Echo Detected and Fourier Transform EPR
In Echo-Detected and FT EPR, microwave pulses are used to drive electronic transitions as the magnetic field is swept. This is in contrast to the more commonly used continuous wave EPR techniques.
In FT EPR, a π/2 pulse is used to transfer equilibrium magnetization to the xy plane.
A Fourier Transform of the resulting FID yields an EPR spectrum of the electron spins that were contained within the frequency bandwidth of the π/2 pulse. Due to the fact that most EPR samples exhibit strong inhomogeneous broadening that extends well past the bandwidth of typical microwave pulses and leads to a very rapid decay of the resulting FID, FT EPR is usually not possible. However, with some room temperature radical signals, the linewidth is narrow enough to excite an entire transition and the FID is long lived enough to extend past the ringdown of the microwave pulses, thus making FT EPR possible.
Echo Detected EPR
In Echo Detected EPR, a Hahn Echo sequence is used and the resulting echo intensity is monitored as a function of the magnetic field.
Echo detected EPR has properties that can make it desirable over conventional CW EPR in certain cases. Often, CW EPR is insensitive to very broad spectra due to the nature of field modulated data collection. Echo detected EPR does not suffer from the same problem and as such, very broad EPR spectra can be easily detected.
Echo Detected EPR Spectrum of a Nitroxide Spin Label