J . Phys. Chem. 1986, 90. 6557-6562 given reaction time in two mixtures at 287 K and in one mixture at 535 K are shown in Table IV. A combination of the data in Tables 111 and IV permits the calculation of the branching ratio p from eq V. The experiments at 287 K lead to a mean value of p = 0.59 f 0.20 for the HCCO yield of the elementary reaction of ethyne with atomic oxygen. The overall 2a error reflects mainly the uncertainties regarding k l , k5 and [HCCO], and includes the uncertainty concerning the change of S with temperature. In the same way, the value p = 0.57 f 0.17 is obtained at 535 K. This result at high temperature, however, has to be corrected for a sampling effect. Because the sampling takes place through a pinhole in the top of a quartz cone, mounted on a water-cooled plate, the temperature at the sampling point will be lower than the reactor temperature. At a reactor temperature of 535 K for instance, measurements indicate a sampling temperature of 400 K. The HCCO mole flux, which at larger distances from the sampling cone is still equal to the quasi-stationary value at the reactor temperature, will tend to adapt to the decreasing temperature when the gas approaches the sampling cone. The reason is that the rate of the HCCO formation process depends much more strongly on temperature than the rates of the HCCO removal reactions.'$28However, the adaption of the HCCO flux proceeds at a limited rate which is closely linked with the HCCO removal rate; the actual extent of the HCCO flux change is largely determined by the temperature profile and by the [O] and [HI profiles. In the experimental conditions at 535 K described in Table IV, a simplified model predicts a HCCO flux decrease of 11% over the cooling region. Correcting for this effect, the branching ratio with 2a error at 535 K is @ = 0.64 f 0.19.
Conclusion Contrary to recent indirect evidence based on H formation in C2H2/0 our direct approach shows that the elementary reaction of C2H2with 0 results for the larger part in the formation of HCCO, with a yield that is nearly independent of temperature: 59% f 20% at 287 K and 64% f 19% at 535 K. Our findings
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are in excellent agreement with very recent theoretical predictions by Harding and Wagner.29 Since only HCCO and CH, are important primary products,' the amount of methylene radicals found in the C2H2 0 reaction is
