42 SECTIONAL ADDRESSES. 



region the stability is such that the luminescence can be observed and 

 measured without difficulty. 



The sudden and complete disappearance of luminescence in the lower 

 instantaneous state when the excitation is stopped must be entirely due 

 to fluorescence, since no phosphorescence is visible. The energy of 

 activation remains stored up and can only be released by raising the 

 temperature. In the intermediate state phosphorescence is always visible 

 to a greater or less extent and in consequence the presence of fluorescence 

 will be recognised by a sudden fall in intensity at the instant when the 

 exciting radiation is cut off. Both these phenomena have been established 

 by Lenard and Klatt's work. 



It must be remembered that the one essential criterion for fluorescence 

 is the existence with a finite stability of an energy level intermediate 

 between the initial level and the super-activated level to which the molecule 

 is raised by absorbing the quantum /(V^. It is by no means necessary that 

 the stability of the intermediate level be sufficiently great for delayed or 

 phosphorescent emission to be visible when the molecule changes from 

 this level to its normal level. The conditions for phosphorescence are 

 far more restricted and rigid, one of these being that the phosphore must 

 be in the solid state. It is, therefore, not surprising that fluorescence is 

 of far more frequent occurrence than phosphorescence. 



Attention has already been directed to the close similarity between 

 the activation processes in photoluminescence and in photochemistry. 

 The principle of fluorescence radiation must also apply to photochemical 

 reactions, in all of which the activating quantum is greater than the 

 actual energy of activation. The course of events must again be that 

 shown in fig. 1, with the simple difference that in photochemistry the 

 existence of the molecule in the energy level C will be established by the 

 occurrence of a chemical reaction, the critical increment of which is hv^. 

 Here again, therefore, the relation should hold that 



where JiWg is the quantum of energy absorbed at the characteristic molecular 

 frequency in the ultra-violet, hv^ is the critical increment and v.^ is the 

 frequency of the fluorescence. It would seem, therefore, that the 

 suggested explanation of fluorescence may be put to a very severe test by 

 the quantitative study of photochemical reactions. Some preliminary 

 observations have been carried out at Liverpool by Mr. Leathwood and 

 these give definite support. The examples selected were not chosen from 

 known photochemical reactions ; rather was it considered desirable to 

 determine whether photochemical reactions take place under conditions 

 when fluorescence is visible and do not take place when fluorescence is not 

 visible. Gas reactions have not been investigated owing to the difficulty 

 of observation of the fluorescence of gaseous systems. 



Some years ago F. 0. Rice investigated the sulphonation of certain 

 phenolic ethers and at the same time he observed the absorption spectra 

 of these substances. A typical instance of the phenomena observed is 

 given in fig. 2, which shows the absorption spectra of anisole. The 

 absorption band A is that exhibited by the ether in alcoholic solution, 

 whilst the absorption band B is that exhibited by the ether in solution 



