Microwave Induced Optical Nuclear Polarization at 75 GHz: A quantitative analysis

Verheij, P.F.A.; Wenckebach, W.Th.; Schmidt, Jan

doi:10.1007/bf03162521

Cited by 7 publications

(4 citation statements)

References 17 publications

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“…In the second mechanism (at higher fields), DNP is limited by the poor efficiency of electron spin-lattice relaxation through phonon scattering (sometimes called a phonon bottleneck). Since our experiments are performed on glycerol/water glasses, rather than single crystals, we expect the phonon lifetime to be on the order of 100 ps [56], rather than the 0.14 µs estimated from the single-crystal experiments [54]. The shorter phonon lifetime makes it unlikely that a phonon bottleneck limits DNP enhancements in our experiments.…”

Section: Discussionmentioning

confidence: 99%

Low-temperature dynamic nuclear polarization at 9.4 T with a 30 mW microwave source

Thurber

Yau

Tycko

2010

Journal of Magnetic Resonance

167

191

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Dynamic nuclear polarization (DNP) can provide large signal enhancements in nuclear magnetic resonance (NMR) by transfer of polarization from electron spins to nuclear spins. We discuss several aspects of DNP experiments at 9.4 Tesla (400 MHz resonant frequency for 1 H, 264 GHz for electron spins in organic radicals) in the 7-80 K temperature range, using a 30 mW, frequency-tunable microwave source and a quasi-optical microwave bridge for polarization control and low-loss microwave transmission. In experiments on frozen glycerol/water doped with nitroxide radicals, DNP signal enhancements up to a factor of 80 are observed (relative to 1 H NMR signals with thermal equilibrium spin polarization). The largest sensitivity enhancements are observed with a new triradical dopant, DOTOPA-TEMPO. Field modulation with a 10 G root-mean-squared amplitude during DNP increases the nuclear spin polarizations by up to 135%. Dependencies of 1 H NMR signal amplitudes, nuclear spin relaxation times, and DNP build-up times on the dopant and its concentration, temperature, microwave power, and modulation frequency are reported and discussed. The benefits of low-temperature DNP can be dramatic: the 1 H spin polarization is increased approximately 1000-fold at 7 K with DNP, relative to thermal polarization at 80 K.

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Section: Discussionmentioning

confidence: 99%

Low-temperature dynamic nuclear polarization at 9.4 T with a 30 mW microwave source

Thurber

Yau

Tycko

2010

Journal of Magnetic Resonance

167

191

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“…This frozen core has also been observed Pr 3+ -doped LaF 3 in optically-detected ESR experiments 42,43 , and in ruby 44 . Combined microwave and optical techniques have been used to analyze the barrier in fluorene-h10 doped with fluorene-d10 45 . Coherent neutron scattering experiments have also been used to probe polarized nuclei close to paramagnetic impurities 46,47 , yielding an estimate of about 1nm for the spin-diffusion barrier.…”

Section: Furman and Gorenmentioning

confidence: 99%

Section: Transport Through the Spin Diffusion Barriermentioning

confidence: 99%

Dynamic Nuclear Polarization and Spin Diffusion in Nonconducting Solids

Ramanathan

2008

Appl Magn Reson

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There has been much renewed interest in dynamic nuclear polarization (DNP), particularly in the context of solid state biomolecular NMR and more recently dissolution DNP techniques for liquids. This paper reviews the role of spin diffusion in polarizing nuclear spins and discusses the role of the spin diffusion barrier, before going on to discuss some recent results. PACS numbers:Microwave irradiation of a coupled electron-nuclear spin system can facilitate a transfer of polarization from the electron to the nuclear spin. In dielectric materials dynamic nuclear polarization (DNP) typically occurs via the solid effect,thermal mixing or the cross effect 1,2,3 . In these systems the electron spins are localized and the non-equilibrium polarization of the bulk nuclei is generated via a two-stage process: a polarization exchange local to the defect; and spin transport to distribute the polarization throughout the sample. Here the DNP process is essentially the inverse of the standard T 1 relaxation mechanism in dielectric solids 4 . While much attention is paid to improving the local polarization transfer efficiency from the electron to the neighboring nuclear spins, usually by incorporating the appropriate electron spins in the sample, the ratelimiting step in efficiently polarizing bulk samples is frequently the spin transport from the defect sites to the bulk. As we attempt to use DNP to enhance nuclear magnetization in an ever-increasing number of systems, it is important to understand the many-spin dynamics that underly the process. As we improve our system model for describing DNP dynamics, we will eventually be able to incorporate recent developments in the theory and practice of optimal control of quantum systems to further improve our techniques 5,6 . The purpose of this paper is to clarify our understanding of the spin dynamics in light of recent experiments in our laboratory. The paper begins with a description of the Bloembergen model and examines the different steps of the transport processes involved in dynamic nuclear polarization, before concluding with a review of recent experimental results.In a strong magnetic field, the Hamiltonian of a nuclear spin system, doped with electron spins is given bycorresponding to the nuclear and electron Zeeman interactions, the nuclear-nuclear and electron-electron dipo- * Electronic address: sekhar@mit.edu lar interactions, the electron exchange interaction and the electron-nuclear hyperfine interaction (including both the Fermi contact and the electron-nuclear dipolar interactions). During the DNP process we could add both microwave and RF fields to irradiate the electronic and nuclear spins as needed. The challenge of dealing with this full Hamiltonian in a systematic quantum mechanical formalism underlies the gaps in our knowledge of the full DNP process. Traditionally researchers have adopted a composite quantum-classical approach in which an isolated defect spin coupled to one or two nuclear spins is treated quantum-mechanically, and a classical approach is us...

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“…These nuclei are ''detuned'' from free bulk nuclei preventing the polarization from dispersing via 1 H-1 H spin diffusion. 23,24 As a consequence, it is necessary to polarize weakly coupled nuclei, leading to very weak transition probabilities for the solid effect ͓Eq. ͑4͔͒.…”

Section: A Laboratory Frame Solid Effect "Lfse…mentioning

confidence: 99%

Solid effect in the electron spin dressed state: A new approach for dynamic nuclear polarization

Weis

Bennati

Rosay

et al. 2000

The Journal of Chemical Physics

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We describe a new type of solid effect for dynamic nuclear polarization (DNP) that is based on simultaneous, near resonant microwave (mw) and radio frequency (rf) irradiation of a coupled electron nuclear spin system. The interaction of the electron spin with the mw field is treated as an electron spin dressed state. In contrast to the customary laboratory frame solid effect, it is possible to obtain nuclear polarization with the dressed state solid effect (DSSE) even in the absence of nonsecular hyperfine coupling. Efficient, selective excitation of dressed state transitions generates nuclear polarization in the nuclear laboratory frame on a time scale of tens of μs, depending on the strength of the electron–nuclear coupling, the mw and rf offset and field strength. The experiment employs both pulsed mw and rf irradiation at a repetition rate comparable to T1e−1, where T1e is the electronic spin lattice relaxation time. The DSSE is demonstrated on a perdeuterated BDPA radical in a protonated matrix of polystyrene.

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Microwave Induced Optical Nuclear Polarization at 75 GHz: A quantitative analysis

Cited by 7 publications

References 17 publications

Low-temperature dynamic nuclear polarization at 9.4 T with a 30 mW microwave source

Low-temperature dynamic nuclear polarization at 9.4 T with a 30 mW microwave source

Dynamic Nuclear Polarization and Spin Diffusion in Nonconducting Solids

Solid effect in the electron spin dressed state: A new approach for dynamic nuclear polarization

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