Broadband Fourier-Transform-Detected EPR at W-Band

We report a true wideband Fourier transform (FT) EPR detection capability at the uniquely high frequency of 94 GHz (W-band). It is based on the quasi-optical HiPER spectrometer developed at the University of St. Andrews, into which we have integrated an arbitrary waveform generator (AWG) that is used to modulate the output from a solid-state multiplier chain prior to amplification, generating up to 1 kW microwave power with 1 GHz instantaneous bandwidth. Benchmark experiments are presented for a standard TEMPOL radical, which comprises a 500 MHz broad EPR spectrum at W-band. Using a single adiabatic chirp pulse, efficient inversion of this spectrum is demonstrated, enabling frequency-dependent studies of the longitudinal magnetization recovery in the time-domain, again via chirp pulse echo detection. From these measurements, an anisotropy in the spin–lattice relaxation time, T 1 , can be determined with ease for TEMPOL. In addition, we implement the FT detection scheme for multi-dimensional (electron–electron double resonance, or ELDOR) experiments, demonstrating the full capabilities of the HiPER spectrometer. As an example, we present a chirp pulse, FT-detected version of the ELDOR NMR technique for the TEMPOL radical.