2-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-resona nce spectra of radicals in disordered solids is presented in detail. I t involves narrow-band pulse excitation during a magnetic field step c ombined with echo detection after a mixing time. The equipment and exp erimental procedures are described, and factors affecting the performa nce of the held-jump coil, spectral resolution, and sensitivity are th oroughly discussed. Simulated spectra, which take into account distrib utions of correlation times, show the spectral features that can be ob served with this technique. These simulations have been improved over previous work by taking into account g-tensor fluctuations, which is t he dominant effect in determining the anisotropy of the electron spin- lattice relaxation. Data for nitroxide radicals in polycarbonate at 11 0 K are analyzed and an orientation averaged nuclear spin-lattice rela xation time of 82 +/- 13 mu s and an electron spin-lattice relaxation time for radicals oriented along the z direction(slowest relaxation) o f 23 +/- 4 mu s are measured. Simulations show that this relaxation is caused by highly restricted liberational motion with a distribution o f correlation times having mean of 0.1 mu s and a width of about 0.8 d ecades in combination with a very narrow mode having a correlation tim e of 10 ps. (C) 1995 Academic Press, Inc.