C. J. p. SPRUIT AND A. SPRUIT-VAN DER BURG 



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energy level by absorption, and from the higher to the ground level by 

 the emission of light. The same is represented in another way in Fig. 

 lb. As long as a number of conditions are fulfilled as to transitional 

 probabilities and spacing of vibrational levels in the two energy states, 

 it may be predicted that there is a mirror symmetry between the 

 emission and the fluorescence spectrum, if plotted versus frequency, 

 as shown in Fig. Ic. 



The reemission of light by the excited molecule is not the only way 



16 18 20 22 24 25 x 10^ cm"' 



Fig. 2. Mirror symmetry as observed in practice. Fluorescence ( F ) and absorp- 

 tion (A) spectra of riboflavin plotted against frequency. Dash line: mirror 

 image of fluorescence spectrum. 



by which this energy can be dissipated, and, as a rule, the molecule 

 returns to the ground level by way of other transitions not involving 

 the emission of a quantum. In this case the energy is lost in the form 

 of heat. Only if special conditions are fulfilled are such radiationless 

 transitions of sufficiently low probability that fluorescence can occur 

 with a measurable quantum yield, and only in relatively rare cases 

 does this yield approach unity. It is an empirical fact that only for 

 the first excited level are the probabilities of these radiationless transi- 

 tions to the ground level sufficiently low to allow for the occurrence 

 of fluorescence. For this reason, only those emission bands are en- 



