ABSTRACT
Spin-echo EPR spectroscopy is a pulse technique that eliminates inhomogeneous broadening by refocusing the dephasing transverse magnetization to give an echo signal. For fast motion, the two-pulse primary echo sequence produces echo amplitudes that decay exponentially at the T2-relaxation time (more generally: the phase-memory time). Partially relaxed nitroxide spin-echo spectra are recorded by monitoring the two-pulse echo amplitude whilst sweeping the static field. When the spin label undergoes rapid, small-amplitude librational motion at low temperatures, the echo-detected spectra have anisotropic lineshapes that change in characteristic ways with echo delay time. The three-pulse stimulated echo produces echo amplitudes with decays that depend exponentially on the second interpulse delay period, which is determined by T1-relaxation and therefore sensitive to much slower motion than is the two-pulse echo. A valuable summary is given in the concluding section of the chapter. Appendices N and P at the end of the book are directly relevant to this chapter.
