40 SECTIONAL ADDRESSES. 



at the calculated frequency v^. The radiation hypothesis was killed 

 because the assumption of the second part was made in ignorance of what 

 molecules can do and cannot do. 



It may be argued that the activated molecular states which are 

 responsible for phosphorescence must be essentially different from those 

 which function in chemical reaction, because their life periods are enormous 

 compared with those of chemical processes. The fact, however, remains 

 that in a series of different energy levels the uplift from a lower to a higher 

 level cannot be achieved by the absorption of radiant energy of the 

 frequency corresponding to the energy difference. The stability of the 

 activated states in the field of phosphorescence and its remarkable varia- 

 tion with temperature are matters of great importance, but too much 

 stress need not be laid upon them at this stage of the argument. A 

 possible explanation will be given later. 



It may be pointed out that there is a close similarity between the 

 effective methods of activation in the fields of photoluminescence and 

 photochemistry. In each the activation is achieved by exposing the 

 inactive molecules to radiant energy of a frequency equal to that of a 

 characteristic absorption band of the inactive state, and this frequency 

 is invariably greater than that calculated from the quantum of activation. 

 Stokes' law, therefore, may be said to apply to photochemistry as well as 

 to photoluminescence. 



In view of the mechanism of activation which is common to photo- 

 luminescence and photochemistry, it is legitimate to inquire into the 

 destination of the excess of the energy absorbed over the critical quantum 

 of activation. The energy quantum absorbed by a single molecule may 

 be denoted by hv^, and the critical quantum of activation by hv^, where 

 V(, is greater than v,, and the question is what happens to the energy 

 difference expressed by /iVg— /iVj. In the case of photoluminescence there 

 is no doubt of the value of hv^, since this may be calculated from the 

 observed emission band, and hv„ is also known from measurements of the 

 absorption band or activating frequency. The course of events during 

 activation may be represented by the diagram shown in fig. 1, where 

 energy content is expressed on the ordinates and time on the abscissae. 



The initial level of a molecule is represented by A and the energy level 

 of the activated state by the horizontal line C. The difference between 

 the two levels is ^v,, and this quantum is radiated when the activated 

 molecule returns to its initial state A. When the molecule in its initial 

 state absorbs the quantum h\ it is raised to the level B, which is higher 

 than the level C. Since the phosphorescent emission is that of the 

 quantum hv^, the molecule after being initially raised to the level A must 

 immediately fall to the C level with the radiation of the energy h^^—h/i. 

 If the initial level A is a definite energy state of the molecule, it is legitimate 

 to assume that the energy difference is radiated as a single quantum hv.^. 

 It may be suggested that this radiation during activation by light of 

 frequency greater than that corresponding to the critical quantum of 

 activation is the origin of fluorescence. Apart from any other argument 

 it is necessary that the radiation of some energy must accompany the 

 activation of a molecule by light if Stokes' law is generally valid, and 

 the view now brought forward is that under certain conditions this energy 



