PHOTOCHEMISTRY 259 



are involved, there is a chance to utilize some of the energy in displace- 

 ments of the atoms within the molecule and in rotations of the molecules 

 around a common center of gravity. These displacements and rotations 

 involve comparatively small amounts of energy of varying size so that 

 by combining them with the electron displacements a series of bands 

 is obtained. Such bands are very common. Whereas the bands will 

 appear under low resolution to be completely absorbing, high dispersion 

 reveals the fact that they are made up of a large number of fine lines. 

 The fine structure of these bands can be detected in emission spectra and 

 in absorption spectra of gases, particularly at low pressures. Only in 

 special cases, however, can the fine structure of absorption bands be 

 detected in solution. The intimate contact of the solvent tends to blur 

 out these lines. Sometimes they can be detected, however, by cooling 

 the system with liquid air. Electronic displacements are not involved 

 in the infra-red bands. 



In the far infra-red the radiant energy is absorbed and converted into 

 rotation of the molecule. Again large dispersion reveals the fact that 

 these bands are made up of a number of discontinuous lines. In the near 

 infra-red, (8000 to 200,000 A) the absorption bands are due to a com- 

 bination of molecular rotation and internal displacements, but they, too, 

 can be resolved into discontinuous lines provided the spectrometer has 

 sufficient resolving power. The centers of the band correspond to the 

 displacements of atoms within the molecule, and additions or subtractions 

 of small amounts of energy absorbed in rotation of the molecule lead to a 

 broadening of the region of absorption and the production of an absorp- 

 tion band. 



LAWS INVOLVED IN PHOTOCHEMICAL REACTIONS 



The most important and the most obvious law in photochemistry is 

 that of Grotthus according to which only that radiation, which is 

 absorbed, is capable of producing chemical reaction. It is clear that those 

 radiations which pass through a system without absorption are incapable 

 of affecting the system. Radiation which is absorbed may, possibly, 

 produce chemical action, but in most cases the radiation is converted into 

 increased kinetic energy of the molecules and the temperature is raised 

 without effecting any chemical change. Photochemical reactions result 

 only when the conditions are such that the activated molecule can pro- 

 duce a chemical reaction. 



The absorption of light in the simplest case follows the differential 

 equation 



^ = « ('> 



which is so frequently met with in physical and chemical phenomena. 

 Integration of this equation gives 



