ENERGY EXCHANGE IN PHOTOREACTIONS 47 



4-4. QUENCHING WITH VIBRATIONAL-ENERGY TRANSFER 



So long as electric quantum numbers alone change on collision, the 

 transfer of energy between excited and unexcited molecules is analogous 

 to the similar processes that have been discussed for atoms. The last 

 part of the discussion in Sect. 4-3 dealt briefly with three rather special 

 transfer mechanisms, all of which are supposed to depend very strongly 

 on the extent to which the resonance requirement is satisfied. Conse- 

 quently conversion of electronic energy into vibrational or external energy 

 is not favored. Most collisions of molecules are more complicated in that 

 many quantum numbers change during energy transfer. The simplest 

 such collision is between an atom and a diatomic molecule, and its treat- 

 ment will be adequate for collisions of larger molecules as long as we 

 concentrate on the reactive degrees of freedom and neglect consideration 

 of more than two vibrational quantum numbers. 



Carlo and Franck (1922) observed that the quenching of mercury reso- 

 nance radiation by hydrogen resulted in the dissociation of the latter. 

 The two primary steps that have received most support, 



Hg(6T;) -1- H2 -^ 2H + Hg(6iSo) + 9.9 kcal (l-43a) 



and 



Hg(6^P;) + H2 ^ HgH + H -h 18.4 kcal, {l-4Sb} 



are reviewed by Noyes and Leighton (1941, p. 327). Proponents of the 

 first reaction have suggested that the marked effectiveness of mercury 

 as a photocatalyst for hydrogen dissociation is in part due to the approxi- 

 mate satisfaction of the resonance requirement for collisions. However, 

 the second reaction is undoubtedly of major significance, and an energy 

 discrepancy of 18 kcal/mole is hardly indicative of close matching of 

 mercury excitation energy and the energy requirement of the reaction. 

 Mercury hydride is metastable and never found in high concentration in 

 these reactions, but that it is involved as an intermediate in the total 

 photoreaction points up the importance of chemical affinity in quench- 

 ing reactions, a fact discussed by numerous authors (Eucken and Becker, 

 1934; Franck and Herzfeld, 1937; Eyring, 1935). Simply on the basis 

 of the free energy of activation, Eq. (1-436) would be expected to pro- 

 ceed more efficiently. Similarly, numerous reactions in which mercury 

 fluorescence radiation is quenched depend on the formation of metastable 

 intermediate compounds containing mercury (HgA, HgK: Oldenberg, 

 1928; HgCH4: Glockler and Martin, 1934). Similar examples involving 

 larger molecules have been illustrated by the quenching of anthracene 

 fluorescence (Sect. 4-3). 



A number of small molecules are effective in deactivating mercury 

 atoms from the 6^P? level to the metastable level 6^Po by causing the 

 redistribution of 5.01 kcal/mole in nonelectronic degrees of freedom. A 



