360 LIGHT AND LIVE 



In a biradical in which two unpaired electrons occur at distances 

 greater than 4 A, the electrons may be regarded as essentially inde- 

 pendent and each of them will give rise to a separate ESR signal. 

 If the two unpaired electrons occur in similar atomic environments 

 (with respect to paramagnetic nuclei) , they may yield identical ESR 

 signals which superimpose; such a signal is indistinguishable from 

 that of an ordinary free radical containing only one unpaired elec- 

 tron. If the two electrons are located in environments which include 

 different paramagnetic nuclei, they may give rise to ESR signals which 

 are distinguishable in g value, and possibly with respect to hyperfine 

 structure, but which are of equal intensity. 



c. Semiconductors 



In physical systems that are capable of conducting electrons (i.e., 

 metallic conductors and semiconductors of various types) , the mobile 

 electrons are unpaired and therefore give rise to an ESR signal. It 

 is characteristic of these systems that the conduction electrons are 

 essentially uncoupled to any other paramagnetic constituents and 

 therefore hyperfine structure is absent. Unfortunately, apart from 

 this negative characteristic (which is of course not unique) ESR sig- 

 nals due to semiconductors cannot be distinguished, on their face, 

 from signals resulting from a free radical. Localized regions of the 

 semiconductor structure (electron traps) may tend to bind an un- 

 paired electron at an energy level slightly below that of the con- 

 duction electron. Such an unpaired electron may become closely 

 associated with magnetic atomic nuclei so that the restiltant ESR 

 signal exhibits hyperfine structvne. Hence an ESR signal due to a 

 trapped impaired electron may be distinguished from the signal due 

 to a conduction electron by the occurrence of hyperfine structure. 

 However, this distinction is not unique, since ordinary free radicals 

 also exhibit both types of signal. 



It has been suggested by Calvin (3) that the light-induced ESR 

 signal observed in chloroplasts at temperatures that are so low as 

 to preclude ordinary chemical reactions must be due to semiconduc- 

 tors. This conclusion is not required by such data. Lewis and Lipkin 

 (9) have shown that in glassy solutions illuminated at liquid nitrogen 

 temperatures, the absorption of light may eject an unpaired electron 

 from an organic molecule, which is then in a free radical form as 

 long as the ejected electron remains trapped in the structure of the 

 surrounding medium. Such a system is not necessarily a semicon- 

 ductor but would give rise to an ESR signal when illuminated at 

 liquid nitrogen temperatures. 



