16 RADIATION BIOLOGY 



above the first excited state. Many different excitation processes may 

 correspond to the same energy, and crossing points are common. The 

 spectra of such molecules begin in the far ultraviolet and are continuous, 

 indicating dissociation (Noyes and Leighton, 1941, p. 327; Sponer and 

 Teller, 1941). Saturated hydrocarbons probably do not fluoresce, and 

 we may conclude that each absorption process is immediately followed 

 by internal conversion whenever excitation is originally to stable states. 

 The extra energy will be in part taken up by other molecules, but the 

 larger amount will be concentrated in the vibrational degrees of freedom 

 of the primary molecules. Dissociation and reaction from stable states 

 can occur only if the energy becomes localized in suitable vibrations. 



Saturated hydrocarbons have no single electrons that are especially 

 loosely held; hence the short wave lengths required for excitation. Mole- 

 cules containing halogen, oxygen, nitrogen, or metal atoms or double 

 bonds usually show ''optical electrons." These may be considered as 

 belonging to a special part of the molecule. Many such molecules — for 

 example, aldehydes, ketones, carboxylic acids, olefins, and amino and 

 cyano compounds — show Rydberg-like spacing of levels in higher exci- 

 tation energies, much like the familiar spectral distributions of atoms 

 and attributable to a localization of excitation in a single atom or group 

 (Sponer and Teller, 1941). Light absorption occurs at wave lengths in 

 the near-ultraviolet or visible bands, and fluorescence is a common accom- 

 panying phenomenon, thus indicating poor crossing to the ground state. 

 The onset of dissociation as wave length is shortened is frequently 

 characterized by predissociation spectra (Sect. 4-1). Most organic mole- 

 cules belong to this second class, i.e., those Avith groups containing local- 

 ized electrons, but there is a third group which, though smaller, is of more 

 importance in the photoinduced processes associated with living systems. 

 These are the molecules in which there exist suitably spaced combinations 

 of atoms or groups of atoms with optical electrons to allow migration of 

 the electrons over large distances within the molecules. Molecules of 

 this type are called "conjugated," meaning that they contain a conju- 

 gated system of electron-rich groups or atoms. Benzene is an important 

 example of this class. The optical electrons associated with a single 

 group can be described approximately in terms of the atomic orbitals, 

 or at least group orbitals. For instance, the x-electrons forming the 

 second electron pair of a nonconjugated double bond can be treated as 

 belonging to this particular bond. In benzene, on the other hand, the 

 TT-electrons of the carbon atoms which form the three double bonds can- 

 not be localized in either of the Kekule structures. Instead, the two 

 Kekule clouds of allowed electron positions overlap. As a result, all six 

 TT-electrons can be pictured as moving through all overlapped clouds and 

 must be treated as belonging to the molecule as a whole; i.e., they move 



