EXCITATION 61 



An excited molecule may also lose energy (11) by a collision of the 

 second kind. In such a process the excited molecule or atom gives up its 

 electronic energy of excitation to its collision partner, and this energy 

 may appear as electronic or oscillational energy of the second molecule 

 or even as translational energy of the system. The amount of energy 

 which appears as either oscillational energy or as kinetic energy of the 

 system is small (not much greater than ^kT) ; and, therefore, the bulk of 

 the energy has to be transferred into the electronic system of the collision 

 partner. If, as a result, the coUision partner has an excited electronic 

 system which is able to emit light, the process is called sensitized fluo- 

 rescence. If, on the other hand, changes of the electronic system occur 

 like transitions of electrons from bonding to antibonding, that is, a 

 singlet-triplet transition causing dissociation, we speak of a sensitized 

 photochemical process. If the atoms in the colliding molecules come 

 into positions during the collision in which by an electronic transition 

 an atom can switch from the first to the second molecule, such processes 

 will occur with great probability, provided the energy originally absorbed 

 by the first molecule is sufficient. One of the most thoroughly studied 

 examples of this type is the interaction of a normal hydrogen molecule 

 with an excited (6 ^P\) mercury atom, resulting in the formation of a H 

 atom and a HgH molecule. 



In addition to emission (fluorescence), direct optical dissociation, 

 simple predissociation, and collisions of the second kind, complex mole- 

 cules may lose energy of excitation as a result of the occurrence of 

 internal conversion (1). This process, like predissociation, depends upon 

 the existence of two electronic levels having in common the same total 

 energy and nuclear configuration. Since there are so many more atoms 

 and degrees of freedom in a complex molecule, it should be expected that 

 the time required for the excited molecule to attain the appropriate 

 configuration will be much greater than the minimum times observed 

 for predissociation in simpler molecules. The usual result of an act of 

 internal conversion will be the transfer of the electronic energy of excita- 

 tion (in all or in part) into generalized oscillational energj^ of some lower 

 electronic state. The occurrence of internal conversion is very probably 

 the explanation of the absence of fluorescence of many complex mole- 

 cules, even when they are in dilute solution or the gaseous state. If the 

 molecule were completely isolated it would, if it did not dissociate, even- 

 tually return to its original state and emit (fluorescent) light. In practice, 

 complete isolation is never attained, and during the long time it takes 

 to reverse internal conversion, the molecule will lose energy by impact 

 with others, thereby losing its ability to come back to the fluorescent 

 state. Momentarily after occurrence of internal conversion the molecule 



