Electron Spin Echoes and Spectral Diffusion in Organic Radical Solids

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Abstract
The echo decay envelopes of the two‐pulse and three‐pulse electron spin echoes have been investigated in radical cations, anions, and neutral radicals randomly oriented in rigid glass matrices. The shape of the two‐pulse echo decay envelopes indicates that the analytical form of the spectral diffusion function is approximately Lorentzian, whereas the three‐pulse echo decay envelopes imply that it is Gaussian in the wings. In most cases, at sufficiently low concentrations spin—spin flips and spin—lattice flips of the matrix nuclei are the dynamic processes responsible for the local field fluctuations which determine the phase memory times, whereas the local hyperfine fields resulting from the interactions with the radical nuclei are essentially static. Investigations of the spin—lattice relaxation using an echo technique show that the electron spin—lattice relaxation times of anions and cations with orbitally degenerate ground states are sufficiently smaller than those with nonorbitally degenerate ground states so that electron spin—lattice relaxation processes can in some cases shorten the phase memory times. Electron spin echospectroscopy experiments are also described in which the echo height is monitored as the magnetic field is swept.