An investigation has been made of the kinetics of the photochemical oxidation of phosphine above the upper explosion limit. The results are summarized by the equationsMercury sensitized reaction —d[PH3]/dt= const. [PH3]/[O2]2intensity.Direct photo‐reaction —d[PH3]/dt= const. [PH3]2/[O2]2.The reactions are shown to be of the chain type and inhibited by oxygen. On the basis of previous knowledge of the propagation of the chains, the question of termination was examined and it is shown that out of six possible cases, only one is in agreement with the experimental facts. This is that the chain carrier, which normally reacts with phosphine, is destroyed in a ternary collision with two oxygen molecules or with one oxygen and one nitrogen molecule (or an argon atom). The efficiency of the collision is about 0.5. In view of these results, it is highly probable that this carrier is an oxygen atom. In the photosensitized reaction one‐fifth of the collisions between oxygen molecules and excited mercury atoms results in the initiation of a chain. These results are shown to be quantitatively consistent with the shape and also the position of the upper limiting explosion curve, except that the limiting pressure is a little higher than that calculated by a strict application of the theory of branched chains. The upper limit is displaced to higher pressures by illumination of the mixture, a phenomenon not predicted by theory. The effect is of short duration, 2 sec. and it is not exhibited at the lower limit. The significance of this result is discussed in relation to the theory of the reaction.
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