PHYSICAL PRINCIPLES OF CHEMICAL REACTIONS 



223 



(Laidler and Shuler, 1951). ^^ The currently available energy values on 

 the basis of which such surfaces as that in Fig. 3-7 are constructed must 

 be deemed of qualitative trustworthiness at best, and, moreover, the 

 assumption of linearity should always be borne in mind in their use. 

 That this assumption restricts the conclusions which may be drawn from 

 study of such a surface is shown by the fact that collisions between H2 

 and Hg*, being wholly random in nature, result in Hg-H-H complexes 

 with all possible central angles, the angle 180° therefore being exceptional. 



Fig. 3-7. Potential surface of HgHo (lowest electronic state; linear configurations; 

 schematic). 



3-3b. The Franck-Condon Principle. The Franck-Condon principle is 

 useful in understanding general features of radiative and colhsion proc- 

 esses involving polyatomic molecules, just as it is in the diatomic case. 

 Thus, the spatial and momentum coordinates of each of the atomic con- 

 stituents of a polyatomic molecule can change only very shghtly during 

 an electronic transition between quantum states. This is manifested 

 in the emission and absorption spectra, however, to a much greater extent 

 in the case of polyatomic molecules, because the coordinates of many 

 atoms having different vibrational frequencies are involved. 



An example is provided by the relations between the absorption and 

 fluorescence spectrum of a polyatomic molecule. In some cases the short- 

 wave-length limit of the fluorescence spectrum is sharp and approximately 



12 Such an unstable intermediate, formed in a collision, is called an "activated 

 complex." The activated complex for a collision between two atoms is a diatomic 

 molecule, for which n = I, and such collisions are interpreted on the basis of the 

 potential curves of the diatomic molecule as in Sect. 3-2c. If even one collision partner 

 is a molecule, the potential surfaces of the activated complex are properly surfaces 

 in a space of four or more dimensions. 



