title: Pulsed electron paramagnetic resonance spectroscopy powered by a free-electron laser.
abstract: 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 other techniques in structural biology have not been able to reveal. 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, 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 gigahertz has, until now, confined high-power pulsed EPR to magnetic fields of 3.5 teslas and below. Here we demonstrate that one-kilowatt pulses from a free-electron laser can power a pulsed EPR spectrometer at 240 gigahertz (8.5 teslas), providing transformative enhancements over the alternative, a state-of-the-art ∼30-milliwatt solid-state source. Our spectrometer can rotate spin-1/2 electrons through π/2 in only 6 nanoseconds (compared to 300 nanoseconds with the solid-state source). Fourier-transform EPR on nitrogen impurities in diamond demonstrates excitation and detection of EPR lines separated by about 200 megahertz. We measured decoherence times as short as 63 nanoseconds, in a frozen solution of nitroxide free-radicals at temperatures as high as 190 kelvin. Both free-electron lasers and the quasi-optical technology developed for the spectrometer are scalable to frequencies well in excess of one terahertz, opening the way to high-power pulsed EPR spectroscopy up to the highest static magnetic fields currently available.
authors.affiliation: Department of Chemistry, University of Southern California, Los Angeles, California 90089, USA.
authors.forename: S
authors.initials: S
authors.lastname: Takahashi
authors.forename: L-C
authors.initials: LC
authors.lastname: Brunel
authors.forename: D T
authors.initials: DT
authors.lastname: Edwards
authors.forename: J
authors.initials: J
authors.lastname: van Tol
authors.forename: G
authors.initials: G
authors.lastname: Ramian
authors.forename: S
authors.initials: S
authors.lastname: Han
authors.forename: M S
authors.initials: MS
authors.lastname: Sherwin
mesh_terms: D000498:Allyl Compounds / Q000737:chemistry; D001554:Benzene / Q000737:chemistry; D003497:Cyclic N-Oxides / Q000737:chemistry; D018130:Diamond / Q000737:chemistry; D004578:Electron Spin Resonance Spectroscopy / Q000295:instrumentation* / Q000379:methods*; D004583:Electrons; D005583:Fourier Analysis; D005609:Free Radicals / Q000737:chemistry; D007834:Lasers; D009589:Nitrogen Oxides / Q000737:chemistry; D013696:Temperature; D013997:Time Factors
chemical_list: D000498:Allyl Compounds; D003497:Cyclic N-Oxides; D005609:Free Radicals; D009589:Nitrogen Oxides; C416323:bis-diphenylene-phenyl-allyl; D018130:Diamond; C039900:nitroxyl; D001554:Benzene; C003959:TEMPO