Measurement of the Diffusion Coefficients of Reactive Species in Dilute Gases

A technique has been developed for measuring the diffusion coefficients of atoms and other reactive species in gases below atmospheric pressure. The technique consists of measuring the rate of dispersion of a pulse of reactive species in a stream of gas flowing rapidly (ca. 10 m s$^{-1}$) down a quartz tube. The reactive species are observed and the profile of the pulses measured by using resonance fluorescence. The technique has been used at room temperature, but in principle measurements could be made at elevated temperatures. Measurements have been made of the rates of diffusion of hydrogen atoms in argon and nitrogen, and values for the diffusion coefficients of 1.61 $\\pm$ 0.04 and 1.35 $\\pm$ 0.03 cm$^2$ s$^{-1}$ respectively, at 1 atmosphere (ca. 10$^5$ Pa) and 294 K, have been obtained. Incidentally to the primary purpose of the experiment, it was observed that the hydrogen atoms spend a small fraction of their time of passage along the tube reversibly adsorbed on its walls. From the measurements, both the partition coefficient, giving the ratio of hydrogen atoms on the walls to those in the gas phase, and the rates of adsorption and desorption can be obtained. This appears to be the first observation of a chromatographic effect for a highly reactive species.