208 RADIATION BIOLOGY 



Burhop, 1952), and is particularly important in systems in which 

 metastable rare gas atoms are present. When possible, the process has 

 high probabihty because it is one of exact resonance; the electron 

 acquires kinetic energy just equal to the difference between the excita- 

 tion energy of A* and the ionization potential of B. 



VI. An example is 



Hg* + H2 ^ Hg + H + H 



Here the dissociation energy of H2 is 4.48 ev, so that 0.38 ev is hberated 

 to kinetic energy of the three products. The presence of H atoms is 

 readily verified by the usual methods. As stated above, it is difficult to 

 distinguish between this process and an indirect one following VII. The 

 combined (quenching) cross section is 6 X 10"^'' cm^ A great many 

 sensitized photochemical reactions, initiated by this process or by VII (or 

 by III with subsequent spontaneous dissociation), have been studied in 

 detail (e.g., decomposition of hydrocarbons). Such studies have the 

 advantages that the initial light-absorption process is well understood 

 and the primary kinetics are simple, so that attention can be focused on 

 later elementary processes. They may also provide photochemical 

 information about a substance in a wave-length region in which it is 

 transparent. 



VII. An example is 



Hg* + H2-^HgH + H 



Since the dissociation energy of HgH is 0.37 ev, the energy excess is 0.75 

 ev, and this is divided between vibrational and rotational energy of the 

 HgH molecule and translational energy of the two products. The pres- 

 ence of HgH is demonstrated by observation of its emission spectrum. 

 The HgH molecule would probably be produced in a high rotational state, 

 but its electronic excitation must be the result of a subsequent collision 

 with another Hg*. It may be mentioned that a collision of type III, 

 with deactivation of Hg* and excitation of H 2 to a high vibrational 

 state, is not possible; it would violate not only the Franck-Condon prin- 

 ciple, but also the spin-conservation rule. 



This reaction is the prototype of a group of photosensitized reactions 

 in which Hg* reacts with a hydrocarbon RH and a free radical R is 

 formed. Such reactions have been extensively studied (cf. Steacie, 1946). 



VIII. An example would be 



Hg* -1- H -^ HgH* 



another proposed process for the excitation of the HgH spectrum during 

 the mercury-sensitized decomposition of hydrogen, the H atoms arising 

 from process VI (or VII). 



IX. This process has been observed with metastable Hg atoms (6^Po) 

 and many varieties of surface. 



