ENERGY EXCHANGE IN PHOTOREACTIONS 



13 



continuum of surfaces exists with a high density of crossing points. 

 Although it is correct to talk of single excited states for diatomic mole- 

 cules, such simplification may be incorrect for polyatomic molecules even 

 when the region of consideration is the very lowest on the energy scale 

 with depth no greater than perhaps 8 ev, as is the case in the present 

 discussion. At least at the top of this small band the density of levels is 

 high for molecules of low symmetry, so that crossing becomes important 

 and the original excited state will have a high probability of undergoing 

 a series of rapid changes to other excited states. In saturated molecules 

 each state may be associated with a single bond, at least to a fair first 

 approximation. By repeated crossing of potential-energy surfaces, elec- 

 tronic potential energy can be passed around among stable states from 

 bond to bond. If there is no mechanism with sufficiently low activation 

 energy whereby the potential energy of stable excited electronic states 

 can be converted into vibrational kinetic energy, dissociation ^^dll not 

 occur, even though the excitation energy exceeds that required for rupture 

 of a bond. Benzene, for instance, fluoresces with high yield in the region 

 2800-2200 A (Sponer and Teller, 1941), whereas the einstein for these 

 wave lengths has the values 120-128 kcal, which are more than adequate 

 to cause the reaction (Roberts and Skinner, 1949) 



C6H6^C6H5 + H; A// = 69.8. 



Dissociation in stable states can be caused only by localization of energy 

 equivalent to chemical reaction as vibrational energy in a single bond or 

 vibrational mode of motion. This 

 excitation energy may be used as 

 free energy of activation or as ther- 

 modynamic energy to raise the prod- 

 ucts to a higher level of free energy. 

 Photoisomerizations, such as those 

 of maleic and fumaric acids (War- 

 burg, 1919) and of 1,2-dichloro- 

 ethylene (Bonino and Briill, 1929), 

 are good examples of photoprocesses 

 in which the energy is used as acti- 

 vation energy. Theoretical treat- 

 ment of this type of photoprocess 

 hasbeengivenby Olson (1931, 1933). 

 The crossing of potential surfaces 

 occurs when the nuclei of a molecule 

 achieve a configuration identical 



with that of another electronic state, as represented in Fig. 1-7. This fig- 

 ure is drawn in a different way from that previously employed to break 

 the combination of potential surfaces for the single electronic states into 



INTERNUCLEAR DISTANCE 



Fig. 1-7. Detailed drawing of a cross- 

 ing point between typical potential- 

 energy surfaces. Dashed lines are usu- 

 ally drawn solid in other figures. 



