Studies on fluorescence and photosensitization in aqueous solution. I.―Introduction

It is well known that the concept of “collisions of the second kind,” first enunciated in precise terms by Klein and Rosseland, has been in the hands of Franck and his co-workers an instrument of great power in forming a theory of the possible ways in which an energy-rich atomic system may assume its normal state otherwise than by radiating its excess energy. This kind of collision is one in which an atom or molecule, which has by some means been raised from its normal to a higher quantum state, loses this additional energy in the course of a collision with a free electron or some other atom or molecule, which in its turn receives an increase in kinetic energy, or assumes a higher state of internal energy. The development of the idea has led to the prediction and confirmation of a number of phenomena in dilute gases, such as sensitized fluorescence and photochemical sensitization, which, taken together, form a firm experimental basis for some of the fundamental concepts of the quantum theory. Now since it is universally believed that in a chemical reaction the true reactants are molecules which have in some way or other become “ activated,” and since there is at present almost complete unanimity that it is necessary to invoke the aid of collisions to account for the velocity of even a unimolecular reaction, it becomes relevant to examine the possibility of extending the concept of collisions of the second kind to the general problems of chemical reaction. Indeed, the essence of the idea appeared in the chemical literature long before its formal announcement and proof by Klein and Rosseland.