272 BIOLOGICAL EFFECTS OF RADIATION 



cient of a photochemical reaction is large, approaching that of an ordinary- 

 thermal reaction, then the reaction contains in it steps which are purely 

 thermal reactions. If the reaction rate does not change at all with tem- 

 perature, then the photochemical reaction is probably free from accom- 

 panying thermal reactions. The majority of photochemical reactions 

 have slight temperature coefficients. 



Wave-length. — The wave-length of light is an important factor in 

 photochemical reactions. Obviously, the light must be of such a wave- 

 length (or frequency) that the radiation is absorbed. If there is no 

 absorption there can be no reaction. If there is absorption, however, it 

 is not necessarily true that chemical reaction follows. As pointed 

 out before, radiation of short wave-length is more likely to produce chemi- 

 cal reaction than radiation of long wave-length simply because the 

 quantum of energy is greater in the light of short wave-length. When 

 the photon of radiation is absorbed many changes may take place. 

 The absorbing molecule may become excited and lose its energy by colli- 

 sion to surrounding molecules thereby increasing their kinetic energy and 

 raising the temperature and dissipating the radiant energy into ordinary 

 heat. The excited molecule may react with another molecule, but a dark 

 thermal reaction may reverse the process in such a way that no result is 

 noticed. The excited molecule may collide with other molecules and 

 transfer its energy in a rather specific manner so that the second molecule 

 will undergo a chemical reaction which can be observed. 



The type of absorption spectrum encountered is often very useful in 

 understanding and predicting photochemical reactions. A discon- 

 tinuous spectrum indicates that the molecule becomes excited by the 

 absorption of light, but a continuous spectrum indicates that the molecule 

 becomes ionized or dissociated, and that fragments are thrown out with 

 kinetic energy. The kinetic energy is not subject to the restrictions of 

 the quantum theory and accordingly there is a random distribution of 

 energies over considerable ranges giving the effect of continuous absorp- 

 tion. The discontinuous spectrum contains fine lines in a spectrogram 

 (images of the slit of the spectrograph) because the displacement of 

 electrons in the atoms or molecules is subject to quantum restrictions, 

 but in case chemical dissociation or ionization takes place, the wave- 

 lengths between these discontinuous lines can also be utilized by a com- 

 bination of displacement and dissociation so that a continuous spectrum 

 is observed. It is obvious that a discontinuous spectrum with many 

 discrete lines will appear as a continuous spectrum when viewed with a 

 spectrograph of low resolving power. Oftentimes spectral gratings of 

 large size are necessary to resolve the spectrum into its fine lines, but in a 

 truly continuous spectrum, involving this disruption of the molecule 

 into fragments, no amount of resolution will rev(>al fine structure in the 

 spectrum. 



