ENERGY EXCHANGE IN PHOTOREACTIONS 



39 



ing reaction, during which their own fluorescence was sensitized thus: 



and 



Hg* + Tl -> Hg + TV 



Tl* -^ Tl + hu. 



(l-35a) 

 (1-356) 



INTERNUCLEAR DISTANCE 



Fluorescence occurred from a number of thallium energy levels lying 

 below the energy of the excited state of mercury. It was strongest, 

 however, when the wave length of fluorescence most closely corresponded 

 to the excitation wave length for mercury. From this it was concluded 

 that the transition energy required 

 by the quencher should be closely 

 equal to the excitation energy avail- 

 able in the primary molecule, as it is 

 in mercury and thallium. This re- 

 quirement appears to be common to 

 many energy-transfer processes and, 

 following the classical definition of 

 resonance, is frequently referred to 

 as the "resonance requirement for 

 energy transfer." Its explanation 

 has been discussed in Sect. 3-4. 



Another well-studied example of 

 a quenching reaction of atoms in 

 which the resonance requirement 

 is markedly demonstrated is the 

 quenching of mercury fluorescence 

 by sodium atoms (Beutler and 

 Josephy, 1929). Only those tran- 

 sitions of sodium corresponding to 

 energies available in excited mer- 

 cury atoms take place in any num- 

 ber. Such processes may be repre- 

 sented in the potential diagrams of 

 Fig. 1-13. A collision between mer- 

 cury and thallium takes place as the 

 configuration point moves from 

 right to left along the upper curve 



< 



INTERNUCLEAR DISTANCE 



Fig. 1-13. Hypothetical potential sur- 

 faces for the transfer of electronic energy 

 from mercury atoms to thallium atoms, 

 demonstrating (a) a negative collision 

 energy and (6) a positive collision energy. 



until the potential-energy barrier 



reverses the direction of travel and the point moves back out to the right, 

 perhaps on the lower curve. In Fig. l-13a all collisions will pass the cross- 

 ing point, so that there is always some probability that the excitation 

 quantum may be transferred. In Fig. 1-136 only those collisions with 

 relative translational energy in excess of a limiting energy Eo, which we 

 call the collision energy, will pass the crossing point. The rate of cross- 



