204 RADIATION BIOLOGY 



state: the result is a transition of the first to a lower excited state and 

 excitation of the second. Thus 



A* + B^ A*' + B* 



This type can be distinguished from III by the properties of the new 

 excited state of A. 



V. An excited atom colUdes with an atom or molecule in its ground 

 state: ionization of the latter takes place. Thus 



A* + 5^ A + 5+ + e 



For this process to occur it is necessary that the ionization potential of 

 B be smaller than the excitation energy of A*. 

 Another possibility is 



A* -{- B ^ AB+ -{- e 



This process has a less stringent energy limitation but is much less 

 common. An example occurs in cesium vapor (Cs^ is formed). 



VI. An excited atom collides with a diatomic (or polyatomic) molecule: 

 the products are a normal atom and a dissociated molecule. Thus 



A*-i-BC^A-\-B + C 



It is necessary, not only that the excitation energy of A* exceed the dis- 

 sociation energy of the molecule, as is demanded by energy conservation, 

 but that it in fact be considerably greater (Franck-Condon principle, q.v.). 



VII. An excited atom collides with a molecule: a chemical change 

 occurs. Thus 



A* -\- BC -^ AB i- C 



This extremely important process is discussed in the following subsection. 



VIII. An excited atom collides with an atom or molecule and a third 

 body: the first two combine to form a larger molecule, the excess energy 

 being removed as translational energy of the third body. Thus 



A* -\- B -\- third body ^ AB -\- third body 



The participation of the third body in the impact is essential, for other- 

 wise the newly formed molecule would be dissociated at once by its 

 excessively great excitation energy (cf., however. Sect. 3-3e). 



IX. An excited atom collides with a surface (wall of a container) : 

 the atom is de-excited and an electron is ejected from the surface. Thus 



A* -\- surface — > A + e 



This process plays an important role in a number of phenomena such as 

 certain types of electrical discharge (e.g., Geiger counters). 



A common classification of collisions of the second kind (Willey, 1937, 



