Room-temperature rate constants for the removal of OH radicals in the gas-phase reaction with acetylene have been determined using pulsed H2O2 photolysis production of OH followed by cw UV-laser long-path absorption detection. The pressure dependences of the rate constants in N-2, O-2 and synthetic air as buffer gases have been investigated in the fall-off range between 1.5 and 100 kPa. In N-2 the pressure dependence can be described by the empirical expression of Gardiner: k = {(k(infinity))(a) + (k(0)[M])(a)}(1/a), using the parameter a = -1/3. This results in the rate constants k(infinity) = (1.07 +/- 0.07) x 10(-12) cm(3) s(-1), and k(0) = (3.9 +/- 1.6) x 10(-29) cm(6) s(-1) for the given pressure range. Smaller effective rate constants were obtained in the presence of O-2, owing to a fast regeneration channel for OH. In terms of the above expression the rate constants k(infinity) = (4.6 +/- 0.2) and (3.1 +/- 0.1) x 10(-13) cm(3) s(-1) and k(0) = (1.8 +/- 0.2) and (1.9 +/- 0.5) x 10(-29) cm(6) s(-1) were obtained for O-2 and synthetic air as buffer gases, respectively. Compared with N-2 there is virtually no difference in the fall-off behaviour in O-2 and synthetic air, indicating that the OH regeneration yield is independent of pressure. On the other hand, this yield was found to decrease with the O-2 mixing ratio. A kinetic model is proposed that qualitatively explains this effect with different chemical properties of non-thermalized OH-acetylene adducts with regard to O-2. This model is confirmed by measurements showing a significantly stronger dependence of the regeneration yield on the O-2 mixing ratio in O-2-He mixtures.
