16 RADIATION niOLOGY 



oitluM- an electron or a proton. An important special case of this type is 

 the photocondnctivity of ionic crystals (AFott and CJnrney, 1940). 

 Althouj^h this latter phenomenon is of vital importance in determininji; the 

 photochemical and optical properties of crystal phosphors, it appears to be 

 too specialized for this discussion. 



When certain compounds dissolved in glassy media at low temperatures 

 are illuminated with ultraviolet light, semi(iuinones are formed hy the 

 ejection of an electron from the absorbing mcjlecule (Lewis and Bigeleisen, 

 1943). In these low-temperature rigid solutions the return of the elec- 

 tron is greatly retarded, and the spectrum of the resulting semi(iuinone 

 can be directly measured. In some instances the semi(iuinone is stabi- 

 lized by the thermal loss of a proton to the surroundings. A number of 

 organic compounds (all of which contain basic nitrogen, sulfur, or o.xygen) 

 exhibit this property. There can be no reasonable doubt ihat a similar 

 photoionization can occur in aqueous solutions. The near-ultraviolet 

 absorption of ferrous ion is very probably due to the transfer of an elec- 

 tron from the central ion to the surrounding shell of water molecules 

 (Zimmerman, 1949). Unless this excited system is stabilized by a 

 secondary reaction, such as the elimination of elementary hydrogen or the 

 reduction of an oxidizing agent, it will return after a short time to its 

 original state. 



The transfer of a proton from an absorbent molecule to a solvent mole- 

 cule or between the two components of a molecular complex has been 

 demonstrated by three independent methods. If aqueous solutions of 

 organic acids or bases are exposed to ultraviolet radiation which does not 

 decompose them, it has been reported (Terent'ev, 1949) that the pH of 

 the solution changes reversibly. In the majority of the cases studied, 

 the pH increased upon illumination by a few tenths of a unit. This sug- 

 gests that the excited molecules are weaker acids than the corresponding 

 normal molecules. 



More detailed and systematic measurements were made by Forster 

 (1950) on solutions of hydroxy and amino derivatives of sulfonated 

 pyrenes. These compounds display either one or the other of two differ- 

 ent absorption spectra and of two different fluorescence spectra, depending 

 on the pH of the solution. The shifts in the emission and absorption 

 spectra do not occur in the same pH range. From an analysis of the.s(^ 

 spectral shifts the ionization constants of the normal and excited mole- 

 cules can be determined. The changes in the ionization constants, due to 

 electronic excitation, are surprisingly great. For example, the ionization 

 constant of hydroxy trisulfonated pyrene is increased by a factor of 10" 

 upon excitation. Since the acid-base equilibrium is realized during the 

 lifetime of the excited (fluorescent) state, this photochemically induced 

 ionization must be classed as a primary step. It does not involve internal 

 conversion. 



