284 BIOLOGICAL EFFECTS OF RADIATION 



The photolysis in the gas phase of propionaldehyde (32) and 

 acetaldehyde (33) has been carefully investigated by Leighton 

 and Blacet, using a mercury arc with a quartz monochromator. Very 

 small quantities of the decomposition products were obtained on 

 account of the weak intensities of the monochromatic light, and 

 the authors developed an excellent technique for the microanalysis 

 of 50 to 100 mm. 3 of these gases, using fused beads of material as solid 

 absorbents. 



Photochemical decomposition and polymerization occur as two 

 independent and simultaneous reactions. Decomposition is independent 

 of pressure, while the apparent polymerization is directly proportional 

 to pressure. The quantum efficiency is independent of the intensity of 

 the light, i.e., the amount of change at a given wave-length is directly 

 proportional to the amovmt of light absorbed. 



The acetaldehyde decomposes chiefly into carbon monoxide, 

 methane, and ethane, together with a little hydrogen. ^ At 3130 A 

 the quantum yield for decomposition is 0.51. At 2654 A it is about 

 0.77, and at 2537 A about 1.02. The absorption spectrum of acet- 

 aldehyde shows distinct structure in the region from 3400 to 3250 A. 

 From 3250 to 3050 A the absorption lines are diffuse and from 3050 

 to 2700 A the lines are without structure. Throughout the whole region 

 from 3400 to 2400 A there is continuous absorption, reaching a maximum 

 at about 2700 A. 



Recent work by Leermakers (30) shows that the quantum yield 

 increases very rapidly at high temperatures, amounting to 310 at 300°C. 

 This large yield appears to be due to a long cham reaction involving free 

 radicals such as ethyl and methyl groups. 



Ammonia. — Ammonia is decomposed by short ultra-violet Kght, which 

 it absorbs at 2000 A and below. A mercury arc does not give sufficient 

 intensity in this region for conducting photochemical measurements. 

 Accordingly a spark of zinc or magnesium is used. Large disks, rotating 

 at right angles, form convenient electrodes between which the spark 

 passes. Either a focal isolation method or a monochromator method can 

 be used. The quantum yield is approximately 0. 14 molecule decomposed 

 per quantum absorbed at 25° (28) . The products are nitrogen and hydro- 

 gen in a 1:3 ratio. The quantum yield is independent of hght intensity 

 and varies but slightly with temperature, giving a value practically twice 

 as large at 200°C. The low quantum yield may be due to recombination 

 of the fragments which are thrown out in the decomposition process; 

 or some of the absorbed energy may be dissipated as heat in vibrations 

 of the atoms within the molecule under conditions where dissociation 

 does not immediately take place. 



Anthracene Polymerization. — When anthracene is dissolved in xylene 

 or other solvent and exposed to ultra-violet light, the anthracene poly- 

 merizes to give double molecules, according to the reaction : 



