Two-dimensional field-step ELDOR. A method for characterizing the motion of spin probes and spin labels in glassy solids

A practical route for obtaining two-dimensional electron double-resonance spectra of radicals in disordered solids is presented in detail. It involves narrow-band pulse excitation during a magnetic field step combined with echo detection after a mixing time. The equipment and experimental procedures are described, and factors affecting the performance of the held-jump coil, spectral resolution, and sensitivity are thoroughly discussed. Simulated spectra, which take into account distributions of correlation times, show the spectral features that can be observed with this technique. These simulations have been improved over previous work by taking into account g-tensor fluctuations, which is the dominant effect in determining the anisotropy of the electron spin-lattice relaxation. Data for nitroxide radicals in polycarbonate at 110 K are analyzed and an orientation averaged nuclear spin-lattice relaxation time of 82 +/- 13 mu s and an electron spin-lattice relaxation time for radicals oriented along the z direction(slowest relaxation) of 23 +/- 4 mu s are measured. Simulations show that this relaxation is caused by highly restricted liberational motion with a distribution of correlation times having mean of 0.1 mu s and a width of about 0.8 decades in combination with a very narrow mode having a correlation time of 10 ps. (C) 1995 Academic Press, Inc.