and air (1 atm) for the low-boiling point products. Xylene and NO were first pressure read in a 105-cm3flask, and then expanded into the reaction chamber. After that, water vapor (60% relative humidity) and air were introduced through four inlets. After 1.5 h, all the gases were mixed homogeneously. After irradiation, sampling was done through the center port attached to the reaction chamber. The reaction mixture was first sampled into a 700-cm3 glass flask and then concentrated in a GC sampling tube. The sampling tube was a quartz U-tube of 5-mm i.d. and about 200-mm length in which Shimalite-Q was packed. This tube was cooled by liquid nitrogen, and, after being subjected to the concentration process, the tube was heated by a nickel-chrome wire and the sample was fed into the GC, GC/MS, and GC/PIMS directly. Sampling for the GC analysis was made periodically every.45-60 min. When sampling had been carried out 7-10 times, the pressure of the reaction chamber was decreased to 700-650 torr. The pressure drop effect on the measurements was calibrated in advance. GC, GC/MS, and GC/PIMS analyses were made by using a 3-m column of 5% SE-30 on Shimalite-W for the high-boiling point products analysis and a 2-m column of Porapak-T for the low-boiling point products. The GC oven temperature was raised from 50 to 200 "C a t a rate of 5 "C/min for the §E-30 column and maintained con0 1980 American Chemical Society
Photooxidation of o-Xylene
Figure 1. FID gas chromatogram for the reaction products of the photooxidation of the o-xylene (10.7 ppm)-NO (2 ppm)-H,O-aiir mixture.
stant at 150 "C for the Porapak-T column. Product identification was performed by comparing the reference mass pattern and GC retention time. The method for. the new GC/PIMS is described in ref 11. In this study an argon lamp with LiF window was used for the photoionization light source. The most powerful characteristics, of this GC/PIMS are (1) ilons formed by photoionization are mostly parent ions, and (2) products can be ana1,yzed without the interference of the large background by water and air because water and air cannot be ionized by argon resonance lines. Research grade o-xylene from Wako Pure Chemical Ind. was used without further purification except degassing. The gases were Pure Air (dew-point lower than -170 "C) supplied from Nippon Sans0 Co. and NO (Research grade, Matheson), both used without further purification. Standard samples of tolualdehyde, dimethylphenol (2,3-, 3,4-, 24-, 3,5-, and 2,6-), dimethylnitrobenzene (nitroxylene) (2,3-, 3,4-, 2,4-, 3,5-, 2,6-, and 2,5-), dimethylbenzoquinone (2,6- and 2,5-), formaldehyde, acetaldehyde, dimethylglyoxal (biacetyl), and glyoxal were commercially available (Wako Pure Chemical Co., Tokyo Kasei Co., and Aldrich Chemical Co.). The relative humidity of all experiments was about 60%, which was measured by the pressure of water vapor in the reaction chamber before air was added.
Result Figure 1 slhows the FID gas chromatogram for the high-boiling point reaction products which are formed in the photooxidation of the mixture of o-xylene (10.7 ppm)-NO (2 ppm)-H,O-air. Biacetyl, tolualde hyde, dimethylphenol (2,3-, 3,4-), o-methylbenzylnitrate, and dimethyliiitrobenzene (mostly 3,4-) were observed as major products and smaller amounts of dimethylbenzoquinone and diniethylinitrophenols were detected. These products were identified by the mass spectra of GC/MS and GC/ PIMS and the GC retention time which were obtained by the standard samples. The low-boiling point products could not be detected by the FID GC, because the GC sensitivities were too low for these products. The main high-boiling point products increased linearly vs. the irradiation time of 5-6 h as shown in Figure 2. Table I shows the concentration ratio of each high-boiling point product formed to o-xylene consumed. In Table I, the initial concentration of o-xylene and NO, total irradiation time, number of samples during the irradiation time, final Concentration of o-xylene, and the averaged ratio of the products formed to o-xylene consumed, which were an average of the number of the samples used, are listed. Since biacetyl was observed by the FID GC, it was listed i n Table I although it should belong to the low-
The Journal of Physical Chemistry, Vol. 84, No.
5, 1980 479
400
The Journal of Physical Chemistry, Vol. 84, No. 5, 7980
100
200
lrradation Time
300 (min)
Flgure 2. Relative yields of the products measured by FID gas chromatograph vs. irradiation time for the photooxidation of the o-xylene (3.5 ppm)-NO (10 ppm)-H,O-air system.
boiling point products. The values listed in Table I were calculated from the peak area of the FID GC peak for each product and the sensitivities of these products to the FID GC which were measured by standard samples. Since a standard sample of o-methylbenzylnitrate could not be obtained, the absolute concentration of o-methylbenzylnitrate was calculated by assuming that the sensitivity of the o-methylbenzylnitrate to the FID GC is equal to that for dimethylnitrobenzene. Since the amount of dimethylbenzoquinone produced was very small, these are not listed in Table I. The calculated ratios of each product formed to o-xylene consumed are largely scattered in Table I. The averaged value of the total high boiling products formed (except biacetyl) was 17.4% of the o-xylene consumed. Of course,
Takagi et at.
it may be possible that there are other high-boiling point products which could not be observed in this measurements and that the above products could not be sampled perfectly; however, the value of 17.4% could show a certain standard for the formation of the high-boiling point products in the photooxidation of o-xylene, although the product yields obtained are regarded as lower limits. The subsequent reactions of the observed products (Le., photolysis and OH radical reactions) could be neglected because each product observed increased linearly against irradiation time (Figure 2). As mentioned before, the FID GC was not used for the measurement of the low-boiling point products. The GC/ PIMS was used to analyze the low-boiling point products. Figure 3 shows an example of the analysis of the lowboiling point products formed in the photooxidation of the o-xylene-NO-H20-air system by the GC/PIMS. Figure 3a shows the TIC chromatogram when the Ar lamp with LiF window was used. Parts b-f of Figure 3 show the mass fragment chromatogram for each mass number together with the mass spectrum for each mass fragment chromatogram peak. Porapak-T (2 m, 150 "C) was used as the GC column. In this case, a measurement of the ionization caused by 70-eV electron impact was attempted, but the analysis was impossible because of the large background of water and oxygen in the sample. Most of the products observed in Figure 3 show the strong signal of the parent ion. From the mass number of the parent ion and the retention time of the GC when standard samples were injected into the GC, formaldehyde, acetaldehyde, glyoxal, methylglyoxal, and biacetyl were analyzed as the reaction products. In addition to these five products, at least two products were present. One product showed a mass number at 56. Acrolein is the most likely compound. However, since the retention time of this product did not agree perfectly with acrolein when a standard sample of acrolein was injected and since the retention time was very sensitive to the amount of water in the sample, we could not determine exactly what compound this product was. Another unidentified product showed a mass number at 70. In addition, it could not be determined whether the signal at m / e 70 was a parent ion or a fragment ion of some compound.
Figure 3. Analysis of the low-boiling point products formed in the photooxidation of o-xylene (100 ppm)-NO (20 pprn)-H,O-air GC/PIMS: (a) TIC chromatogram; (b-e) mass fragment chromatogram for each m l e and mass spectrum for each peak.
system by the
The Journal of Physical Chemistry, Vol. 84, No. 5, 1980 401
Photooxidation of o-Xylene
TABLE I[: Ratio of the Amount of the Low Boiling Point Products Formed in the Photooxidation of the 0-Xylene- N 0-H[,0- Air System' ratio (normalized to biacetyl) initial concn, ppml irradn (CHOP time, o-xylene 98.8 98.8 99.2 96.0 104.3 106.4 av
NO 19.6 19.6 19.5 19.8 19.8 19.8
min 341 600 510 321 392 346
CH,O 0.49 1.08 2.31 not measured 0.60 0.42 0.98 i 0.79
CH,CHO 0.09 0.31 0.38 0.47 1.16 0.14 0.26 t 0.15
(CHO 1 2 1.97 2.51 3.10 5.07 5.40 3.44 3.28 t 1.31
(CH3CO) (CH,CO), 0.25 1 0.27 1 0.95 1 0.92 1 0.45 1 0.30 1 0.52 t 0.33 1
' Measurements were done with the GCIPIMS. TABLE 111: Ratio of the Amount of the Low Boiling Point Products Formed to the Amount of o-Xylene Consumed -.(CHO )CH,O CH,CHO (CHOL (CH,CO) (CH,COL total 0.26 3.28 0.52 1 av ratio of low boiling point products 01.98 (from 'Table 11) 0.26 1.57 0.14 0.07 0.85 A(product)/- A(o-xylene) 0.25 318 418 ratio of C atom no. to o-xylene 1 18 218 218 0.053 0.130 0.445 0.018 0.213 yields ncirmalized by C atom no. 0.031
The absolute concentrations of the five low-boiling point products (CH20,CH3CH0, (CH0)2,(CHO)(CH3CO),and (CH3CO),) produced could be calculated from the peak area of the mms fragment chromatogram for each product under the same sensitivity of the GC/:PIMS, and from the sensitivities of these products to the GC/PIMS which were determined by the injection of standard samples. Since a standard sample of methylglyoxal could not be obtained, the meain value of the sensitivity of glyoxal and biacetyl was used for the sensitivity of the methylglyoxal. The calculated ratios of the amount of the low boiling products normalized to the biacetyl produced are shown in Table 11. Since the analytical sensitivity of the GC/PIMS was lower than that of the FID GC and GC/MS by electron impact with 70 eV, experiments in this case were done under high initial concentrations of o-xylene and NO in order to obtain a large amount of products. Further, since a large amount of sampling of the products was done once for the irradiated sample, one samplirng was done for each irradiation. The averaged ratio of the amount of lowboiling point products for the six measurements 'was calculated and listed in the bottom row of Table 11.
Discumion When the data in 'Tables I and I1 are combined, the ratios of'the amount of the low-boiling point products formed to the amount of o-xylene consumed could be estimated. As shown in Table I, since the averaged ratio of biacetyl formed to o-xylene consumed is 0.26, the ratio of the other low-boiling point product formed to o-xylene consumed (A(products)/-A(o-xylene))l could be calculated by using: the average ratio of the amount of low-boiling point products in Table 11. The result is shown in Table 111. In this case the total amount of A(product)/-A(oxylene) !was 1.57, which means that the concentration of low-boiling point products formed is 157% of the concentration of o-xylene consumed (as mentioned before, this value was 17.4'%for the high boiling products). This value of 157% is not unreasonable, because the carbon numbers of the low-boiling point products are smaller than that of o-xylene. The yields of the low-boiling point products formed to o-xylene consumed normalized by the carbon atoms number are shown in the bottom row of Table 111. The total of the values in the bottom row was 0.445. This value of 0.445 means that 44.5% of the o-xylene consumed could be collected as low-boiling point products.
.-
As shown in Tables I and 11, the initial concentraticlns of o-xylene and NO differed greatly between the high- and low-boiling point product measurements. It, may be unwise to combine two experimental results obtained under dlifferent experirnental conditions. Especially, under high hydrocarbon and low NO initial concentration, the fiormation and the accumulation of ozone occurred earlier in the photooxidation process. It is well known that glyoxal, methylglyoxal, and biacetyl could be produced in the ozone-aromatic compound reaction.14 As mentioned before, Nojima et al.g detected glyoxal, methylglyoxal, and biacetyl in the photooxidation of xylenes in the NO2-air system and they suggested that these three compounds were formed because of the ozone reaction. As shown in Figure 2, however, the amount of biacetyl produced increased linearly from the beginning of the irradiation. This linear increase shows that biacetyl would not be formled by the ozone reaction. If it was formed by the ozone-oxylene reaction, the formation of biacetyl should be delayed after irradiation. For the other low-boiling point products, the irradiation time dependency could not be measured, but the formation mechanism of glyoxal and methylglyoxal should be the same as that of biacetyl. Further, the rate for the reaction of ozone-aromatic compounds is thought to be very slow in the gas phase. At this stage, we feel that none of the low-boiling point products were formed by the ozone reaction but rather by cleavage of the aromatic ring due to OH-radical reaction. The ratios of the high-boiling point products formed were not very different from the results by Katou and Hanai* except, for the amount of nitro compounds. Since the initial corrcentration of NO differed greatly, it is not meaningful to compare the production of nitro compounds in these results with the results of Katou and Hanai. On the other hand, the ratio of the amount of glyoxal to methylglyoxal to biacetyl formed differed greatly in these results from the results by Nojima et al. The ratio of glyoxal, methylglyoxal, and biacetyl formed is 3.3:0.5:1 from our result and 0.2:2.5:1from the result of Nojima et al. If these products are formed by cleavage of the aromatic ring and the cleavage occurs with equal probability to all C-C bonds in the aromatic ring, then the above ratio should be 3:2:1. In our calculation, the averaged value of the sensitivity of the GC/PIMS for glyoxal and biacetyl was used for the sensitivity of methylglyoxal. It is possible that the small amount of methylglyoxal from our data
482
The Journal of Physical Chemistry, Vol. 84, No. 5, 1980
Takagi et al.
Scheme I CH3
&3:
I H
02
OH
,
(13)
‘0 H
resulted from an overestimation of the sensitivity of methylglyoxal. In the NO-K20-air system, the reaction of o-xylene should be initiated mainly by OH radicals produced by reactions 1 and 2.15 Oxygen atoms are also formed by the NO + NO, + HzO e 2HONO (1) HONO hu (290-400 nm) OH NO (2) photodissociation of NOz as shown in reaction 3. However, NO2 + hv NO + 0 (3) the contribution of oxygen atoms to attack o-xylene may be not important because the rate for the reaction of OH radicals with o-xylene16is about 100 times faster than that for the reaction of oxygen atoms with o-xy1ene.l’ The production mechanism of the high-boiling point products could be explained by the same mechanism as proposed previously in the photooxidation of toluene13and ethylbenzene.l2 If OH radicals produced in reaction 2 abstract hydrogen atoms from o-xylene, tolualdehyde and methylbenzyl nitrate identified as the high-boiling point products could be produced. In addition if the reaction proceeds via OH radical addition to the aromatic ring, dimethylphenols and dimethylnitrobenzenes should be produced. The ratio of hydrogen atom abstraction in the reaction of OH radicals with o-xylene has been estimated to be 10-35% by Perry et a1.I8 This ratio would appear to be consistent with the value in this study, which could be calculated to be about 8% (tolualdehyde + o-methylbenzyl nitrate). If the photooxidation of xylene proceeds only via the process to form high-boiling point products, the number of NO molecules which are converted photochemically to NOz when one xylene molecule is consumed should be less than two. In the previous study, this number was measured to be larger than two,1° which means that cleavage of the aromatic ring occurred in the photooxidation of xylenes and that fragment radicals formed by the ring rupture played the roll of converting NO to NOz.
-
+
-+
+
Although the mechanism for the cleavage of the aromatic ring in o-xylene is not clear, it could be explained, for example, by the OH-radical mechanism (Scheme I). After ring rupture, reaction could be speculated to proceed to form glyoxal and biacetyl. In this case, if the O2 adduct in reaction 5 was formed at the 2 position in the aromatic ring of o-xylene and the reaction proceeds in the same manner as reactions 6-11 two methylglyoxal and one glyoxal could be produced. According to the result for the reaction of oxygen atoms with o-xylene of Grovenstein and Mosher,lg the OH adduct via reaction 4 could be formed at the 1,3, and 4 positions on the aromatic ring of o-xylene. Therefore, ring cleavage may occur by many possible routes. Of course, reactions could be terminated forming nitrates and nitrites. In this study, however, such nitrates and nitrites of the ring-ruptured compound, and compounds such as A and B, could not be identified. Recently Darnall et a1.20reported the formation of biacetyl in the NO, photooxidation of o-xylene. According to their results, the fraction of the reaction of OH radicals with o-xylene yielding biacetyl was calculated to be 0.18. This value is consistent with our value of 0.26. Further, they reported that the biacetyl formed disappeared because of photooxidation after it passed through a maximum. This disappearance of biacetyl was not observed in our experiments as mentioned before because experimental conditions (light intensity, initial concentrations of NO, and o-xylene, and especially conversion of o-xylene in the photooxidation reactions) were different. Although the proposed reaction mechanisms to form biacetyl also differed, both mechanisms are speculative and, at this stage, we could not say which mechanism is correct. The formation route of acetaldehyde is not clear, but the formation of acetaldehyde is consistent with the result that the formation of PAN was found in the photooxidation of alkylbenzenes in the NOz-air system.‘ There should be many formation mechanisms of formaldehyde. It could be explained, for example, by the reactions via methyl radicals,z1reactions 15-17. The same experiments
-
+ O2
(15) CH302 + NO CH30 +- NOz (16) CH30 NO or O2 CH,O IlNO or H02 (17) CH3
+
403
J. Phys. Chem. 1900, 84, 483-488
CH302
(6) J. M. Heuss and W. A. Glasson, fnviron. Sci. Techno/., 2, 1109 (1968); S.L. Kopczynski, R. L. Kuntz, and J. J. Bufalinl, ibid., 8, 648 (1975). (7) S.L. Kopczynski, Int. J . Air Water Pollut., 8, 107 (1964). (8) T. Katou and Y. Hanai, Bulletin of the Instfiute of Environmental Science and Technology (in Japanese), Vol. 2, Yokohama National University, 1976, p 1. (9) K. Nojima, K. Fukaya, S. Fukui, and S.Kanno, Chemosphere, 5, 247 (1974). (10) N. Washida, G. Inoue, H. Akimoto, and M. Okuda, Bull. Chem. SCC. Jpn., 51, 2215 (1978). (11) N. Washida, H. Akimoto, H. Takagi, and M. Okuda, Anal. Chen?., 51, 910 (1978). (12) M. Hoshlno, H. Akimoto, and M. Okuda, Bull. Chem. SOC.Jpn., 51, 718 (1978). (13) H. Aklmoto, M. Hoshino, G. Inoue, M. Okuda, and N. Washida, Bull. Chem. Soc. Jpn., 51, 2496 (1978). (14) P. S.Bailey, Chem. Rev., 58, 925 (1958); W. P. Keaveney, R. V. Rush, and J. J. Pappas, Ind. Eng. Chem., Prod. Res. Devel., 8, 89 (1969). (15) W. H. Chang, R. J. Nordstrom, J. G. Calvert, and J. H. Shaw, Envirtn. Sci. Technol., 10, 674 (1976); C. H. Wu, C. C. Wang, S.M. Japar, L. I.Davis, Jr., M. Hanabusa, D. Killinger, H. Niki, and 8. Weinstock, Int. J. Chem. Kinet., 8, 765 (1976); R. A. Cox and R. G. Derwent, J. Photochsm., 6, 23 (1976/77). (16) D. A. Hansen, R. Atkinson, and J. N. Pitts, Jr., J . Phys. Chem., 79, 1763 (1975). (17) R. Atkinson and J. N. Pitts, Jr., J. Phys. Chem., 78, 1780 (1974). (18) R. A. Perry, R. Atkinson, and J. N. Pitts, Jr., J . Phys. Chem., 111, 296 (1977). (19) E. Orovenstein, Jr., and A. J. Mosher, J. Am. Chem. Soc., 92, 3810 (1970). (20) K. R. Darnall, R. Atkinson, and J. N. Pitts, Jr., J . Phys. Chem., 113, 1943 (1979). (21) H. A. Wiebe, A. Villa, T. M. Hellman, and J. Heicklen, J. Am. Chem. SOC.,95, 7 (1973); W. A. Glasson, Environ. Sci. Technol., 9, 1048 (1975); P. M. Cox, R. G. Derwent, P. M. HoL, and J. A. Kerr, J. Chem. SOC.,faraday Trans. 1 , 72, 2044 (1976).
+
were done for m- and p-xylene. For the high-boiling point products, products were almost the same found in the case of o-xylene except for the difference in the isomers of the products. In the low-boiling point products, biacetyl could not be found and the amount of methylglyoxal was increased. In the case of m- and p-xylene, since biacetyl, which should be measured with both FID GC and GC/ PIMS, was absent, results for the high- and low-boiling point product measurements could not be combined.
References and Notes (1) W. A. Lonneiman, T. A. Bellar, and A. P. Altshuller, fnviron. Sci. Technol., 2, 1017 (1968); A. P. Altshuller, W. A. Lonneman, F. D. Sutterfield, and S. L. Kopczynski, ibid., 5, 1009 (1971); W. A. Lonneman, S.L. Kopczynski, P. E. Darley, and F. D. Sutterfiekl, ibM., 8, 229 (1974). (2) N. Ito, K. Nakano, S. Izumikawa, T. Hirono, MI. Funashima, K. Asakuno, H. Kobayashl, M. Hayafuku, H. Yokoto, and T. Ohdaira, “Fbsearch and Survey 01 Photochemical Smog in Tokyo”, 3rd report, The Tokyo Metropolitan Research Institute for Environmental Protection (in Japanese), 1974, p 307; T. Chikamoto and K. Sakoda, J. Jpn. Soc. Air Pollut. (in Japanese), 12, 389 (1977). (3) K. R. Darnall, A. C. Lloyd, A. M. Winer, and J. N. Pitts, Jr., Environ. Sci. Technol., 10, 692 (1976). (4) R. E. Huie and J. T. Herron, Prog. React. Kinet., 8, I (1975). (5) National Research Council, Committee on Medical and Biologic Effects of Environmental Pollutants, ”Ozone and Other Photochemical Oxidant!?.. Aerosol”, Washington D.C., National Academy of Sciences, 1977.
Positraniurn Formation and Quenching in Argon-Oxygen Mixtures’ Richard L. Klobuchar2 and Paul J. Karol* Department of Chemistry, Carnegie-Mellon fJniversity, Pittsburgh, Pennsylvania 152 13 (Received August 8, 1979; Revised Manuscript Received October 12, 1979) Publication costs assisted by the National Science Foundation
The cross section for the quenching of orthopositronium by molecular oxygen was determined to be (1.0 f 0.1) X cm2. Both lifetime measurements and the two-photon coincidence rate difference method were used. The latter technique also provided a measurement of the fraction of positrons which form positronium in argon-oxygen mixtures as a function of composition at a total pressure of 5.60 atm.
Introduction Two of the major directions in the field of positronium chemistry in the past several years have been the study of the formation (or inhibition of formation) of positronium and the quenching of orthopo~itronium.~The various experimental techniques measure the annihilation of positronium into two or three annihilation photons. Quenchiing of long-lived orthopositronium (0-Ps) may be conveniently grouped into three processes (conversion, pickoff, and chemical reaction) each possessing a characteristic reaction rate (K, A,, and Ach). The cross section (u) for each process may be obtained from the reaction rates with elementary relationships K =
A, = nu+
(1) (2)
Xc. = n u , h u
(3)
nuku
0022-3654/80/2084-0483$01 .OO/O
where Uk, up, and Uch are the effective cross sections for conversion, pickoff, and chemical reaction, respectively; n is the concentration of the species affecting the desired removal; u is the average velocity of thermalized positronium (v = 7.6 X lo6 cm s-l at 25 “C). The individual rate constants can be deduced from observed changes in twophoton or three-photon annihilation rates by application of appropriate annihilation relationships. Alternately, since the three processes “shorten” the lifetime of positronium, the rate constants may be deduced from changes in the logarithmic slope of decay curves obtained in lifetime measurements. Quantitatively, quenching cross sections for chemical reactions are typically on the order of cm2,while cross sections for conversion and pickoff in a number of gasses are on the order of and cm2,re~pectively.~,~ O2 and NO are examples of gases which quench via conversion and noble gases via pickoff mechanisms. Compounds 0 1980 American
Chemical Society
