On the law and mechanism of monomolecular reaction

1. The object of the present paper is to work out an expression for the rate of monomolecular reaction on the basis of the idea that radiation is the cause of such reactions. The whole position of the radiation hypothesis of chemical reactivity up till now has been fully discussed by Harned. I only wish to draw attention to the fact, as pointed out by Langmuir, and Lewis and McKeown, that a great similarity exists between photo-electric emission of electrons and photo-chemical reaction. The true analogue of the thermo-chemical reaction should be sought, however, in the phenomenon of thermionic emission of electrons. It has long been shown experimentally by Richardson and others that the thermionic emission of electrons is vastly in excess of the total photo-electric emission at any temperature T. In the same way we should expect that the amount of thermo-chemical reaction in a system at a given temperature should be greater than the total photo-chemical reaction by black body radiation at the same temperature. Becker has shown that the distribution of velocities among the photo-electrons emitted from a metal by the action of black body radiation at a temperature T is similar to that found amongst the electrons emitted thermally from the hot metal at the same temperature T. It is thus natural to assume that the thermionic emission of electrons from a hot body is really due to the radiation in equilibrium with it. Richardson║ has recently given a very interesting discussion on the photo-electric theory of thermionic emission of electrons. Owing to the well-known difficulties the old view of the freely-moving electrons in a metal has, in recent years, been replaced by that of a lattice structure— a metal being considered to be constituted of interlaced lattices of ions and electrons. Such a view of metallic electrons precludes them from sharing in kinetic energy according to the equipartition law. It is rather more rational to imagine that the metallic electrons do exist in some modified quantum orbits, and are bound to the ions by a certain potential energy. If this view of the electronic structure in metals be accepted, then we have to look to radiation as the only controlling factor in the emission of electrons from hot bodies. The writer has tried to show that the law of thermionic emission derived on the basis of radiative mechanism is in good agreement with experiment. Lewis and McKeown have pointed out that “the concept of matter and radiation being at one and the same temperature means that as a result of absorption and emission, the system as a whole maintains a certain distribution of energy among all frequencies.” If by some process a set of frequencies are removed the system tends to make good the loss by a corresponding reverse process, provided the velocity of the process be not too large to make it physically impossible to keep the system at a fixed temperature by means of a thermostat. In my view the resemblance of photo-electric emission and photo-chemical reaction with thermionic emission and thermo-chemical reaction respectively arises from both kinds of processes being due to radiation. But the distinction lies in the fact that one is due to the action ofhigh temperature radiation on acold system, while the other is brought about by the action of radiation in temperatureequilibrium with the system itself. 2.The Range of Frequencies of Radiation capable of bringing about a Chemical Reaction . Up till now it has been usually assumed that a single frequency, or rather a narrow range of frequencies, is capable of bringing about a chemical change. But experiments have shown that photo-chemical reactions are produced by the action of light of a wide range of frequencies. The simplest of all chemical reactions is the breaking up of atoms into ions and electrons, and it is widely known that the photo-electric action in various elements, both in solid and vapour phase, are brought about by all frequencies of radiation above a certain limiting frequency. The familiar reaction of practical photography is also known to be produced by light of a great variety of wave-lengths. It is, therefore, evident that a more complete theory of chemical reactivity should involve a summation of a number of frequencies, or, what is more plausible, an integration over a whole range of frequencies above a certain limiting value.