PHOTOCHEMISTRY 257 



In all of these cases where molecules can receive an abnormal amount 

 of energy from an external source, the reaction may proceed in ways which 

 would not be expected from ordinary thermal activation. The situation 

 is illustrated by the dotted lines at E in Fig. 2, where the second hump 

 represents the number of molecules containing a large amount of energy. 

 It is evident that the large number of molecules having this high energy 

 cannot come from ordinary thermal collisions and the Maxwell distribu- 

 tion of velocities, but must come from some external source as just 

 described. 



Extra energy for this activation may be introduced not only by colli- 

 sions with photons, electrons, or other materials of high energy but also 

 by intermediate chemical changes. Under these conditions the reaction 

 is said to be catalytic and the extra material which accelerates the reaction 

 is called the catalyst. The catalyst is often effective in mere traces and 

 it is not consumed in the course of the reaction. Catalysis is quite 

 specific and the prediction of catalytic properties is, in our present state 

 of knowledge, more of an art than a science. One important branch of 

 catalysis has to do with reactions at the surface of a solid; but catalysis 

 may be effected also by gases, liquids, or dissolved material. The 

 catalyst effects the loosening of the bonds and alters the reaction so 

 that it is not necessary to supply the complete energy of activation 

 as shown in Fig. 1. 



TYPES OF RADIATION 



There are a great many different kinds of radiation depending on the 

 wave-length. These have been discussed on page 14. The shorter 

 the wave-length the more energy is contained in a given unit of radiation. 

 The radio waves and the infra-red waves do not contain enough energy 

 to effect ordinary chemical reactions. X-rays contain sufficient energy 

 to completely ionize the molecules and to produce chemical reactions, but 

 they are so penetrating that only a small portion is absorbed in a reaction 

 system of ordinary dimensions. Ultra-violet and visible light, particu- 

 larly in the blue end of the spectrum, may be very effective in bringing 

 about chemical reactions. 



Radiation in the ultra-violet and visible is due to a displacement of 

 outer electrons in atoms or molecules and the return of these displaced 

 electrons to their normal states. X-rays are due to a similar cause, 

 except that the displacements of inner electrons close to the nucleus of the 

 atom are involved. Infra-red radiation involves the displacements of 

 atoms in the molecule. These displacements may be caused by heat, and 

 all solids give about the same distribution of radiation among the different 

 wave-lengths at a given temperature. Figure 4 shows a typical curve of 

 so-called black-body radiation. The amount of radiation is plotted 

 against the wave-length. A black body is defined as a solid material 



