ACTION OF ULTRAVIOLET LIGHT ON VIRUSES 



147 



a molecule which has a method of motion which is equivalent 

 to the proper electric dipole. 



In the case of molecular absorption of ultraviolet light, the 

 vibrations of atoms as a whole in the molecule are of far too low a 

 frequency. Hence, the only suitable means of absorption is by a 

 transition from one electronic state to another, which, as it 

 primarily involves single electrons, has the proper frequency 

 range. Inevitably associated with this electronic transition is 

 some kind of vibration, and the combination of the two gives 



Fig. 6.1. Electronic absorption from one state of vibration to another in 

 which an electron is in an excited state. This is the major process of ultra- 

 violet absorption, 



rise to an electronic-vibration band. Rotation is also possible, 

 although in a large protein or nucleic-acid-like molecule it is 

 probably not so important. 



Using the familiar representation of potential hollows in 

 which vibration can occur, such a transition is represented as 

 in Fig. 6.1. Three important features need to be remembered. 

 The first is that, inasmuch as an electronic transition changes 

 the state of the electron cloud around an atom, there may be a 

 great weakening of the bond joining this atom to a neighbor. 

 The second is that the vibrational levels, and indeed all levels, 

 are broadened by the thermal agitation and electric-field overlap 

 of the near neighbors. For these reasons, ultraviolet absorption 



