234 RADIATION BIOLOGY 



migration by disturbing the resonance. Thus the effect is observed with 

 certainty chiefly in "hard" crystals for which hv^ib of the lattice atoms is 

 relatively great and, even at room temperatures, the lattice vibrations are 

 therefore rare. In organic crystals or in ordered arrays of organic 

 molecules exciton movement is of relatively minor occurrence. 



Electrons which by Hght absorption (or energy transfer from high- 

 energy radiation) are thrown into so-called conductivity bands of a 

 crystalline insulator can also migrate over distances which again are the 

 greater the more perfect the lattice is and the less it vibrates. This 

 process is responsible for the so-called internal photoelectric effect and 

 for photoconductivity of irradiated crystals. The migrating electron 

 will either recombine quickly with the atomic group or ion from which it 

 was removed, or it will be trapped in the lattice for a certain period. If it 

 is released somewhat later from the trap by a thermal fluctuation, it again 

 has a chance to recombine, and if this process of recombination (directly 

 or by "sensitized fluorescence") results in light emission, the emission 

 will be observed at a more or less long time after the irradiation (crystal 

 phosphorescence). Electron migration can also, of course, bring about 

 chemical reaction at a site distant from that at which the electron was 

 originally liberated. Whether electron migration in arrays of organic 

 molecules may be of biological importance cannot yet be ascertained. 

 No case has been observed thus far which can with certainty be ascribed 

 to this process. 



In this discussion, the problem of energy migration in a system of 

 conjugated double bonds has been omitted. However, there is no reason 

 not to regard such a system, if it be long enough, as in a sense a micro- 

 scopic metal, and the existence of a pseudometallic conductivity band 

 makes it understandable that certain of the electrons in such molecules 

 belong to the molecule as a whole. 



4. REACTIONS FOLLOWING IONIZATION 



4-1. THE IONIZATION PROCESS, AND THE ROLE OF IONIZATION 

 IN PHOTOCHEMICAL REACTIONS 



Ionization of an atom is the consequence of absorption of a photon 

 having energy hv greater than the ionization potential of the atom. The 

 region of the absorption spectrum in which this process occurs starts at 

 the edge of convergence of the discrete absorption spectrum (Fig. 3-1), and 

 is one of continuous absorption. An atom will also be ionized in a col- 

 Hsion with a moving charged particle such as an electron or an a particle 

 (or even in a collision with a neutral atom or molecule, although energy 

 transfer in such a collision is comparatively improbable) if the energy 

 transferred to it exceeds the ionization potential. Both optical and 

 impact ionization are entirely analogous to the corresponding electronic- 



