Spin Resonance of the Conduction Electrons ESR

Making use of electron-spin resonance in the radical-ion salts, in particular the diffusion constant D and the hyperfine interaction constant A of the electron spins can be measured. One thus obtains independent information on the dynamics and the spatial distribution of the conduction electrons within the stacks. 

In contrast to the ESR spectra of radical ions in solution, all traces of hyperfine structure are thus missing in the ESR spectrum of these crystals. The reason for 

The long transverse relaxation times could be exploited to determine experimentally the difEision constant D of the charge carriers [36], [34]. For this measurement, Hahn sequences and other electron-spin-echo experiments were carried out in applied magnetic fields Bo, with a constant magnetic field gradient G superposed parallel to Bo. When the spins difiuse within the time interval 2r, i.e. the time between the first 90° pulse and the echo afier the 180° pulse at time r, they arrive at locations where the coherence of the spin precession is destroyed owing to the field gradient This leads to an additional decay of the echo amplitude A(r)  

With the value of the Fermi velocity Vp from Table 9.3, we obtain from Dy an independent value of the scattering time ry = Dy/v = 1.3 lO s. It is to be sure somewhat different from the values determined from the optical reflection spectra and from the conductivity (Table 9.4), but the differences are negUgible considering the different counterions and the very diverse methods and approximations in the data analyses. 

The extremely narrow ESR Une (Fig. 9.23a) could be made use of to measure the hyperfine coupling of the conduction electronen to the nuclear spins [35] due to the polarisation of the nuclear spins in the appUed magnetic field Bq, the average hyperfine field of aU the nuclear spins is superposed onto the applied field and leads to a smaller shift Avpsii of the ESR resonance frequency vesr in comparison to the ESR resonance frequency without hyperfine coupling or without nuclear-spin polarisation  

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