PHOTOCHEMISTRY 



U 



A^B* 



A+B 



r 



[a) 



states of the excited electroiiic state that can be reached directly by the 

 absorption of a photon. This is illustrated for a diatomic molecule in Fig. 

 1-1, in which the ordinate r represents the distance between the nuclei and 

 U is the potential energy of the molecule. In Fig. 1-la, the eciuilibrium 

 separation for the nuclei is about the same in the excited state as it is in 

 the ground state. The probable transitions from the ground state can be 

 represented by vertical arrows lying 

 between a and 6 since they corre- 

 spond to small changes of position or 

 momentum. In this way, a simple 

 application of the Franck-Condon 

 principle demonstrates that direct 

 optical dissociation is improbable for 

 a molecule whose potential-energy 

 curves are of the type illustrated in 

 Fig. 1-la. In the gas phase at low 

 pressure, where the life expectancy 

 of an excited molecule is short com- 

 pared to the time interval between 

 collisions, such a molecule would 

 emit one quantum for each one ab- 

 sorbed. As is suggested by the vari- 

 ations in the length of the downward 

 arrows, the fluorescence or emission 

 spectrum is much more complex than 

 the corresponding absorption spec- 

 trum. In a condensed phase or in a 

 gas at higher pressure, there is a high 

 probability of the excited molecule 

 losing its excess oscillational energy 

 by collisions of the second kind 

 (Franck, 1922) during the lifetime of 

 the excited state. As a result, prac- 

 tically all the transitions, corresponding to the fluorescence, will start 

 from the lowest vibrational level of the excited state. Stokes's law 

 holds under these conditions, and the fluorescence spectrum is shifted 

 to the red side of the absorption spectrum. Freciuently the fluorescence 

 spectrum is approximately a mirror image of the absorption spectrum 

 (Lewschin, 1931). 



u 



A+B 



'B 



r 



(b) 



Fig. 1-1. Potential-energy diagrams for 

 a diatomic molecule. 



DIRECT OPTICAL DISSOCIATION 



In the case illustrated by Fig. 1-1 b, the molecule is nuich less stable in 

 its excited than in its ground state and, correspondingly, the equilibrium 

 separation of the nuclei is increased. It follows, from the Franck-Condon 



