ABSTRACT
In principle electron spin resonance may be used to examine any system possessing unpaired electrons including atoms, free radicals, radical ions and transition metal salts. This chapter considers the effect of placing the system of electrons in a uniform magnetic field. In order to achieve detectable absorption or emission, there must be a net population difference between the two energy states since the transition probabilities for stimulated emission and absorption are equal. The chapter discusses homogeneous broadening arises directly from the relaxation processes. There are two types of relaxation studies which are performed: spin label and spin probe experiments. The use of spin labels and spin probes, which are sensitive to the environment of the radicals, provide unique information on the structure and dynamics of polymers in which they are located. Interaction of the nuclear spin magnetic moment with the electron spin results in additional splitting of the electron energy levels.
