92 



LIGHT AND LIFE 



is indispensable if the radiative transfer of the fluorescence is to be 

 avoided, so that the depolarization observed may be unequivocally 

 attributed to radiationless transfer of the excitation among the phenol 

 molecules. Fig. 8 shows the different results obtained when thick and 

 thin layers of phenol solutions are used. It has been found empiri- 

 cally (18) and it can be analytically derived (26) that the reciprocal 

 of the linear polarization of the fluorescence is a linear function of 

 the concentration. The figure shows that such a relation does hold 



28r- 



24 



20 



T '^ 



CONCENTRATION DEPOLARIZATION 

 OF PHENOL SOLUTIONS 



5 rnm LAYER 



30/x LAYER 



C 



Fig. 8. Concenuation dcpolaiizalion of phenol solutions. Ordinate: 1/1', recipro- 

 cal of the linear polarization. Abscissa, molarity of indole. 



for very thin layers, but that in thick layers an extra source of de- 

 polarization is found, one which can be attributed to radiative trans- 

 fer, or, as it has been graphically described for the gas phase case, 

 the imprisonment of the emitted radiation. The slope of the plot 

 of the reciprocal of the polarization (1/F) against molar concen- 

 tration (C) permits the calculation of the constant R^ which is de- 

 fined as the distance between two parallel molecules at which the 

 excited state has a 50 per cent chance of migration from the ex- 

 cited to the unexcited one. The distances thus derived may be com- 

 pared with those jjredicted by the theory of Forster (5) on the 



