PHYSICAL PRINCIPLES OF CHEMICAL REACTIONS 219 



which destroy molecules in state B (radiation, quenching, etc.) most of 

 the excited B molecules will indeed predissociate; if it is much greater, 

 predissociation will be unimportant ; if it is comparable to one or several 

 others the various processes will compete according to their relative prob- 

 abiUties. The type of predissociationre presented by Fig. 3-6 is only one 

 of several different but related varieties (Herzberg, 1950). 



The necessity that energy be conserved shows that radiationless transi- 

 tion at a crossing of two potential curves cannot occur unless the energy 

 of the intersection point Ues above the dissociation limit for one of the 

 curves (the oscillational energy of the latter thus being nonquantized). 

 Indeed, it should be noted that although two potential curves "intersect," 

 neither of the two respective nearby vibrational levels will exactly match 

 the intersection point in energy (except in the event of a highly improb- 

 able coincidence), so that in general many vibrational periods are required, 

 on the average, for the transition to take place (cf. Fig. 3-8 and discussion 

 on p. 227). The exact lifetime depends on a number of more intimate 

 features of the two electronic states, in addition to the disparity in vibra- 

 tional energy. 



Predissociation in a particular electronic state of some molecule is 

 manifested by two striking features of the band spectra arising from 

 transitions originating or terminating in that state. The first is a diffuse- 

 ness in the component lines of the band: each Une is broadened, often 

 enough to obscure the rotational structure entirely, leaving only the 

 vibrational structure. This behavior is just as anticipated if the lifetime 

 of a given level is shortened (by predissociation) to a value which is 

 shorter than one rotational period (whereby the rotational quantization 

 is destroyed and replaced by a continuum) but longer than a vibrational 

 period (as explained). The second is a pronounced weakening, and often 

 a complete absence, of emission bands originating in the predissociating 

 state. The same band will be observable in absorption, for there the 

 level referred to is the final state, and predissociation merely broadens 

 the lines: but emission requires the state to persevere for the time, of 

 order of magnitude 10"* second, required for radiation, and predissocia- 

 tion can usually compete favorably with this relatively slow process. 



From the chemical point of view, predissociation is a delayed dissocia- 

 tion. As such it is more susceptible to external influences such as col- 

 hsions (ciuenching), and the dissociation products may be formed accord- 

 ing to more complicated kinetic laws. It plays an important role in 

 many reactions induced by various radiations. 



3-2g. Collisions of the Second Kind. Excited diatomic molecules can 

 undergo many varieties of collisions of the second kind. On the whole, 

 very few examples of such collisions have been studied in detail or are 

 fully understood, a fact which reflects the far greater complexity of the 



