  Electron paramagnetic resonance (EPR) spectroscopy interrogates unpaired
electron spins in solids and liquids to reveal local structure and dynamics;
for example, EPR has elucidated parts of the structure of protein complexes
that have resisted all other techniques in structural biology. EPR can also
probe the interplay of light and electricity in organic solar cells and
light-emitting diodes, and the origin of decoherence in condensed matter, which
is of fundamental importance to the development of quantum information
processors. Like nuclear magnetic resonance (NMR), EPR spectroscopy becomes
more powerful at high magnetic fields and frequencies, and with excitation by
coherent pulses rather than continuous waves. However, the difficulty of
generating sequences of powerful pulses at frequencies above 100 GHz has, until
now, confined high-power pulsed EPR to magnetic fields of 3.5 T and below. Here
we demonstrate that ~1 kW pulses from a free-electron laser (FEL) can power a
pulsed EPR spectrometer at 240 GHz (8.5 T), providing transformative
enhancements over the alternative, a state-of-the-art ~30 mW solid state
source. Using the UC Santa Barbara FEL as a source, our 240 GHz spectrometer
can rotate spin-1/2 electrons through pi/2 in only 6 ns (vs. 300 ns with the
solid state source). Fourier transform EPR on nitrogen impurities in diamond
demonstrates excitation and detection of EPR lines separated by ~200 MHz.
Decoherence times for spin-1/2 systems as short as 63 ns are measured, enabling
measurement of the decoherence time in a frozen solution of nitroxide
free-radicals at temperatures as high as 190 K. Both FELs and the quasi-optical
technology developed for the spectrometer are scalable to frequencies well in
excess of 1 THz, opening the possibility of high-power pulsed EPR spectroscopy
up to the highest static magnetic fields on earth.
