90 



LIGHT AND LIFE 



transitions would be expected to have such a relation, and on the 

 basis of the relative position, oscillator strength, and polarization, 

 the 270 m^ and 295 mjx peaks can be provisionally identified with 

 these two transitions. 



At wavelengths longer than 300 ni/^ the polarization rises very 

 rapidly, considerably surpassing the value for the 270 m^ peak. It 

 is possible to account for this increase without assuming the existence 

 of an independent electronic transition at this and longer wavelengths, 

 as is shown by the Franck-Condon scheme of Fig. 6. G-S^ and G-So are 

 the two L transitions already postulated. Wavelengths shorter than 

 270 m^ correspond to points above V._, where excitation falls upon 



POTENTIAL ENERGY SURFACES 

 IN INDOLE 



^ Fluorescence 

 Level 



Fig. 6. Potenlial energy surfaces in indole. 



both bands but predominantly upon G-S,, the main absorption band. 

 Here the polarization increases monotonically. Between V^ and Vi 

 the excitation to G-.S, decreases, while that to G-So reaches a maxi- 

 mum at 28fi ni/x. Points below }\ are those where excitation of G-So 

 is no longer possible, but excitation to G-S'„ although very weak, is 

 still Franck-Clondon allowed. The decrease observed in the polariza- 

 tion spectrum at still longer wavelengths than 305 m^t has probably 

 a different origin. Excitation here is probably restricted to mole- 

 cules which arc in levels above the zero vibrational level of the ground 



