ultraviol?:t absorption spectra 



169 



configuration of the individual molecules and thus the energy associated 

 with a particular electronic transition will be influenced in a condensed 

 system by the electric and magnetic fields associated with nearby mole- 

 cules. Since the spatial orientations involved will be random (except in 

 crystals) and will be varying, owing to thermal motion, there will result 

 a statistical distribution of electronic configurations and of transition 

 energies of the absorbing molecules, thus broadening the observed absorp- 



60.000 



60,000 



40,000 





20,000 - 



12 3 4 5 



INTERATOMIC DISTANCE r, A 



Fig. 5-3. Illustration of the Franck-Condon principle. Horizontal line.s within the 

 well of each potential-energy curve represent various vibrational-energy levels. A 

 transition from the ground state (V") to the excited state (V) would most probably 

 leave the molecule in the second excited vibrational level (point A) since the inter- 

 atomic distance cannot change appreciably within the duration of the transition. 

 (Reproduced by permission of the publishers front Practical Spectroscopy, by George R. 

 Harrison, Richard C. Lord, and John R. Loofbourow, copyright, 1948, by Prentice-Hall. 

 Inc.) 



tion band. These effects will be reduced if the fields involved are reduced 

 (as in nonpolar solvents) or if the extent of the variations due to thermal 

 motion is reduced, as in spectra of substances at low temperatures (Sin- 

 sheimer et al., 1950a). 



For many substances in solution, the effects described widen the indi- 

 vidual absorption bands associated with a given electronic transition so 

 as to produce a fusion of these bands into an apparently single band of 

 considerable breadth. The individual bands, representing transitions 

 from vibrational energy states accompanying the normal (lowest energy) 

 electronic state to various vibrational states accompanying the excited 



