PHYSICAL PRINCIPLES OF CHEMICAL REACTIONS 225 



petitive processes (predissociation, internal conversion, and of course 

 deactivation by radiation or collision). The greater the surplus energy, 

 the shorter will be the time reciuired for a certain amount to be concen- 

 trated in any particular degree of freedom (Franck, Sponer, and Teller, 

 1932), and therefore the faster will be any possible dissociation. 



Predissociation of a polyatomic mole ;ule can occur when the excita- 

 tion energy of the state exceeds an energy at which the potential surface 

 of the state intersects the potential surface of some other electronic 

 state in the region where the levels of the latter are continuous: the con- 

 figuration point in its motion may reach a point common to the two sur- 

 faces, and then the transition, with subsequent motion of the configura- 

 tion point on the second surface, will always be possible (the probability 

 depending on various intricacies of the two surfaces such as spin, sym- 

 metry, etc.). Intersections of (n -\- l)-dimensional potential surfaces for 

 excited states occur much more commonly than do intersections of the 

 two-dimensional curves of diatomic molecules. Predissociation in a poly- 

 atomic molecule will therefore be much more common, but also much 

 slower, than in a diatomic molecule; it differs from direct dissociation 

 only in the respect that the moving configuration point must change from 

 one potential surface to another. This change, which is a radiationless 

 electronic transition, will be much more probable (once the proper con- 

 figuration has been attained) than in the diatomic case, for the intersec- 

 tion of two potential surfaces is an (n + l)-dimensional space curve, so 

 that the quantum energy of the initial vibrational level will always 

 "match " some energy of the intersection curve. (On the other hand, the 

 transition from discrete to diffuse structure, as hv increases within a given 

 band above the lowest energy at which the surfaces intersect, will be more 

 gradual, because of the randomness of the motion of the configuration 

 point.) Thus both direct dissociation and predissociation are much more 

 probable for polyatomic than diatomic molecules, although for both proc- 

 esses the time between the light absorption act and the actual dissociation 

 is much longer for polyatomic than for diatomic molecules. The former 

 fact contributes to the preponderance of diffuse over discrete structure in 

 the electronic spectra of polyatomic molecules. The latter is borne out 

 (indirectly) by a variety of experiments on fluorescence yields, and may be 

 connected in some cases to the observation that dissociation of metastable 

 polyatomic ions can be observed in a mass spectrograph, whereas it has 

 never been found (i.e., is too fast) for a diatomic molecular ion. 



The great likelihood of dissociation processes follomng absorption of 

 visible light or ultraviolet radiation by polyatomic molecules in the gas- 

 eous state is extremely important for their photochemistry. Thus expo- 

 sure of NHs to ultraviolet radiation (for which the spectrum is 

 entirely diffuse) always decomposes the molecule, even at extremely low 

 pressure. 



