30 



RADIATION BIOLOGY 



these processes (Kassel, 1932, p. 313; Eyring, 1935; Boehm and Bon- 

 hoeffer, 1926; Rabinowitch and Wood, 1936a,b; Hilferding and Steiner, 

 1935). In terms of the requirement for matching of quantum magni- 

 tudes in energy exchanges, the observations on stabiHzing colUsions can 

 be correlated with the fact that the size of vibrational quanta decreases 

 with increasing quantum number. The higher this number, the better 

 the probability for transfer into translational energy. 



Third bodies that are effective in stabiHzing recombination are, of course, 

 equally efficient in providing activation energy for unimolecular reactions. 

 Excited molecules have a much higher probability of losing energy on 

 the next collision than of gaining additional amounts (Roessler, 1935). 

 Molecules with few internal degrees of freedom are consequently acti- 

 vated from the median-energy population. The upper regions of poten- 

 tial energy are involved in activating collisions, and hence, as observed, 

 chemical affinity plays a very important role (Glasstone et at., 1941, 

 Chap. 5). 



Intramolecular Eriergy Migration. Molecules with many internal 

 degrees of freedom may store energy preceding a final unimolecular 

 reaction step. Requirements for activating collisions will then be less 

 strict, since the process can occur through several lower-energy collisions. 



0.7 



0.8 0.9 1.0 I.I 12 



DISTANCE BETWEEN THE ATOMS 6 AND C ( ^^p), '^ 



Fig. 1-11. Calculated trajectory for migration of vibration in the hypothetical mole- 

 cule H3. {From Hirschfelder et al., 1936.) 



That is, the slow process may shift from transfer to the molecule to 

 transfer within the molecule. Internal migration of energy is extremely 

 complicated and has not yet been dealt with satisfactorily. The repre- 

 sentation of such processes involves trajectories that cross and recross 

 the stable potential wells found on such surfaces as that of Fig. 1-4. 

 The appropriate well for any molecule will have as many dimensions 

 and as many valleys as there are independent vibrations. In Fig. 1-11 



