Nonadiabatic Reactions. Chemiluminescence

It is pointed out that chemiluminescent reactions are necessarily nonadiabatic, since emission of radiation can only occur by transition from an upper to a lower potential energy surface. The restriction imposed by the second law of thermodynamics on the photon yield is, $$\mathrm{[open\; phi]⩽\Delta F/E,}$$ where $\mathrm{[open\; phi]}$ is the ratio of quanta evolved to molecules reacted, ΔF is the free energy decrease in the reaction, and E is the energy of the photon emitted. Upper potential energy surfaces are reached either by activation by radiation, which may be that of a black body at the temperature of the reaction, or by the ``crossing'' of potential energy surfaces. From Audubert's data on the radiation produced by the thermal decomposition of sodium azide approximate surfaces which account for the observations are constructed.