High-Field DNP and ENDOR with a Novel Multiple-Frequency Resonance Structure V. Weis, M. Bennati, M. Rosay, J. A. Bryant, and R. G. Grifﬁn 1 MIT/Harvard Center for Magnetic Resonance, Francis Bitter Magnet Laboratory and Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 Received May 5, 1999 We describe a new triply tuned (e  , 1 H, and 13 C) resonance structure operating at an electron Larmorfrequency of 139.5 GHz for dynamic nuclear polarization (DNP) and electron nuclear double-resonance (ENDOR) experiments. In contrast to conven- tional double-resonance structures, the body of the microwave cavity simultaneously acts as a NMR coil, allowing for increased efﬁciency of radiofrequency irradiation while maintaining a high quality factor for microwave irradiation. The resonator design is ideal for low--nuclei ENDOR, where sensitivity is limited by the fact that electron spin relaxation times are on the orderof the RF pulse lengths. The performance is demonstrated with 2 H ENDOR on a standard perdeuterated bis-diphenylene-phenyl-allyl stable radical. In DNP experiments, we show that the use of this reso- nator, combined with a low microwave power setup (17 mW), leads to signiﬁcantly higher 1 H signal enhancement (  400  50) than previously achieved at 5-T ﬁelds. The results emphasize the importance of optimizing the microwave B 1 ﬁeld by improving either the quality factor of the microwave resonator or the micro- wave power level. © 1999 Academic Press INTRODUCTION Electron paramagnetic/nuclear magnetic resonance (EPR/ NMR) multiple irradiation techniques are powerful spectro- scopic tools for enhancing sensitivity and spectral resolution in conventional NMR and EPR experiments. In a typical dynamic nuclear polarization (DNP) experiment, microwave irradiation is applied to the sample at or close to the electron Larmor frequency in order to transfer polarization from the electron to the nuclear spins. For all polarization mecha- nisms (1–6) the NMR signal enhancement increases with the microwave magnetic ﬁeld strength. Similarly, in pulsed electron nuclear double resonance (ENDOR) the NMR spec- trum is detected by monitoring either the electron spin echo or the electron free induction decay and therefore taking advantage of the high polarization of the electron spins (7, 8). Both double-resonance experiments require high mi- crowave as well as high RF ﬁeld strengths at the sample position for optimal performance. In the past decade, the general trend of performing double- resonance experiments (ENDOR and DNP) at increasingly higher ﬁelds has led to the implementation of pulse experi- ments at ﬁelds of 3–10 T (9 –14). Experimental limitations for the implementation of pulse techniques derive from the paucity of high-frequency, high-power microwave (W) sources. For DNP experiments at 140 GHz, we have successfully intro- duced a cyclotron resonance maser (15) that provides a high output power of 10 –100 W. Nevertheless, routine EPR/EN- DOR spectrometers are based on commercially available low- power W sources, such as GUNN and IMPATT diodes (70 mW output power), due to their easy operation, maintenance, and small size. Because of their lower power these sources require high-quality resonator designs in order to achieve rea- sonable pulse lengths ( t /2  200 ns) in pulsed EPR/ENDOR experiments. Several designs of RF transparent microwave resonance structures for double resonance have been used at X-band EPR frequencies (8 –10 GHz) (16 –21). Initially, slow wave helices were successfully used to increase the microwave magnetic ﬁeld at the sample position (16, 17). In special cases, it was found that helices can lead to higher spin sensitivity than resonant cavities, although the W quality factor is reduced by nearly an order of magnitude due to radiation losses of the open structure (20). An application of this particular design at high W frequencies is not possible for two reasons. First, the resonance frequencies of the helix are ﬁxed by its geometry and do not allow tuning to the microwave frequency of the source. Second, if 2a  / 2, where a is the radius of the helix and  the free space wavelength of the W, the helix no longer acts as a slow wave structure propagating only the fundamental mode. Instead, higher modes and radiative waves are excited without concentrating the electromagnetic energy inside the helix to produce intense microwave magnetic ﬁelds (see case (b) in Ref. (20)). 1 To whom correspondence should be addressed. E-mail: grifﬁn@ccnmr. mit.edu. Journal of Magnetic Resonance 140, 293–299 (1999) Article ID jmre.1999.1841, available online at http://www.idealibrary.com on 293 1090-7807/99 $30.00 Copyright © 1999 by Academic Press All rights of reproduction in any form reserved.