Photosynthesis in the Shortest Uhraviolet 



A. N. TERENIN 

 University of Leningrad 



The primitive atmosphere of the Earth, deprived of oxygen, was exposed to 

 intense irradiation by the shortest ultraviolet wavelengths present in the solar 

 radiation, but now screened off by oxygen and the ozone layer. 



Investigation of the upper atmosphere with rockets equipped with spectral 

 apparatus, and computation [i] have in fact revealed a high intensity of the solar 

 radiation in the range of wavelengths shorter than 1850 A, strongly absorbed 

 by oxygen of the atmosphere and therefore not reaching the Earth's surface. 



It was interesting to have some information about what kinds of organic 

 compounds could be synthesized photochemically under the action of such a 

 shortest ultraviolet spectral range, known as the Schuman region. The study of 

 the photochemical dissociation of simple gas molecules under the action of light 

 quanta in this range was started some 20 years ago by Harteck and Groth in 

 Germany [2], by Leighton & Steiner in U.S.A. [3], and by us in U.S.S.R. [4]. 



In Fig. I are shown in a very simplified and schematic manner the spectral 

 regions of absorption and photochemical sensitivity of some gases, which could 

 form the primitive terrestrial atmosphere, namely ammonia, water, methane and 

 carbon monoxide. The absorption by oxygen, begiiming from 1850 Â on to the 

 shorter wavelengths, shows that under ordinary laboratory conditions only 

 ammonia can be subjected to photochemical change. The shorter ultraviolet 

 wavelengths necessary to decompose water, methane and carbon monoxide are 

 not transmitted through the air surrounding the radiation source and require 

 either a vacuum on their path, or the substitution of the air by a gas transparent 

 in this range, as for example hydrogen, to study their action. 



In Fig. I alongside the wavelengths (given in Â) there is also given the scale 

 of the energy values of the corresponding quanta, expressed, as is usual with 

 physicists, in electron-volts. The sharp limits of the photochemically active 

 ranges indicate approximately the thresholds, from which there begins the split- 

 ting of gas molecules into primary fragments — atoms and radicals, as shown in 

 the figure. In addition, the dashed horizontal arrows indicate the final, stable 

 products, formed as a result of subsequent reactions of these primary particles. 

 The resulting quantum yield of the photolysis of CO under irradiation by 

 A 1295 Â is near to i [5]; for NH3 and A 1470 Â it is ca. 0-5 [15], and for H2O 

 vapour under irradiation by A 1650 A it can amount to 03, according to recent 

 data [16]. For the photolysis of CH4 the quantum yield is equal to 0-35-0-5 as 

 judged from hydrogen evolution [6]. Of much help for the identification of these 

 particles has been the photodissociation into emitting radicals, found by us in 

 1936 [4], which occurs at somewhat shorter thresholds, as indicated by an 

 asterisk in Fig. i at the radicals concerned. The method of luminescent fragments 



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