PHOTOCHEMISTRY 5 



the expression must be integrated over all frequencies which correspond 

 to the electronic transition under consideration. 



Equation (1-1) is based on the assumption that an excited molecule can 

 lose its energy of excitation only by emitting a photon. If the energy can 

 be lost in any other way, either spontaneous or induced, the lifetime of the 

 excited state will be correspondingly reduced. When the system is illu- 

 minated with light of constant intensity, a steady-state condition will pre- 

 vail, and the concentration A^'.v of molecules in the excited state will 

 be constant. If "intensities" are expressed in photons absorbed per 

 cubic centimeter per second and the rate Vc of the nonradiative disap- 

 pearance of excited molecules in corresponding units, then 



labs = Ifi + ?'c = {kfi + k,)NN. 



The coefficient k^ can be a function of added substances, but, for any given 

 solution, kfi -\- k^ is a constant and is equal to the reciprocal of the mean 

 life r of the excited state under these special conditions. Since 



it follows that 



(1-2) 



where ro is the natural mean life or the life which the excited state would 

 have if the emission of fluorescent light was the only possible degradative 

 process. The ratio of Ifi/ labs is called thfe fluorescence yield. 



PREDI8S0CIATI0N 



The fluorescence yield, and correspondingly the mean life, of an excited 

 molecule may be reduced by a process known as "predissociation." 

 This process is possible when a stable vibrational level of an electronically 

 excited state overlaps a dissociation region of another state. For a dia- 

 tomic molecule this corresponds to the crossing of the potential-energy 

 curves of two excited states. Under these conditions, when both the 

 energy and the nuclear configuration are the same in the two states, 

 the molecules can cross over from the stable, quantized state into the 

 unstable, nonquantized state. While the energy of the primary excited 

 state must be greater than the thermochemical energy of dissociation, it 

 may be much less than that required for direct optical dissociation. The 

 probability of such a transition may have any value from unity to prac- 

 tically zero. It is determined by the Franck-Condon principle and by 

 certain selection rules. The chemical detection of the resultant atoms or 

 radicals is the most sensitive test for the occurrence of predissociation. 

 The weakening of the fluorescence intensity is a direct measure of the 

 probability of predissociation. The disappearance of the rotational 

 structure of an absorption band indicates that the probability of the cross- 



