HYSCORE and DEER with an upgraded 95GHz pulse EPR spectrometer.

The set-up of a new microwave bridge for a 95 GHz pulse EPR spectrometer is described. The virtues of the bridge are its simple and flexible design and its relatively high output power (0.7 W) that generates pi pulses of 25 ns and a microwave field, B(1)=0.71 mT. Such a high B(1) enhances considerably the sensitivity of high field double electron-electron resonance (DEER) measurements for distance determination, as we demonstrate on a nitroxide biradical with an interspin distance of 3.6 nm. Moreover, it allowed us to carry out HYSCORE (hyperfine sublevel-correlation) experiments at 95 GHz, observing nuclear modulation frequencies of 14N and 17O as high as 40 MHz. This opens a new window for the observation of relatively large hyperfine couplings, yet not resolved in the EPR spectrum, that are difficult to observe with HYSCORE carried out at conventional X-band frequencies. The correlations provided by the HYSCORE spectra are most important for signal assignment, and the improved resolution due to the two dimensional character of the experiment provides 14N quadrupolar splittings.