PHYSICAL PRINCIPLES OF CHEMICAL REACTIONS 203 



(or ion or molecule) loses the whole or part of its excitation energy in an 

 impact with an electron, or with an atom (or ion or molecule) initially in 

 its ground state. Also included is the special case in which an ionized 

 atom loses its ionization energy, i.e., is neutralized (cf. Sect. 4-3a). If 

 the atom initially excited is fluorescent, collisions of the second kind 

 always manifest themselves by a quenching of that fluorescence. In 

 addition to energy exchange, one or a group of atoms may be transferred 

 between the colhding partners. The following types of coUision of the 

 second kind for a neutral excited atom are known. (The symbols 

 A*, B*, etc., denote atoms in excited states; A, B, etc., denote the same 

 atoms in their ground states; e denotes an electron.) Examples of most 

 of them are given in Sect. 3-le. 



I. An excited atom coUides with a slow electron: the products are a 

 normal atom (or an atom in a lower excited state) and a faster electron. 

 Thus 



A * + e — > A + e (with greater kinetic energy) 



This process is important in certain types of gaseous discharge. 



II. An excited atom coUides with an atom (or molecule) in its ground 

 state: the products are the two atoms having no electronic excitation but 

 greater translational energy. Thus 



A* + B -^ A -\- B (both with greater kinetic energy) 



This process usually has exceedingly low probability at normal tempera- 

 tures (for a reason to be explained in Sect. 3-2c). A special case is that 

 in which A* is merely brought to a lower excited state as a result of the 

 impact. Here the probability may be quite appreciable at normal tem- 

 peratures if the difference in excitation energy of the two excited states of 

 A that are involved is very small — say, not greater than a few tenths ev. 



III. An excited atom collides with an atom (or molecule) in its ground 

 state; the result is the de-excitation of the first and excitation of the 

 second. Thus 



A* -^ B^ A + B* 



This process can occur only if atom B possesses an excited state with 

 excitation energy lower than or equal to (or only very slightly in excess of) 

 the excitation energy of A*. If that energy is smaller, the energy hber- 

 ated in the reaction appears as augmented translational energy of A and 

 B*. If it is sHghtly greater, the needed energy may be provided by 

 initial kinetic energy of A* or B. If the B* atoms are not quickly 

 deactivated in other impacts of the second kind, they will radiate their 

 own characteristic spectrum, a striking phenomenon called sensitized 

 fluorescence. 



IV. An excited atom collides with an atom or molecule in its ground 



