G. WEBER 



93 



basis ol ihc overlap ol emission aiul al)sorption bands and the os- 

 cillator strength ol absorption of the acceptor molecule. An exact 

 comparison of the experimental and theoretical values is made diffi- 

 cult by the tact that Forster's theory utilizes the idealized model ot 

 two molecules isolated from the rest of the molecules in the solu- 

 tion, while in practice the value determined corresponds to a proba- 

 bility of repeated transfers not between isolated pairs of molecules 

 but over a chain of molecules which cannot be broken up into in- 

 dependent pairs. Comparison of the values experimentally determined 

 with those expected from Forster's theory shows that the overlap 

 of absorption and emission (29) and the absolute oscillator strength 

 of the absorption are indeed the parameters controlling the value 

 of Ro, ^vhich has the magnitude predicted by the theory. Neverthe- 

 less, the theory predicts a spread of values between 10 and 100 A, 

 while in practice the observed values appear to cover a more re- 

 stricted range, between 15 and 40 A. For phenol, Ro = 17 A. while 

 the calculated value does not exceed 11 A. It must be stressed 

 that these differences can in no case be attributed to experimental 

 error, since R^ is determined from the sixth root of the slope of the 



Polarization of Indole 



230 240 250 260 270 .2BO 290 30D 3IO 



A 



Fig. 9. Polarization spectra of indole at different concentrations. 



