166 



RADIATION BIOLOGY 



\arious substances considered may aid still further in identification of 

 the primary receptor or receptors. 



PARAMETERS OF ABSORPTION SPECTRA 



Any absori)tion spectrum, however complex, may he regarded as a 

 summation of a set of individual absorption bands, each corresponding to 

 a transition between two particular electronic configurations (Fig. 5-1) 

 (Sheppard et al., 1941; Wulf and Dcming, 1938). It is usually possible 

 to group these individual bands, each group consisting of transitions 

 invoking nearly the same energy difference. The dilTcrcnt bands within 

 a group then represent transitions involving a common change in basic 

 electronic configuration, together with varied associated secondary 

 changes in the distribution of energy among the molecular vibrations. 



475 500 525 



550 



575 600 625 650 675 700 725 750 

 FREQUENCY X IO-'2 



Fig. 5-1. Resolution of the absorption spectrum of merocyanine in hexane into six 

 bands, each representing a particular vibrational transition associated with the funda- 

 mental electronic transition. (Sheppard et al., 1941.) 



The parameters of an absorption spectrum are properly the sum of the 

 parameters of the individual bands. Any individual band (representing 

 a transition between two distinct electronic states plus vibrational states) 

 may be described by three parameters: (1) the position in the electro- 

 magnetic spectrum, (2) the breadth of the electromagnetic spectrum 

 occupied by the band, and (3) the intensity of absorption. 



POSITION IN THE ELECTROMAGNETIC SPECTRUM 



The position of an absorption band in the electromagnetic spectrum Js 

 dependent on the energy difference between the initial and the e.xcited 

 electronic configurations since this energy dilTerence must be supplied by 

 the absorbed photon, the energy of which is related to its frequency (v) 

 by Planck's relation 



E = hv = ^^ (5-1) 



