PHOTOCHEMISTRY 295 



spectrum is very complex, with many discrete lines and some continuous 

 spectrum. In ultra-violet light and in the violet of the visible spectrum 

 nitrogen dioxide is decomposed by the light that it absorbs, giving nitric 

 oxide and oxygen. As the decomposition proceeds, nitric oxide and 

 oxygen accumulate and the reverse reaction takes place thermally. 

 This reverse reaction involves a triple collision between two nitric oxide 

 molecules and one oxygen molecule; accordingly the reaction is quite 

 slow. For short exposures this reverse reaction can be neglected, particu- 

 larly at the lower pressures. When nitrogen pentoxide gas is mixed with 

 nitrogen dioxide, the nitrogen pentoxide reacts immediately with the 

 nitric oxide formed by the photodecomposition of the nitrogen dioxide, 

 producing more nitrogen dioxide. In this way the reverse reaction of 

 nitric oxide is prevented and more accurate measurements of the photo- 

 decomposition of nitrogen dioxide can be obtained by determining the 

 amount of accumulated oxygen. 



This reaction has been investigated carefully in three different 

 laboratories (13, 25, 37) and all are in substantial agreement. 



A quantum yield of 2, obtained with ultra-violet Ught by extrapolat- 

 ing to pure nitrogen dioxide, is explained on the basis of a collision between 

 an activated molecule of nitrogen dioxide and a second molecule to produce 

 two molecules of nitric oxide and one of oxygen as given by the following 

 equations : 



NO2 + /?v = NO2 



NO* + NO2 = 2N0 + O2 

 The asterisk (*) indicates an excited molecule. 



o 



The lower quantum yield of about 0.5 at 4050 A is explained on the 

 assumption that the energy of a quantum at this wave-length is not quite 

 sufficient to bring about the chemical reaction. Part of the energy of 

 activation must come from thermal collisions, and not every molecule 

 that absorbs a photon contains sufficient thermal energy to effect reac- 

 tion. At 4360 A there is no decomposition, i.e., the quantum yield is 

 zero. 



Norrish (37) has studied the relation of fluorescence of nitrogen 

 dioxide to this reaction. At the longer wave-lengths he obtained a 

 fluorescence, but at the shorter wave-lengths, where dissociation occurs, 

 no fluorescence is detectable. 



Nitrogen Pentoxide. — Nitrogen pentoxide absorbs radiation below 

 3000 A and it is decomposed with a quantum efficiency of 0.6 of a molecule 

 decomposed per quantum absorbed (25). The decomposition of nitrogen 

 pentoxide at longer wave-lengths when sensitized by nitrogen dioxide 

 has been explained in the preceding section. It is clear that this sensi- 

 tized reaction is really a chemical reaction of the nitrogen pentoxide 

 with one of the chemical products resulting from the photodecomposition 

 of the nitrogen dioxide. 



