108 



RADIATION BIOLOGY 



II' disrupt ion is less likel}', tlir inolccule may remuiu iiituct for several 

 luiiulrod or thousand vibrational periods, or lO^'^-lO"'^ sec. The spec- 

 tral hroadeniii}; in this case will not he so great and will give rise to 

 so-called "predissociation " l)ands (Rice and Teller, 1949). 



The duration of the excited state may be reduced because of a high 

 probability of a transition to a third electronic state (Fig. 5-2). The dura- 

 tion of this state will have no influ- 

 ence on the width of the original 

 absorption band. A number of such 

 instances of transitions to a "triplet" 

 state have been reported (Kasha, 

 1947). Transitions from this triplet 

 state to the original ground state are 

 of very low probability so that the 

 molecule may retain energy as an 

 excited triplet state for appreciable 

 lengths (seconds) of time (McClure, 

 1949). 



Under conditions of appreciable 

 intermolecular contact (solutions and 

 solids) the energy of excitation may 

 be rapidly dissipated by conversion 

 to vibrational energy which, in turn, 

 is simply transferred by collisions or 

 electromagnetic damping to neigh- 

 boring molecules and ultimately ap- 

 pears as thermal energy (Massey, 

 1949). Such dissipative effects, 

 which reduce the duration of the ex- 

 cited state, are in part responsible 

 for the broadening of absorption 

 bands of substances in solution as 

 compared to their vapor absorption 

 spectra. 



If the molecule retains its excita- 

 tion energy for a time comparable 

 with the probability^ of transition 

 from the excited to the ground state, 

 the energy will be reradiated as "fluorescence." This fluorescence radia- 

 tion may then escape, or it may, under appropriate conditions, be reab- 

 sorbed by other chromophores within the solution or biological system 

 (Arnold and Oppenheimer, 1950; Forster, 1948; Franck and Livingston, 

 1949). 



In addition to these primarily intramolecular factors, the electronic 



INTERATOMIC DISTANCE ALONG 

 CRITICAL COORDINATE 



Fig. 5-2. Illustration of the possibility 

 of a radiationless transition from the 

 initial excited electronic state (T*) to a 

 second excited state (T), in this 

 instance a triplet state which would 

 have a long duration and from whicih 

 return to the ground state could occur 

 by delayed emission of radiation or 

 phosphorescence (P). The curves rep- 

 resent the variation of potential energy 

 of the molecule as a function of the inter- 

 atomic separation for a diatomic mole- 

 cule. (Kasha, 1947; copyright, 1947, 

 by The Williams and Wilkins Company.) 



