Absorption Spectrum of Vibrationally Excited N2in Active Nitrogen

The Bakerian Lecture, 1966 The progress of photochemistry exemplified by reactions of the halogens

It is appropriate for us to contemplate the growth of understanding of chemical kinetics which has taken place since the time of Planck. Understanding starts with the recognition by Stark in 1908 of the photon as an elementary particle in chemical reactions, and with the conception of the chain reaction by Bodenstein in 1913. Time was needed for the realization that the atom and the free radical are more often than not the entities responsible for chain propagation; time for Franck in 1924 to amplify Nernst’s conception of dissociation as the primary photochemical process; time for Semenov to give us the understanding of degeneracy in branched chain reactions; by 1930 the pattern had taken shape. When nuclear reactions were discovered their kinetic description needed no other modification than a change of time scale. In the evolution of chemical kinetics, the photochemistry of the halogens is not only typical but basic. From our early struggles with the hydrogen-chlorine reaction, through kinetic spectroscopy to population inversion of excited states and laser action, there is an unbroken thread, woven from the halogen staple, which knits together the fabric of chemical reaction mechanisms.