220 RADIATION BIOLOGY 



reacting system. Only those excited states which do not immediately 

 dissociate can show these reactions, of course, and again metastable states 

 are the most likely to do so. Several actual cases are now presented. 



In the flame of burning hydrogen (H2-O2 reaction), excited OH mole- 

 cules are known to be present. One of their reactions which has been 

 postulated is 



OH* + H2 -^ O + H + H2 



This process is endothermal by 7.6 kcal per mole, but at the high tem- 

 perature of the flame this is readily provided by the thermal energies of 

 the reactants. 



In the afterglow of excited nitrogen, the following reactions are con- 

 sidered to provide some of the excited N atoms (the excited state of N2 

 is a metastable state) : 



2N?(3S+) ^ N2 + 2N*(2Z)) 

 and 



2N?('S+) -^ N2 + N*(2P) + N(4S) 



In the photochemical formation of ozone by ultraviolet irradiation of 

 O2, the following is one of the elementary processes: 



0* + O2 ^ 0, + o 



[It is interesting that the "intermediate complex" (cf. Sec. 3-3a) is a 

 transient state of the known molecule O4.] 



Both the resonance rule and the spin-conservation rule (Sect. 3-1 c) are 

 applicable in certain cases to collisions of the second kind involving 

 molecules. Some care must be exercised in use of the resonance rule, 

 however, in order that the Franck-Condon principle not be violated 

 (Oldenberg, 1952). 



3-3. ELEMENTARY PROCESSES 

 INVOLVING EXCITED POLYATOMIC MOLECULES 



3-3a. Potential Surfaces. The total energy of a polyatomic molecule in 

 one of its quantum states is, just as in the case of a diatomic molecule, the 

 sum of the energies of the electronic system, of vibration, of rotation, and 

 of translation. The last of these can again be disregarded in most of the 

 considerations. The chief differences between the patterns of possible 

 energies for a polyatomic and those for a diatomic molecule arise from : 

 first, the much more intricate fields which establish the conditions for 

 binding of the outer atomic electrons in the molecule ; second, the greatly 

 multiplied number of possibilities for vibration, since each atom or group 

 of atoms may vibrate with respect to any other; and third, the greater 

 possibilities of mutual interactions between various vibrations, rotations, 

 and electronic configurations. Thus the characteristics of the energy 

 levels of a polyatomic molecule are extremely complicated. 



