In the recent developments of chemical kinetics there have been many indications of a type of reaction known as a chain reaction, in which each act of transformation gives rise to a product which, on account of its active chemical nature or high energy content, causes further molecules to be transformed without special activation. Under certain conditions the chain may "branch," and the number of centers from which reaction proceeds increases indefinitely unless the chains are broken. In a gaseous reaction there may be a definite concentration at which the rate of initiation of chains just begins to exceed the rate at which they are broken. At this point a stable condition ceases to be possible. However slow the reaction may be on one side of the limiting concentration, on the other side its rate increases continuously with time till explosion results. The acceleration may require an imperceptible fraction of a second only, since the time scale on which molecular collision processes occur is a very minute one. The remarkable phenomenon to which this gives rise, namely an abrupt transition from negligibly slow reaction to explosion, is exemplified in the union of oxygen with phosphorus vapour and with sulphur vapour at low pressure, and in the combination of hydrogen and oxygen under certain conditions. The present paper deals with a similar phenomenon in the oxidation of phosphine.
Low-Energy Molecular Collision Processes in Space