24 RADIATION BIOLOGY 



previously emphasized, can be represented by figures of the type of Fig. 

 1-10, though the potential-energy surfaces would have to be calculated 

 for a different "best" collision configuration. Such surfaces would be 

 noteworthy only in that they lie generally at a lower potential energy 

 than surfaces appropriate for vibrational collision. Important gateways 

 would be found to have lower collision energies or heats, since the amount 

 of mutual perturbation of the electrical fields of the collisional partici- 

 pants necessary for the transfer of the small rotational quanta is small, 

 and close approach is unnecessary. The gateways will, because of their 

 position, also probably be rather wide. Both entropy and heats of col- 

 lision will consequently be such as to favor rapid energy exchange and 

 hence rapid thermal equilibration among rotational and translational 

 degrees of freedom. Anomalous persistence of rotation has been observed 

 only in molecules containing hydrogen: H2 (Smith, 1936) and HgH 

 (Rieke, 1936, 1937; Beutler and Rabinowitch, 1930; Oldenberg, 1931; 

 Wood and Gaviola, 1928; Gaviola and Wood, 1928). The behavior of 

 hydrogen is, as we have previously mentioned, due to the large rotational 

 quanta, which require gateways for collision at high potential energies 

 and thus make for poor energy exchange with translational degrees of 

 freedom. In transient phenomena of very high speed, there may be 

 hiadequate time for the 1-100 collisions required to achieve normal 

 rotation-translational equilibrium. It has been indicated that the rota- 

 tional degrees of freedom of some molecules are inert in shock-wave 

 propagation (Lewis and Van Elbe, 1939; Greene et al., 1951). Landau 

 and Teller (1936) have treated equiUbration of rotational degrees of free- 

 dom. They neglect chemical affinity of the reacting partners. Their 

 treatment is a good first approximation because the collision complexes 

 are easily formed in low-energy reactions of this type. 



3-3. ADIABATIC PROCESSES OF VIBRATIONAL-ENERGY EXCHANGE 



Low-lying equipotential lines are seldom greatly distorted in contour 

 maps for collision processes similar to those shown in Fig. 1-10. This 

 situation is the result of negligible chemical interaction of the collision 

 partners at distances larger than the order of a kinetic-theory diameter. 

 Collision processes that have their principal gateways in the regions of 

 these fines do not depend on chemical affinity. For instance, rare gases 

 should be as good as highly reactive halogen atoms in establishing rota- 

 tional equilibrium. On the other hand, exchange collisions involving 

 vibrational degrees of freedom, with the exception of heavy molecules 

 like iodine (Roessler, 1935), generally have their important gateways 

 high on the potential barrier, i.e., high on the barrier that restricts chemi- 

 cal reaction. The latter is, of course, the same barrier that limits col- 

 lision, for, according to the present picture, a colfision is no more than 

 an incomplete chemical reaction. The potential height of the gateway 



