75 



MECHANISMS OF ENERGY DEGRADATION AND CHEMICAL CHANGE: 

 EFFECTS OF ELECTRONIC EXCITATION 



Henry Linschitz 



So far in the Conference I think we have been able at least to ask a few 

 straight-forward and more or less clearly defined questions regarding the com- 

 plex problem of the fate of thermal electrons in water. I hope that the same 

 clarity of asking questions can be maintained in this phase of the conference, 

 because here we really go off the deep end and get to problems that are even 

 more complex than those we have been talking about so far. Among other top- 

 ics, I hope that we can get around to some matters to which Dr. Pollard has 

 been vigorously trying to draw attention, and which are certainly relevant to 

 the problems of radiobiology. 



The problem to be discussed is the role played by electronic excitation in 

 radiobiology. In attempting to treat the effects which ensue following the exci- 

 tation of electron systems in molecules, we are confronted with all the basic 

 problems of photochemistry plus a few further complicating factors. In the 

 first place, the excitation is anything but monochromatic. If we regard the ex- 

 citation as taking place by radiation arising from the various components of the 

 field in the neighborhood of the rapidly moving charge, it is clear that the 

 whole spectrum of excited states can be obtained. Since the excitation process 

 will also involve direct impact with relatively high energy particles or recom- 

 bination effects, the excited states that will be reached will be even more 

 varied than those that can be reached by ordinary optical processes. Finally 

 one has to take into account the role of the local distribution of excited mole- 

 cules or fragments in the system. Let us list very briefly the various process- 

 es that can ensue after excitation of the electronic system or systems of the 

 stopping medium. 



First of all, of course, we can get simple fluorescence. Let me take just 

 one moment to talk in some detail about this, because the processes encoun- 

 tered here must be clearly seen before trying to deal with the more complex 

 effects. The initial excitation of a complex molecule will lead to some excited 

 vibrational level of the upper state. For strong optical transitions, the life- 

 time with respect to radiation will lie usually between 10"8 and 10"9 seconds, 

 and in any case, will not be much less than 10~9 seconds. States reached by 

 particle excitation, which may have different multiplicities than the ground 

 state, will have lifetimes much longer than 10"^ seconds. These radiative life- 

 times are to be contrasted with times of the order of 10" 13 seconds, which are 

 required for typical molecular vibrations to occur. Thus, a great many vibra- 

 tions may take place during the natural radiative lifetime of even the shortest- 

 lived electronic excited states. In liquid or solid phase the vibrating molecule 

 is always in contact with its neighbors and the vibrational energy transfer will 

 be efficient. Hence, in condensed phase, the first process which occurs, in 

 this simple case, is dissipation of the vibrational energy of the upper state, 

 this energy going into local heating of the surroundings while the molecule, in 



