PHOTOCHEMISTRY 283 



The acetone vapor was partially removed by evacuation and a series 

 of transmissions was measured at different partial pressures. The 

 logarithm of the percentage transmission was then plotted against the 

 partial pressure of the acetone vapor, as shown in Fig. 5 on page 261. It 

 is clear from the diagram that the absorption is greatest at the 2650 A 

 line, and the straightness of the lines indicates that the absorption follows 

 Beer's law — as it should in the case of a simple absorbing system. 



In one typical experiment the radiation was absorbed at the rate of 

 85,200 ergs/sec. over a period of 23,000 sec. This absorption of light 

 resulted in an increase of pressure amounting to 30.4 mm. From 

 the known volume of the cell (about 60 cc.) it can be calculated that 

 5.28 X 10^^ molecules of acetone were decomposed; calculating the 

 radiation in terms of number of quanta absorbed during the whole expo- 

 sure, it was found that on the average 0.17 molecule of acetone was 

 decomposed for each quantum absorbed. With a total of eight separate 

 determinations, the average deviation from this averaged value was 0.02. 

 The quantum yield increased at low intensities and decreased at low 

 pressures. 



The photolysis of acetone has been investigated recently by Norrish, 

 Crone, and Saltmarsh (38). The quantum yields obtained were prac- 

 tically the same as those already described. These investigators found 

 that the spectrum shows discontinuity at the longer wave-lengths from 

 3325 to 2950 A. Below 2950 A it is continuous. Ultra-violet light 

 produces in acetone vapor a brilliant green fluorescence. At low intensi- 

 ties the fluorescence appears to be produced only by exciting light that 

 falls in this discontinuous region. 



The mechanism by which acetone decomposes under the influence 

 of ultra-violet light is a matter of considerable interest. The fluorescence 

 of acetone suggests an excitation of the molecule, whereas the continuous 

 spectrum suggests decomposition directly into fragments. Several 

 different mechanisms can be suggested, but there seems to be an 

 increasing amount of evidence, in this and similar reactions, which favors 

 photodissociation into free radicals (methyl groups) as one of the 

 principal primary reactions. 



The spectrum of acetone vapor throughout the ultra-violet has been 

 carefully measured and interpreted by Noyes, Duncan, and Mann- 

 ing (41). The theory of the decomposition of ketones and aldehydes has 

 been discussed by Norrish and his coworkers. 



Aldehydes. — Formaldehyde is decomposed by ultra-violet light 

 between 3650 and 2540 A, according to the reaction: 



H2CO + /ii/ = Ho + CO 

 In the longer wave-lengths of this region the quantum efficiency is 

 0.7 molecule per quantum of absorbed light. At the shorter wave-lengths 

 it is 0.9, and in the neighborhood of 3100 A it is 1.1 (39). 



