Use of Catalytic Probes to Determine Atom Concentrations and Atom Diffusion Coefficients

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Abstract
A simple analysis is presented for the perturbation of a stream of partially dissociated gas by a catalytic probe capable of completely removing all the atoms from the stream. The results of this analysis were confirmed by experiments on discharged O2, which showed that for linear flow rates of several meters per second the atom concentration reaches 99% of the unperturbed value within 2.5 cm upstream from the probe. It is shown that such a probe can be used to measure atom concentrations, since the flux to the probe remains finite even though the atom concentration at its surface is virtually zero. No appreciable errors are introduced into relative concentration measurements required to obtain first order rate constants. Under suitable experimental conditions, absolute concentrations can be obtained which are negligibly different from the unperturbed values regardless of the order of the recombination processes. A method is described for using such a probe to obtain accurate values of atomic diffusion coefficients which does not require accurate knowledge of heterogeneous or homogeneous recombination rates.