The recombination of oxygen atoms at surfaces

Abstract

Linnett and Marsden's apparatus, with some modifications, was used for determining the accommodation coefficients for the removal of oxygen atoms at surfaces. The results at room temperature for 28 surfaces (elements, oxides and halides) are presented and discussed. The conclusions may be summarized as follows. (i) For most of the surfaces the decay of atom concentration down the side tube is, within our experimental error, exponential, showing that the wall removal-process is usually first order. In the few cases where the decay appears not to be strictly exponential the surface may not be uniform.(ii) The range of values for the recombination coefficient is from 10-4 to 2 X 10-1. (iii) The activity cannot be related to any single parameter associated with the surface material though some of the comparisons can be understood on the basis of a particular mechanism.Linnett and Marsden192 measured the efficiencies of the recombination of oxygen atoms on various surfaces using the method Smith 3 had employed for hydrogen atoms. Pyrex (Hysil) glass, potassium chloride, lithium chloride, molybdenum trioxide and lead monoxide were examined; the effect of varying the temperature was also studied. Here the same general method was used. Metal and non-metal, oxide and salt surfaces were examined at room temperature and their efficiencies calculated. The efficiencies y were expressed as a fraction of the collisions of oxygen atoms at the surface, assumed smooth, which led to their recombination. The object of examining many materials was to see whether the relative efficiencies would indicate the mechanism of removal.The apparatus was similar to Linnett and Marsden's; 1 certain important differences are described in the next section. EXPERIMENTAL GAS SUPPLYDry electrolytic oxygen was used, the gas being passed through a trap cooled in liquid air and then over phosphorus pentoxide. The oxygen flow was controlled by a capillary tube into which a tungsten wire, almost filling the tube, could be moved by a screw. At this valve, the pressure dropped from about 760 to 0.05 mm Hg. Pressures were measured on a McLeod gauge. ELECTRIC DISCHARGEAn electrodeless discharge was used to produce atoms.1 The discharge tube of Pyrex, 4.2 cm diam. and 70 cm long, had two side-arms attached 55 cm from the end at which gas entered. The discharge coil 8 an diam. consisted of six turns of 6 mm &am. copper tubing. The r.f. generator was the oscillator and output stages of a converted RCA high frequency communications transmitter, type ET-4336-B. The frequency was controlled at 14mc/sec, and the output at 50-100 W. The discharge was coupled to the output coil by a single coupling loop and parallel feeder. The output circuit was series tuned by a condenser. To reduce stray radiation, the discharge coil, tuning condenser and parallel feeder were enclosed in an earthed screen of perforated zinc sheet. SIDE ARMS AND SURFACESOne side arm was 14.5 mm diam. and 55 cm long, and the other, 12 mm diam. and 35 cm long. Both were fitted with water jackets main...

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