Pulsed high-frequency/high-field EPR

Abstract

Publisher Summary This chapter discusses some of the general ideas on continuous-wave high-field electron paramagnetic resonance (EPR) spectroscopy and the technical and the theoretical aspects specific (or different) for pulsed high-field EPR. There are some special features of the combination of high excitation frequency and pulsed excitation that might overcome some of the restrictions still limiting the applications of pulsed EPR at lower frequencies. The chapter discusses the theoretical aspects of the field dependence of the electron spin Hamiltonian—Zeeman interaction with anisotropic g matrix, nuclear hyperfine interactions, dipolar interactions, exchange interaction, and relaxation processes and mechanisms. The chapter also discusses the various applications of high-field/high-frequency EPR—measurements of principal g values on disordered samples, spin-polarized transient spectra of coupled radical pair systems, high-field EPR on Mn2+ centers in proteins, high-field electron spin-echo envelope modulation (ESEEM) nitrogen nuclei, and libration of molecules studied by pulse echo experiments. Generally, the EPR spectrum at usual X-band frequencies is dominated by inhomogeneous line broadening mechanisms that are not well defined or specific. This may be unresolved hyperfine couplings to many nuclei, distributions and distortions in surrounding, crystal strains, and so on. Pulsed experiments at these frequencies can rely on only the one-dimensional time domain information content to unravel and assign the responsible mechanism and processes.