42 RADIATION BIOLOGY 



lational energy to produce high velocities. Atoms moving with very high 

 velocities are termed "hot" and may in rare instances lose their energy 

 in subsequent collisions to produce excited electronic states. Geometrical 

 and momentum restrictions favor rapid redistribution of the extra energy 

 into smaller translational quanta, and it is well known that electronic 

 and, indeed, vibrational excitation occurs rarely as a result of collisions 

 with hot atoms produced in chemical reactions or by quenching. In 

 stellar atmospheres, conditions such as high relative concentration of hot 

 atoms and long periods between collisions may be such as to make hot 

 atoms important. It is also possible that detonation processes may 

 partly depend on reactions induced by these atoms (Sanger et at., 1949) 

 (Sect. 5). The very hot atoms produced by recoil in nuclear disinte- 

 gration (Szilard-Chalmers effect) probably cause reaction on their first 

 coUision in most cases (Friedman and Libby, 1949). Such atoms are, 

 of course, rare in nature, though it is possible that gene mutation may be 

 produced in this way by cosmic-ray-induced nuclear reactions. 



Hot-atom reactions are probably responsible for the luminescence that 

 occurs when meteors enter the earth's atmosphere (Bobrovnikoff, 1942). 

 In Fig. 1-135 the lowest electronic curve (hypothetical), corresponding 

 to the ground states of both atoms, is drawn. If the unexcited atoms 

 are accelerated to very high approach velocities, the configuration point 

 will rise on the lowest curve to the crossing points. Depending on con- 

 ditions at these points, excited thallium and excited mercury atoms can 

 be produced at relative rates that bear no relation to the thermal dis- 

 tributions of populations found at thermal equilibrium. Normal col- 

 lision activation would form more excited thallium than mercury atoms, 

 but conditions at crossing points could, under nonequilibrium conditions, 

 reverse the distribution, thus favoring greater luminescence from mercury 

 radiation than from that of thallium. The vaporized atoms from meteor 

 surfaces are hot atoms with respect to the constituents of the atmosphere 

 and probably produce the well-known radiation of meteors passing 

 through the upper air. As in other such luminescence processes, the 

 relative intensity of lines will, in part, be determined by conditions at 

 crossing points. 



We have mentioned that the theories of Kallmann and London and 

 others for electronic-energy exchange under conditions of very close reso- 

 nance predict large distances of particle separation over which transfer 

 may take place. Indeed there is a variety of experiments indicating the 

 validity of these theories. Forster (1949) found that irradiation of trypa- 

 flavin in methanol solution sensitized the fluorescence of rhodamine B 

 molecules. When the molecules were, on the average, 70 A apart, 50 per 

 cent of the energy was lost in this way. There is no evidence that the 

 process is viscosity-dependent; however, as we have seen, lack of such a 

 dependence does not establish the fact that the participants in quench- 



