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FT EPR and Spin Echo Techniques
The basic signals generated in pulse-EPR are the Free Induction Decay (FID) and the Electron Spin Echo (ESE). Both signals can be used to detect the EPR spectrum. If the EPR spectrum is narrow enough (less than 100 G) it can be fully excited by the microwave pulses. In these cases the Fourier transform of the FID or ESE yields the EPR spectrum. It is, however, often the case in EPR that the spectrum is too broad to be fully excited. In this situation the spectrum is recorded by monitoring the FID/ESE as a function of the external magnetic field. Any spectrum that is in the range of the magnetic field can be detected in this way.
- Nitroxide spin label spectrum obtained by FFT of the FID
- ESE detected field sweep spectrum of a copper complex.
- A typical inversion recovery experiment. The magnetization is inverted by a pulse and the recovery to thermal equilibrium is monitor by a spin echo.
Relaxation times provide insight into molecular motion, phase transitions and distances between paramagnetic centers. The FID and ESE are the prime detectors in relaxation measurements. A 2-pulse echo decay, for example, is used to determine the spin-spin relaxation time (T2), a property which gives information on molecular motion. To measure the spin-lattice relaxation time (T1) the spin system can be inverted or saturated. At a certain time-delay after inversion/saturation an FID or ESE then monitors the state of the magnetization.
