B.— CHEMISTRY. 



41 



can be radiated as a single quantum of fluorescence. The really essential 

 condition for this to take place is that the molecule can exist for a finite 

 period of time at the energy level C. 



In all cases of photoluminescence the criterion exists for the radiation 

 of excess energy as a quantum of fluorescence, since the phosphorescent 

 emission gives direct evidence for the existence of the molecule in the 

 energy level C in fig. 1. Fluorescence, therefore, should always be 

 exhibited during the photo-activation of a phosphore. Lenard and Klatt 

 in their investigations of photoluminescence recorded the fact that in 

 general the intensity of the luminescence showed a sudden and marked 

 diminution at the instant the exciting radiation was removed. It will 



B 



^ 



f^i 



kV-hy^^hV. 



T" 



kv, 



JL 



Jj£. 



k 



Fig. 1. 



be remembered that they defined two ' instantaneous ' states, characteristic 

 of each emission band, when the luminescence vanished completely at the 

 instant the activation was stopped. These two states are determined by 

 the temperature, and there lies between them an intermediate state when 

 true phosphorescent emission with measurable persistence is observed. 

 There is no doubt that in the lower instantaneous state the stability of the 

 activated molecules is so great that the phosphorescent emission is too 

 small to be observed. There is also no doubt that in the upper instan- 

 taneous state, which has a very small temperature range immediately 

 below the upper temperature limit, the stability of the activated state is 

 so small that the whole of the phosphorescent emission takes place within 

 a fraction of second after activation has ceased. In the intermediate 



