Exposure-Dependent Surface Recombination Efficiencies of Atomic Oxygen

Surface recombination efficiencies $\mathrm{(\gamma )}$ on metals have been measured as a function of exposure time in steady Oseen flows which provide a step‐function increase, or pulse, in oxygen‐atom concentration. Cylindrical, circumferentially coated, thin‐film resistance thermometers were used to determine the rate of heat transfer. The relationship between the surface and free stream concentration of oxygen atoms was obtained by means of the mathematical analogy between heat and mass transfer. It was possible to use relatively high partial pressures of atomic oxygen and carrier gas (1% atomic oxygen in a flow pressure of 4–6 mm Hg of an Ar–O₂ mixture) which are important to the accuracy of the measurements of many of the parameters involved. Under these conditions, the dependence of $\gamma$ on exposure time was obtained as a function of time for periods up to several seconds. Each of the metals investigated exhibits a markedly different time history of $\gamma$ during the period of initial gas–solid interaction, and the $\gamma$ of each metal, after reaching an initial peak value, eventually reaches a final but different stable‐state value. The final values are 0.12, 0.03, 0.017, 0.0085, 0.007, and 0.005 for Ag, Cu, Fe, Ni, Al, and Au, respectively, which values are generally lower than those previously reported. Reasons for these differences include the elimination of excess heating due to excited species from the discharge which generates the atom flow.