FAST FLOW STUDIES OF ATOMIC CARBON KINETICS AT ROOM-TEMPERATURE

The reactions of atomic carbon with OCS, NO, O2, N2O, SO2, and H2S were studied at room temperature in a fast-flow reactor. The atomic carbon, which was obtained from the microwave dissociation of CO diluted in He, was homogeneously mixed with the reactant molecules in a section of flow tube far from the discharge. The pseudo-first-order decay of atomic carbon was determined from the decay of CS ultraviolet chemiluminescence produced by the C + OCS reaction. As electronically excited CS is rather long-lived, it is shown how long-lived chemiluminescence can be used for obtaining kinetic data. In flow experiments, the rigorous determination of pseudo-first-order reaction rate constants is given by solving the differential continuity equation of the decaying reactive species in the flow. However, in most studies such an effort is not undertaken and rate constants are determined assuming plug flow for the reactive species because it allows an easy conversion of decay distances into reaction times. In the present study both approaches have been used. The plug-flow rate constant values were found to be significantly smaller than those given by the solution of the continuity equation. Our study thus provides a new example of the danger of the plug-flow approximation when conditions justifying it are not fulfilled. Specifically, the plug-flow assumption requires low homogeneous and wall-depletion rates of the reactive species with respect to its diffusion rate in the carrier gas. In our experiments none of these conditions was fulfilled and, in particular, the atomic carbon wall removal was found to be very efficient. Rate constants were determined for the first time for the reactions with OCS, SO2, and H2S. For the reactions with O2, NO, and N2O, previous studies, essentially performed by flash photolysis, gave a large scatter of data. Our values do not match any of those data. However, agreement for relative rate coefficients is found with Husain's latest values, obtained at the lowest flash-lamp energy, which are furthermore the closest to ours. Our reaction rate constant values, given by the solution of the continuity equation of the atomic carbon in the flow, are (in 10(-11) cm3 molecule-1 s-1 units): 10.1 +/- 0.7 with OCS; 2.7 +/- 0.2 with NO; 1.6 +/- 0.2 with O2; 0.85 +/- 0.16 with N2O; 6.9 +/- 1.7 with SO2; and 8.3 +/- 1.8 with H2S.