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Pulsed electron paramagnetic resonance spectroscopy powered by a free-electron laser

Nature volume 489, pages 409–413 (2012)Cite this article

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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 reveal1,2,3,4. EPR can also probe the interplay of light and electricity in organic solar cells5,6,7 and light-emitting diodes8, and the origin of decoherence in condensed matter, which is of fundamental importance to the development of quantum information processors9,10,11,12,13. 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.

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Figure 1: Towards high-frequency, high-power pulsed EPR spectroscopy.
Figure 2: Rabi oscillation measurements with BDPA.
Figure 3: Fourier-transform EPR measurements with diamond.
Figure 4: Hahn echo measurements with TEMPO.

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Acknowledgements

This work was supported by the NSF (CHE-0821589, DMR-0520481 and DMR-0703925) and the W. M. Keck Foundation. We thank D. Enyeart, S. El Abbadi, N. Krauss, K. Akabori, M. Anholm, T. Visher, J. Bricker and G. Kontsevich for support of the development and operation of the FEL.

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Authors and Affiliations

  1. Department of Chemistry, University of Southern California, Los Angeles, 90089, California, USA

    S. Takahashi

  2. Institute for Terahertz Science and Technology, University of California, Santa Barbara, 93106, California, USA

    S. Takahashi, L.-C. Brunel, G. Ramian, S. Han & M. S. Sherwin

  3. Department of Physics, University of California, Santa Barbara, 93106, California, USA

    D. T. Edwards & M. S. Sherwin

  4. National High Magnetic Field Laboratory, Florida State University, Tallahassee, 32310, Florida, USA

    J. van Tol

  5. Department of Chemistry and Biochemistry, University of California, Santa Barbara, 93106, California, USA

    S. Han

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Contributions

S.T. and M.S.S. contributed to the writing of the manuscript. M.S.S., S.H., L.-C.B. and J.v.T. conceived the development of the FEL-powered EPR spectrometer. The development was carried out by S.T., D.T.E., G.R., L.-C.B., S.H. and M.S.S. S.T., D.T.E., J.v.T. and M.S.S. conceived the EPR experiments. The measurements were carried out by S.T., D.T.E. and L.-C.B.

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Correspondence to M. S. Sherwin.

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The authors declare no competing financial interests.

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Takahashi, S., Brunel, LC., Edwards, D. et al. Pulsed electron paramagnetic resonance spectroscopy powered by a free-electron laser. Nature 489, 409–413 (2012). https://doi.org/10.1038/nature11437

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