Magic-angle sample spinning electron paramagnetic resonance - Instrumentation, performance, and limitations

An electron paramagnetic resonance (EPR) setup for line narrowing experimen ts with fast sample spinning at variable angles between the rotation axis a nd the static magnetic field is described and applied in the magic-angle sa mple spinning (MAS) EPR experiment at X-band frequencies (9.5 GHz). Sample spinning speeds up to 17 kHz at temperatures down to 200 K can be achieved with rotors of 4-mm outer and 2.5-mm inner diameter without severe losses i n microwave amplitude compared to standard pulse EPR probeheads. A phase cy cle is introduced that provides pure absorption MAS EPR spectra and allows one to distinguish between positive and negative frequency offsets (pseudoq uadrature detection), Possible broadening mechanisms in MAS EPR spectra are discussed. It is demonstrated both by theory and by experiment that the MA S EPR experiment requires excitation bandwidths that are comparable to the total spectral width, since otherwise destructive interference between cont ributions of spins with similar resonance offsets suppresses the signal. Ex perimental observations on the E-1'; center in gamma -irradiated silica gla ss and on the SO3- radical in gamma -irradiated sulfamic acid are reported. (C) 2000 Academic Press.