ABSTRACT
Nonlinear EPR refers to CW-spectroscopy at radiation powers where the spectrum is appreciably saturated. The aim is to access spin–lattice relaxation, which extends the timescale of conventional spin-label EPR spectroscopies to longer times. The most familiar nonlinear technique is progressive-saturation of the conventional spectrum at increasing microwave powers. A variety of nonlinear techniques involves detecting the EPR signal 90°-out-of-phase with respect to the field modulation used for phase-sensitive detection. (In the absence of saturation, the out-of-phase signal has zero intensity.) Of these displays, the out-of-phase signal of the normal first-harmonic absorption is sensitive to spin–lattice relaxation alone, not to motion. The out-of-phase second harmonic absorption is the classic saturation-transfer spectrum; these lineshapes are sensitive to very slow rotational motion in the microsecond T1-regime. A valuable summary is given in the concluding section of the chapter.
