216 RADIATION BIOLOGY 



energy is the same concept and has identical origin with that of thermal 

 activation in ordinary chemistry. 



It will be obvious that the same reasoning for the reverse process shows 

 a great improbability (or, equivalently, high activation energy) for the 

 collision of the first kind: 



Hg + He + kinetic energy -^ Hg* + He 



Another illustrative application shows the impossibility of quenching 

 of the Hg* resonance radiation by transferring the entire excitation 

 energy to vibrational energy of a diatomic molecule; even though there 

 may be one energy level, corresponding to high vibrational (and perhaps 

 rotational) excitation of the latter (Fig. 3-2) which nearly coincides with 

 the Hg* level (Fig. 3-1), the transition would require sudden changes in 

 the momenta of the two atomic constituents of the molecule. 



A vivid manifestation of the Franck-Condon principle is found in the 

 upper atmosphere of the earth. Here, molecular oxygen and nitrogen 

 both absorb ultraviolet radiation from the sun and are excited or ionized. 

 Because most of the relevant excited and ionized states of oxygen have 

 values of Ve significantly different from that of the ground state, absorp- 

 tion of light usually leads to dissociation; in the case of nitrogen, how- 

 ever, the relevant equilibrium internuclear separations differ only 

 slightly and direct dissociation is rare. This simple arg\iment immedi- 

 ately explains the observation that oxygen is completely dissociated 

 above an altitude of about 110 km, whereas molecular nitrogen pre- 

 dominates to much greater heights and is noted at altitudes as great as 

 1100 km. 



As discussed above, the Franck-Condon principle determines the most 

 favorable conditions for a transition. Actually, small quantitative devia- 

 tions do occur: slight movements of the heavy constituents are possible 

 and for complete understanding it is necessary to have a detailed theoret- 

 ical treatment giving the probability distribution. This is a difficult 

 problem, however, and, although it has been solved for such cases as some 

 radiative transitions of diatomic molecules (Coolidge, James, and Present, 

 1936), very httle can be done with the more complex systems to which 

 the Franck-Condon principle is just as applicable — and perhaps, at the 

 present time, even more useful because detailed theory is impossible. 



3-2d. Luminescence. The principles governing fluorescence of atomic 

 vapors apply also to diatomic molecules, but the spectra involved must 

 evidently be much more complex. Thus the so-called resonance spec- 

 trum, which, as in the atomic case, follows absorption of monochromatic 

 radiation, consists of many Unes: even if a single vibrational level of the 

 upper electronic state is selected (by use of light of a single wave length), 

 there can occur a transition to any of several vibrational levels of the 



