PHOTOCHEMISTRY 17 



Tereniii and Kariakin (1947) prepared thin films by subliming simul- 

 taneously acridine and an organic acid (e.g., succinic acid) onto a plate 

 kept at — 180°C. When irradiated with wave length 3660 A, the newly 

 prepared films exhibited the violet fluorescence which is characteristic of 

 neutral acridine. When the material had stood for some time at — 180°C, 

 or for a much shorter time at room temperature, this fluorescence was 

 replaced by the green emission which is given by acridinium salts. A 

 short irradiation with wave length 2537 A restored the film to its violet 

 fluorescent state. On standing in the dark the fluorescence again became 

 green. These changes could be repeated indefinitely. The effect of 

 irradiation with actinic light must be the transfer of a proton across the 

 hydrogen bond from the amino group to the carboxyl ion. This reaction 

 is essentially similar to those which were studied in aqueous solution. 

 They all involve the transfer of photons across hydrogen bonds. Terenin 

 (1947) has suggested that processes of this type may play an important 

 role in the photochemistry of biological systems. 



SECONDARY STEPS 



The photochemical secondary steps, those simple reactions which trans- 

 form the primary products into the stoichiometric reaction products, are 

 identical with the reaction steps which are responsible for the observed 

 kinetics of thermal (dark) reactions (Laidler, 1950, Chap. 7). The indi- 

 vidual steps are kinetically simple — commonly unimolecular, bimolecular, 

 or termolecular chemical reactions. Under some conditions the kinetics 

 of the over-all reaction are influenced by the rates of the diffusion, adsorp- 

 tion, or desorption processes. 



BIMOLECULAR STEPS 



Bimolecular reactions involving a reactive entity and a normal molecule 

 are of special importance in the mechanisms of photochemical reactions. 

 Reactions between stable molecules and atoms have been extensively 

 investigated. The following reactions are typical examples whose exist- 

 ence has been reasonably well established (Steacie, 1946; Polanyi, 1932; 

 Laidler, 1950): 



Na -\- Clo -^ NaCl + CI, 



CI + H2 ^ HCl + H, 



H + CH4 -^ H2 + CH,3, 



OH + H2O2 -^ H2O + HO... 



Similar reaction steps such as 



H + CH3 -^ CHo + Ho, 



which invohc a radical in place of the stable molecule, appear to occur 

 with equal facility. Reactions of this general type, which do not change 



