Relaxation-Resolved ESR Detected by the Spin-Echo Method

5 Relaxation-Resolved ESR Detected by the Spin-Echo Method 

Although the entire ESR spectrum of radical species in solids cannot be obtained by the Fourier transform of the ESE signal because of a large spectral width, it can be obtained by monitoring the ESE intensity while sweeping the external magnetic field very slowly. The shape of the ESR spectrum thus obtained is not necessarily the same as that of the cw ESR spectrum. The ESE method detects only the radical species which is not phase-relaxed at the time of detection, so that the radical species with very fast phase relaxation rate cannot be observed by the ESE-detected ESR. 

A typical example is seen for 1-hydroxyethyl radical trapped in a y-irra-diated ethanol matrix at 77 K [16]. As is shown in Fig. 2, the cw ESR spectrum of the radical is composed of five lines due to hyperfine interactions with one a proton and three P protons of a methyl group. The hyperfine interaction depends on the location of the P protons with respect to the p. orbital of the unpaired electron. However, the observed hyperfine coupling constant is the same for all the p protons because of the quick rotation of the methyl group in the time scale of the cw ESR measurement. On the other hand, the ESE-detected ESR spectrum is composed of four lines due to the hyperfine interactions with 

Since the ESE-detected ESR measures an ESR spectrum as a function of t2 and ti, we can observe the ESR spectrum as a function of the longitudinal and the phase relaxation degrees. We call this method relaxation-resolved ESR. The relaxation resolved ESR can be used for differentiation of overlapped ESR spectra from each other, when the relaxation rates of the coexisting radical species are not the same. This provides us a powerful means for investigating the nature and behavior of radicals in polymers, as described in Sect. 4. 

---