Electron Spin Resonance in the Study of Radiation Damage 243 



electron with H, the potential energy of the electron is slightly greater for one 

 of the orientations than for the other. The difference in energy for the two 

 orientations is equal to the microwave quantum energy hv which will induce 

 the spin vector to flip over from one orientation to the other. The classical 

 Larmor precessional frequency of the electron spin vector about the direction 

 of Hh equal to the absorbed microwave frequency. Thus the precessing electron 

 is in tune with, or at resonance with, the microwave radiation. 



In normal organic matter about us, the electrons are all — or nearly all — 

 in the lowest orbital levels, with the maximum limit of two electrons in each 

 molecular orbital. According to the Pauli principle, two electrons can share 

 an orbital only if their spins are aligned in an antiparallel manner. If it is 

 assumed that the spin vector of one electron flips over in an imposed field, 

 that of its orbital mate must flip in the opposite direction at the same time, 

 thus preventing any observable absorption or emission of radiation. To produce 

 an observable electron spin resonance in normal organic matter, one must 

 by some means lift electrons out of the completely filled orbitals of the ground 

 level. Strong ionizing quanta, such as those of x-rays, can eject electrons from 

 ground molecular orbitals with sufficient energy to free them entirely from the 

 parent molecule. If a molecule loses a single electron through ionizing irradia- 

 tion, the ionized molecule — if it holds together — will have a single unpaired 

 electron in one of its orbitals. This electron is now free to flip over in an 

 external field without the opposite flipping of a partner. The singly ionized 

 molecule is thus paramagnetic and can execute electron spin resonance. Further- 

 more, the electron which is knocked away from one molecule may become attached 

 to a neighboring molecule and thus convert it into a negatively charged radical. 

 Since the latter molecule is presumed to have all its bonding orbitals filled, 

 the new arrival must go into an orbital of higher energy and remain unpaired. 

 Thus resonance of electrons on negatively charged molecules might Hkewise 

 be detected. If the electron is ejected with sufficient energy it may, of course, 

 ionize several molecules before coming under the control of a particular molecule. 

 The end result is the same, however, except that a single quantum has, in 

 effect, been able to ionize more than one molecule. Two types of charged 

 radicals are thus produced. If the barrier to the return passage of the electron 

 between the molecules is high, as is the case in most organic solids, a sufficiently 

 high concentration of charged radicals can be built up in this way to give a 

 detectable electron spin resonance. The molecules may be small ones such 

 as the amino acids or long-chain macromolecules such as the proteins or 

 nucleic acids. The only requirement is that the separated electrons cannot 

 easily become paired again, i.e. that the radicals produced by ionizing radiation 

 have a lifetime sufficiently long for a detectable quantity to be built up. 



II. NATURE OF INFORMATION CONTENT IN 

 ELECTRON SPIN RESONANCE 



If the spin of an odd electron of a radical were entirely free from perturbing 

 influence of its environment, its resonance would be a single, sharp, isotropic 

 line with a g factor of 2.0023. Not much information is contained in such 

 a simple signal, although one could measure the lifetime of the radical from 



