Q-Band Pulsed Electron Spin-Echo Spectrometer and Its Application to ENDOR and ESEEM

Abstract A “Q”-band (ν 0 ∼ 35 GHz; actually, K a band) homodyne electron spin-echo (ESE) spectrometer has been constructed and is described in the first section of this report. The second reformulates the expressions describing the performance of an ESE spectrometer so that they can better apply to the samples of interest in our laboratory, frozen-solution metalloproteins whose broad EPR envelopes are determined by g anisotropy. Some issues that play a central role in determining ESE performance, most especially for pulsed ENDOR, yet have not previously been described and are not quantifiable in straightforward ways, are also discussed. The third section presents Mims and Davies pulsed ENDOR and ESEEM applications and includes comparisons with a previously constructed X-band ESE spectrometer. When the same sample tube is used in both spectrometers (2.5 mm O.D. thin-walled quartz), for the applications discussed, the Q-band spectrometer exhibits sensitivity enhancements over X band that vary in understandable ways from about 10- to 3-fold, depending on the experiment.