206 EADIATION BIOLOGY 



"cross section" as a measure of the probability that a certain event 

 occurs in a binary colHsion, cf. Massey and Burhop (1952).] In a few 

 cases it has been possible to describe in some detail the interaction of 

 A*, B, and C as the reaction proceeds, and to deduce the reaction rate 

 (Laidler and Shuler, 1951). 



The type of exchange reaction discussed here is intimately related to 

 that in which an unexcited "free" atom or radical A reacts with BC to 

 form AB and C; indeed, the latter type is merely a special case of the 

 former. Exchange reactions of free atoms and radicals are steps in the 

 mechanisms of many thermal as well as photochemical and radiation- 

 chemical reactions. 



An illustrative example of an exchange reaction. is discussed in the 

 following subsection and again in Sect. 3-3a. 



3-le. An Example: Reactions of Excited Mercury Atoms. The two 

 strongest absorption lines of Hg vapor are the resonance lines 1850 A and 

 2537 A. Both lines are also strongly emitted by a quartz Hg lamp ; the 

 former, however, is ordinarily at least partly absorbed by the quartz wall 

 of the lamp, or by any interposed air, and is therefore difficult to work 

 with; the latter is the strongest ultraviolet line emitted by such a lamp, 

 and is the most popular source of ultraviolet light. The upper level 

 (6^Pi) of the 2537 A line (cf. Fig. 3-1) is one of a "triplet" group, the 

 other two of which (6^Po and 6^P2) are metastable and are, therefore, 

 not produced directly by light absorption. 



The 6^Pi excited state of Hg (to which the notation Hg* will hence- 

 forth specifically refer) can also be produced by collisions with electrons. 

 If the energy of the electrons in a beam is constant, it is found that no 

 excitation occurs if this energy is less than the excitation energy of Hg* 

 (4.86 ev) . If it equals or exceeds this amount, abundant excitation occurs 

 and is demonstrated by a pronounced radiation of the 2537A line. Some 

 of the many varieties of collisions of the second kind which Hg* has been 

 shown to undergo are now briefly reviewed. The Hg* may be produced 

 either by light absorption or by electron impact in a discharge. All 

 the processes are, of course, accompanied by a quenching of the Hg* 

 fluorescence. 



I. Hg* + e — > Hg -\- e (with correspondingly greater kinetic energy). 

 This process occurs in mercury discharges in which both Hg* and free 

 electrons are abundant. 



II. An example would be 



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



This process, with rare gas atoms, does not occur with appreciable prob- 

 ability at ordinary temperatures, a fact in harmony with the dictate of 

 the Franck-Condon principle against conversion of electronic energy into 



