CARBON DIOXIDE REDUCTION IN ULTRAVIOLET LIGHT 83 



or even above the level where ozone is formed (about 50 km. above the 

 surface), since no rays with wave lengths < 290 mn are available below 

 this layer. From the point of view of artificial photosynthesis, or of 

 natural photosynthesis under the present terrestrial conditions, it appears 

 entirely irrelevant whether traces of formaldehyde can be formed by 

 ultraviolet illumination of carbonate solutions or not. In dealing with 

 photochemical reactions, it must be kept in mind that the energy available 

 in one quantum, particularly a quantum of ultraviolet light, is much 

 larger than the activation energy required for most, if not all chemical 

 reactions. Thermal reactions take place when the energy of molecular 

 vibrations, together with the collision energy, are just sufficient to bring 

 the reacting molecules over the top of an "activation pass" in the many- 

 dimensional relief map representing the potential energy of the reacting 

 system as a function of its various configuration co-ordinates. This 

 favors a uniform fate for all these molecules, which all drop into the 

 same "potential valley." Activation by light absorption, on the other 

 hand, often breaks molecules into free atoms and radicals, thus lifting 

 the system onto a high energy plateau from which it can descend into 

 many different valleys, corresponding to more than one set of reaction 

 products. In the rearrangement of radicals formed from carbon dioxide 

 and water by the absorption of quanta with an energy of 150 kcal per 

 einstein, a few may err into the shallow potential trough of formaldehyde, 

 and miss the opportunity to drop into deeper valleys, representing more 

 stable configurations. In this way, traces of formaldehyde may be 

 formed by an entirely accidental side reaction, which can have nothing 

 in common with the highly specific and purposeful mechanism of 

 photosynthesis. 



The same consideration applies to experiments in electric discharge tubes, irradiation 

 with x-rays, and other treatments which break the molecules and afford the opportunity 

 for the rearrangement of the broken pieces into all kinds of new patterns. The formation 

 of formaldehyde by silent electric discharges and corona discharges in carbon dioxide — 

 described by Losanitsch and Jovitschitsch (1897), Berthelot (1898, 1900), Lob (1905, 

 1906), Gibson (1908), Holt (1909), Moser and Isgarishev (1910) and Lunt (1925)— 

 had for a wliile aroused much interest as an approach to the problem of artificial photo- 

 synthesis. In our opinion, the only aspect of these observations which may conceivably 

 be of importance for the understanding of photosynthesis, is the contribution of atmos- 

 pheric discharges to the first synthesis of organic matter on earth. 



The results discussed above show that, even when the molecules of carbon dioxide 

 and water are broken to pieces and allowed to recombine at random, the chance that 

 they will form formaldehyde and oxygen is very small. The same also seems to be 

 true for mixtures of carbon dioxide and hydrogen, despite the fact that in tWs case, 

 the system CH2O and H2O represents as deep a potential trough as did the system 

 CO2 + 2 H2 in its initial state. Thiele (1908) found no formaldehyde after the ultra- 

 violet illumination of hydrogen-carbon dioxide mixtures, while Berthelot and Gaudechon 

 (1910), Coehn and Sieper (1916) and Mezzadroli and Babes (1929) asserted that some 



