S-band (2-4 GHz) pulse electron paramagnetic resonance spectrometer: Construction, probe head design, and performance

The design and construction of a combined pulse/continuous wave electron pa ramagnetic resonance (EPR) spectrometer operating in the 2-4 GHz frequency range (S band) are presented. Details of the microwave bridge are described including a Q-tune arm for adjustments of the microwave coupling during hi gh-power pulse excitation. Factors affecting the microwave resonator constr uction and the frequency and Q tuning are discussed. The distribution of th e electromagnetic field in a bridged loop-gap resonator with a large sample volume for EPR and electron-nuclear double resonance experiments and exper iments with B-0-field steps are determined using numerical calculations. On e- and two-dimensional pulse echo experiments are presented and compared wi th the results obtained with a pulse X-band (8-10 GHz) spectrometer. For we akly coupled nuclei the echo modulation depth increases by about one order of magnitude when lowering the microwave frequency from X band to S band. ( C) 2000 American Institute of Physics. [S0034-6748(00)04207-6].