CaltechAUTHORS: Combined
https://feeds.library.caltech.edu/people/Weitekamp-D-P/combined.rss
A Caltech Library Repository Feedhttp://www.rssboard.org/rss-specificationpython-feedgenenThu, 23 May 2024 19:55:59 -0700Fourier Transform Multiple Quantum Nuclear Magnetic
Resonance
https://resolver.caltech.edu/CaltechAUTHORS:20120730-082639528
Year: 1978
DOI: 10.1039/FS9781300049
The excitation and detection of multiple quantum transitions in systems of coupled spins offers, among other advantages, an increase in resolution over single quantum n.m.r. since the number of lines decreases as the order of the transition increases. This paper reviews the motivation for detecting multiple quantum transitions by a Fourier transform experiment and describes an experimental approach to high resolution multiple quantum spectra in dipolar systems along with results on some protonated liquid crystal systems. A simple operator formalism for the essential features of the time development is presented and some applications in progress are discussed.https://resolver.caltech.edu/CaltechAUTHORS:20120730-082639528Selective Excitation of Multiple-Quantum Coherence in Nuclear Magnetic Resonance
https://resolver.caltech.edu/CaltechAUTHORS:WARprl79
Year: 1979
DOI: 10.1103/PhysRevLett.43.1791
Wideband selective n-quantum excitation in the NMR of coupled spins is demonstrated for the first time. By a combination of multiple pulse averaging and phase shifts ϕ a pure n-quantum excitation operator can be produced (n=2π / ϕ). This allows enhancement of normally weak n-quantum transitions. Selective excitation of the zero- and four-quantum transitions in benzene illustrates this approach. Extensions to selective absorption of only groups of n photons in other regimes of spectroscopy are straight-forward, in principle.https://resolver.caltech.edu/CaltechAUTHORS:WARprl79Theory of selective excitation of multiple-quantum transitions
https://resolver.caltech.edu/CaltechAUTHORS:WARjcp80
Year: 1980
DOI: 10.1063/1.440403
The question of whether a molecule can be made to absorb and emit photons only in groups of n is treated. Pulse sequences are introduced which in effect selectively induce the absorption of only groups of n photons. This causes only n-quantum transitions even when many other transitions might be resonant. The technique involves repeated phase shifts of 2pi/n in the radiation to build up the selected coherences and cancel all other coherences, and is applicable to a wide range of spectroscopic systems. Coherent averaging theory is extended to describe selective sequences and demonstrates that n-quantum selectivity is possible to arbitrarily high order in the average Hamiltonian expansion. High-order selectivity requires many phase shifts, however, and for this reason the residual nonselective effects of sequences which are selective to only a finite order are calculated. Selective sequences are applied to the multiple-quantum NMR of oriented molecules, where in combination with time reversal sequences they produce a much more efficient transfer of the population differences into selected coherences than is obtainable by normal wideband pumping. For example, the 10-quantum transition in a 10-spin system can be enhanced by more than four orders of magnitude. Experiments on selective excitaiton of the 4-quantum transitions in oriented benzene verify the expected enhancement.https://resolver.caltech.edu/CaltechAUTHORS:WARjcp80Determination of dipole coupling constants using heteronuclear multiple quantum NMR
https://resolver.caltech.edu/CaltechAUTHORS:WEIjcp82
Year: 1982
DOI: 10.1063/1.444180
The problem of extracting dipole couplings from a system of N spins I = 1/2 and one spin S by NMR techniques is analyzed. The resolution attainable using a variety of single quantum methods is reviewed. The theory of heteronuclear multiple quantum (HMQ) NMR is developed, with particular emphasis being placed on the superior resolution available in HMQ spectra. Several novel pulse sequences are introduced, including a two-step method for the excitation of HMQ coherence. Experiments on partially oriented [1−13C] benzene demonstrate the excitation of the necessary HMQ coherence and illustrate the calculation of relative line intensities. Spectra of high order HMQ coherence under several different effective Hamiltonians achievable by multiple pulse sequences are discussed. A new effective Hamiltonian, scalar heteronuclear recoupled interactions by multiple pulse (SHRIMP), achieved by the simultaneous irradiation of both spin species with the same multiple pulse sequence, is introduced. Experiments are described which allow heteronuclear couplings to be correlated with an S-spin spreading parameter in spectra free of inhomogeneous broadening.https://resolver.caltech.edu/CaltechAUTHORS:WEIjcp82Zero-Field Nuclear Magnetic Resonance
https://resolver.caltech.edu/CaltechAUTHORS:WEIprl83a
Year: 1983
DOI: 10.1103/PhysRevLett.50.1807
In polycrystalline samples, NMR "powder spectra" are broad and much structural information is lost as a result of the orientational disorder. In this Letter Fourier-transform NMR in zero magnetic field is described. With no preferred direction in space, all crystallites contribute equivalently and resolved dipolar splittings can be interpreted directly in terms of internuclear distances. This opens the possiblity of molecular structure determination without the need for single crystals or oriented samples.https://resolver.caltech.edu/CaltechAUTHORS:WEIprl83aDelayed four-wave-mixing spectroscopy in molecular crystals: A nonperturbative approach
https://resolver.caltech.edu/CaltechAUTHORS:WEIpra83
Year: 1983
DOI: 10.1103/PhysRevA.27.3089
The delayed or time-domain four-wave-mixing experiment is treated in the regime of intense near-resonant pulses. The interaction with the radiation during both pump and probe pulses is considered to all powers of the electric field amplitude. Analytical results are obtained for an effective four-level system. These include the dependence of the coherence amplitudes on the ratio of the pump-field intensities when there is a large vibrational discrepancy between ground and excited electronic states and a general solution for the unitary time development during the probe pulse. For the first time, delayed coherent anti-Stokes Raman scattering is detected from highly dilute (10-ppm) guest molecules. Illustrative examples are presented for the system of pentacene in benzoic acid at low temperature. Vibronic-free induction decay and the effect of field inhomogeneity across the beam profile are found to be essential for understanding the observed intensity and spectral distribution of the signal beam in the region of optimum pulse intensity.https://resolver.caltech.edu/CaltechAUTHORS:WEIpra83Total spin coherence transfer echo spectroscopy
https://resolver.caltech.edu/CaltechAUTHORS:GARjcp83
Year: 1983
DOI: 10.1063/1.445692
The sensitivity of multiple quantum NMR transitions to magnetic field inhomogeneity and the relative phases and amplitudes of multiple quantum lines are discussed. The technique of total spin coherence transfer echo spectroscopy (TSCTES) is described and experimentally demonstrated. The TSCTES method allows multiple quantum spectra to be obtained which are free of inhomogeneous magnet broadening, yet remain sensitive to spin–spin couplings and chemical shift differences. The method takes advantage of the properties of the total spin coherence, the unique transition between the extreme eigenstates of a coupled spin system. Experimental results are reported for partially oriented acetaldehyde and are analyzed in terms of irreducible tensor operators. Limitations on the method and extensions to heteronuclear spin systems are also discussed.https://resolver.caltech.edu/CaltechAUTHORS:GARjcp83Vibrational dephasing in molecular mixed crystals: A picosecond time domain CARS study of pentacene in naphthalene and benzoic acid
https://resolver.caltech.edu/CaltechAUTHORS:DUPjcp83
Year: 1983
DOI: 10.1063/1.445753
Multiresonant time-domain coherent anti-Stokes Raman scattering (CARS) experiments have been employed in a study of the decay of vibrational coherences of pentacene doped into naphthalene and benzoic acid. In all cases, the CARS decay is found to be exponential, which indicates that the electronic and vibronic inhomogeneities in this system are strongly correlated. The temperature dependence of vibrational dephasing shows no effect of coupling to the lowest-frequency librational mode of pentacene that is known to dominate electronic dephasing. This surprising result can be understood on basis of a dephasing model where rapid coherence exchange exists between a cold vibrational transition and a corresponding near-resonant librationally hot one. For the 767 cm^−1 vibrational transition, oscillations of the CARS signal as a function of delay are shown to arise from interference at the detector with a nearby naphthalene host signal. An inconsistency with a previously reported spontaneous Raman study is resolved by showing that the signal observed there is actually site-selected fluorescence.https://resolver.caltech.edu/CaltechAUTHORS:DUPjcp83Fourier transform pure nuclear quadrupole resonance by pulsed field cycling
https://resolver.caltech.edu/CaltechAUTHORS:BIEjcp84
Year: 1984
DOI: 10.1063/1.446915
We report the observation of Fourier transform pure NQR by pulsed field cycling. For deuterium, well resolved spectra are obtained with high sensitivity showing the low frequency nu0 lines and allowing assignments of quadrupole couplings and asymmetry parameters to inequivalent deuterons. The technique is ideally applicable to nuclei with low quadrupolar frequencies (e.g., 2D, 7Li, 11B, 27Al, 23Na, 14N) and makes possible high resolution structure determination in polycrystalline or disordered materials.https://resolver.caltech.edu/CaltechAUTHORS:BIEjcp84Nutation Sequences for Magnetic Resonance Imaging in Solids
https://resolver.caltech.edu/CaltechAUTHORS:CHOprl85
Year: 1985
DOI: 10.1103/PhysRevLett.55.1923
Novel radio-frequency NMR pulse sequences are presented and their application to imaging of solids with use of rf field gradients is discussed. The sequences cause a nuclear spin to precess about the static field direction at a rate proportional to the strength of certain of the pulses. This forced precession is independent of the resonance offset and of couplings to other spins. The pulse-sequence design is described by means of coherent averaging theory and is confirmed experimentally and numerically.https://resolver.caltech.edu/CaltechAUTHORS:CHOprl85Zero field NMR and NQR
https://resolver.caltech.edu/CaltechAUTHORS:ZAXjcp85
Year: 1985
DOI: 10.1063/1.449748
Methods are described and demonstrated for detecting the coherent evolution of nuclear spin observables in zero magnetic field with the full sensitivity of high field NMR. The principle motivation is to provide a means of obtaining solid state spectra of the magnetic dipole and electric quadrupole interactions of disordered systems without the line broadening associated with random orientation with respect to the applied magnetic field. Comparison is made to previous frequency domain and high field methods. A general density operator formalism is given for the experiments where the evolution period is initiated by a sudden switching to zero field and is terminated by a sudden restoration of the field. Analytical expressions for the signals are given for a variety of simple dipolar and quadrupolar systems and numerical simulations are reported for up to six coupled spin-1/2 nuclei. Experimental results are reported or reviewed for 1H, 2D, 7Li, 13C, and 27Al nuclei in a variety of polycrystalline materials. The effects of molecular motion and bodily sample rotation are described. Various extensions of the method are discussed, including demagnetized initial conditions and correlation by two-dimensional Fourier transformation of zero field spectra with themselves or with high field spectra.https://resolver.caltech.edu/CaltechAUTHORS:ZAXjcp85Transformation of Symmetrization Order to Nuclear-Spin Magnetization by Chemical Reaction and Nuclear Magnetic Resonance
https://resolver.caltech.edu/CaltechAUTHORS:BOWprl86
Year: 1986
DOI: 10.1103/PhysRevLett.57.2645
A method of obtaining very large nuclear-spin polarizations is proposed and illustrated by density-operator calculations. The prediction is that chemical reaction and rf irradiation can convert the scalar parahydrogen state into polarization of order unity on the nuclear spins of the products of molecular-hydrogen addition reactions. A means of extending the resultant sensitivity enhancement to other spins is proposed in which the transfer of order occurs through population differences not associated with magnetization.https://resolver.caltech.edu/CaltechAUTHORS:BOWprl86Chemical exchange and quantum exchange
https://resolver.caltech.edu/CaltechAUTHORS:20201104-193125384
Year: 1990
DOI: 10.1007/978-3-642-76072-3_94
The mutual chemical exchange rate has been derived with the rovibrational lattice treated quantum-mechanically. Our formulation contrasts sharply with previous descriptions of the role of tunnelling in NMR lineshapes.https://resolver.caltech.edu/CaltechAUTHORS:20201104-193125384Dihydrogen addition studies by the Pasadena effect
https://resolver.caltech.edu/CaltechAUTHORS:20210123-170330175
Year: 1990
DOI: 10.1007/978-3-642-76072-3_77
Several experiments and theoretical development of the PASADENA effect (parahydrogen and synthesis allow dramatically enchanced nuclear alignment) to solution and solid-state NMR are presented.https://resolver.caltech.edu/CaltechAUTHORS:20210123-170330175New approaches to ultrasensitive magnetic resonance
https://resolver.caltech.edu/CaltechAUTHORS:20181213-143630057
Year: 1991
DOI: 10.1117/12.44229
Spectroscopic methods tend to exhibit an inverse correlation between sensitivity and the ability to discriminate between similar structures. Were they obtainable with adequate sensitivity, magnetic resonance spectra could resolve structural controversies involving the nature of clusters, ions, semiconductor defects and catalytic intermediates. This paper describes several novel approaches to magnetic resonance, which have in common that the spins are coupled to other degrees of freedom in order to obtain nonequilibrium polarization and/or greater detection sensitivity. The methods under development include single-ion electron spin resonance (ESR) detected by ion trapping frequencies, catalyst NMR detected by the branching ratio to different spin symmetry species, and semiconductor nuclear magnetic resonance (NMR) detected via the circular polarization of luminescence.https://resolver.caltech.edu/CaltechAUTHORS:20181213-143630057Time-sequenced optical nuclear magnetic resonance of gallium arsenide
https://resolver.caltech.edu/CaltechAUTHORS:BURprb91
Year: 1991
DOI: 10.1103/PhysRevB.44.9035
A method of optical detection of nuclear magnetic resonance is demonstrated in which optical nuclear polarization, spin resonance, and optical detection are separated into distinct sequential periods and separately optimized by varying the optical, rf, and static fields. Experiments on the bulk 69Ga resonance of GaAs show that sites imperceptibly perturbed by the optically relevant defect are optically observable with the rf applied in the dark. A signal-to-noise analysis is given that relates the sensitivity to readily measured material properties and indicates applicability to dilute defects.https://resolver.caltech.edu/CaltechAUTHORS:BURprb91Can nuclear magnetic resonance resolve epitaxial layers?
https://resolver.caltech.edu/CaltechAUTHORS:BURjvstb92
Year: 1992
DOI: 10.1116/1.586233
The recently demonstrated technique of time-sequenced optical nuclear magnetic resonance in GaAs has made possible the detection of spectra free of the line shape distortions that accompanied earlier steady-state methods with an improvement in sensitivity as well. This work examines the possibility of even higher spectral resolution by means of selective averaging with radio frequency-optical multiple-pulse techniques with the aim of isolating the site-specific changes in the spin Hamiltonian associated with excitation to localized states of the conduction band, as in quantum wells. Simulations are presented to evaluate the approach proposed. It is concluded that such experiments are capable of the sensitivity and resolution to resolve individual epitaxial layers in high-quality structures and would provide unprecedented detail on the electronic structure and its uniformity by way of the nuclear quadrupole and spin-averaged hyperfine interactions.https://resolver.caltech.edu/CaltechAUTHORS:BURjvstb92Optical Larmor Beat Detection of High-Resolution Nuclear Magnetic Resonance in a Semiconductor Heterostructure
https://resolver.caltech.edu/CaltechAUTHORS:MARprl95
Year: 1995
DOI: 10.1103/PhysRevLett.75.1364
A new method of optical nuclear magnetic resonance, Larmor beat detection, is described and demonstrated on a III-V semiconductor heterostructure. Modulation of the circular polarization of luminescence at the difference between two nuclear spin precession frequencies is induced by rf pulses. One isotope provides a spin-locked reference field, while NMR transients of a second isotope are observed optically in real time. Order-of-magnitude improvement in resolution and sensitivity over previous techniques is obtained, revealing weak electric field gradients in single epitaxial structures.https://resolver.caltech.edu/CaltechAUTHORS:MARprl95Quantum Statistical Corrections to Dynamic Nuclear Magnetic Resonance
https://resolver.caltech.edu/CaltechAUTHORS:20141125-082827667
Year: 1999
DOI: 10.1126/science.283.5398.61
A quantum statistical treatment of the chemical exchange between molecular eigenstates or conformations revealed previously unsuspected dynamic terms in the spin Hamiltonian operator that describes fast exchange. These terms resulted from the effect of nuclear spin on rotational and vibrational relaxation. With the traditional theory, an interpretation of new carbon-13 nuclear magnetic resonance measurements of the chemical shift of methylcyclohexane in solution showed fast-exchange equilibrium constants that were inconsistent with the slow-exchange free-energy difference and were spread over a range of 30 percent for the various carbon-13 positions. Modeling of the new terms indicated that they have the correct magnitude and temperature dependence to reconcile these inconsistencies.https://resolver.caltech.edu/CaltechAUTHORS:20141125-082827667Method for atomic-layer-resolved measurement of polarization fields by nuclear magnetic resonance
https://resolver.caltech.edu/CaltechAUTHORS:KEMjvstb00
Year: 2000
DOI: 10.1116/1.1305287
A nuclear magnetic resonance (NMR) method of probing the dielectric response to an alternating electric field is described, which is applicable to noncentrosymmetric sites with nuclear spin I>1/2. A radio-frequency electric field induces a linear quadrupole Stark effect at a multiple of the nuclear Larmor frequency. This perturbation is applied in the windows of an NMR multiple-pulse line-narrowing sequence in such a way that the resulting nonsecular spin interactions are observed as first-order quadrupole satellites, free of line broadening by the usual dominant static interactions. A simulation of the 69Ga spectrum for the nuclei within the two-dimensional electron gas of a 10 nm quantum well predicts resolution of individual atomic layers in single devices due to the spatial dependence of the polarization response of the quantum-confined carriers to the applied field. This method is part of a more general strategy, perturbations observed with enhanced resolution NMR. Experimentally realized examples in GaAs include spectrally resolving electron probability densities surrounding optically relevant point defects and probing the changes in radial electric field associated with the light-on and light-off states of these shallow traps. Adequate sensitivity for such experiments in individual epitaxial structures is achieved by optical nuclear polarization followed by time-domain NMR observed via nuclear Larmor-beat detection of luminescence.https://resolver.caltech.edu/CaltechAUTHORS:KEMjvstb00The PASADENA Effect at a Solid Surface: High-Sensitivity Nuclear Magnetic Resonance of Hydrogen Chemisorption
https://resolver.caltech.edu/CaltechAUTHORS:20170406-074218760
Year: 2001
DOI: 10.1021/ja010572z
The use of parahydrogen as a high-sensitivity spin label for nuclear magnetic resonance (NMR) was first proposed and then demonstrated to provide orders-of-magnitude signal enhancement in species which are formed from the molecular addition of hydrogen. The phenomenon known as the PASADENA effect (parahydrogen and synthesis allow dramatically enhanced nuclear alignment) derives from the fact that deviation of the parahydrogen mole fraction in a sample of H_2 from its statistical high-temperature limit of 1/4 is associated with an inherent form of spin order. Upon molecular addition of the two protons from a single H_2 molecule into coupled, magnetically inequivalent environments, this order is manifested as large nonequilibrium spin population differences across allowed NMR transitions.https://resolver.caltech.edu/CaltechAUTHORS:20170406-074218760Observation of force-detected nuclear magnetic resonance in a homogeneous field
https://resolver.caltech.edu/CaltechAUTHORS:MADpnas04
Year: 2004
DOI: 10.1073/pnas.0405232101
PMCID: PMC516476
We report the experimental realization of BOOMERANG (better observation of magnetization, enhanced resolution, and no gradient), a sensitive and general method of magnetic resonance. The prototype millimeter-scale NMR spectrometer shows signal and noise levels in agreement with the design principles. We present H-1 and F-19 NMR in both solid and liquid samples, including time-domain Fourier transform NMR spectroscopy, multiple-pulse echoes, and heteronuclear J spectroscopy. By measuring a H-1-F-19 J coupling, this last experiment accomplishes chemically specific spectroscopy with force-detected NMR. In BOOMERANG, an assembly of permanent magnets provides a homogeneous field throughout the sample, while a harmonically suspended part of the assembly, a detector, is mechanically driven by spin-dependent forces. By placing the sample in a homogeneous field, signal dephasing by diffusion in a field gradient is made negligible, enabling application to liquids, in contrast to other force-detection methods. The design appears readily scalable to µm-scale samples where it should have sensitivity advantages over inductive detection with microcoils and where it holds great promise for application of magnetic resonance in biology, chemistry, physics, and surface science. We briefly discuss extensions of the BOOMERANG method to the µm and nm scales.https://resolver.caltech.edu/CaltechAUTHORS:MADpnas04PASADENA Hyperpolarization of Succinic Acid for MRI and NMR Spectroscopy
https://resolver.caltech.edu/CaltechAUTHORS:20170512-131816890
Year: 2008
DOI: 10.1021/ja7101218
PMCID: PMC2662769
We use the PASADENA (parahydrogen and synthesis allow dramatically enhanced nuclear alignment) method to achieve ^(13)C polarization of ∼20% in seconds in 1-^(13)C-succinic-d_2 acid. The high-field ^(13)C multiplets are observed as a function of pH, and the line broadening of C1 is pronounced in the region of the pK values. The ^2J_(CH), ^3J_(CH), and ^3J_(HH) couplings needed for spin order transfer vary with pH and are best resolved at low pH leading to our use of pH ∼3 for both the molecular addition of parahydrogen to 1-^(13)C-fumaric acid-d_2 and the subsequent transfer of spin order from the nascent protons to C1 of the succinic acid product. The methods described here may generalize to hyperpolarization of other carboxylic acids. The C1 spin−lattice relaxation time at neutral pH and 4.7 T is measured as 27 s in H_2O and 56 s in D_2O. Together with known rates of succinate uptake in kidneys, this allows an estimate of the prospects for the molecular spectroscopy of metabolism.https://resolver.caltech.edu/CaltechAUTHORS:20170512-131816890Fluorine-19 NMR Chemical Shift Probes Molecular Binding to Lipid Membranes
https://resolver.caltech.edu/CaltechAUTHORS:20170717-151917105
Year: 2008
DOI: 10.1021/jp800646k
The binding of amphiphilic molecules to lipid bilayers is followed by ^(19)F NMR using chemical shift and line shape differences between the solution and membrane-tethered states of −CF_3 and −CHF_2 groups. A chemical shift separation of 1.6 ppm combined with a high natural abundance and high sensitivity of ^(19)F nuclei offers an advantage of using ^(19)F NMR spectroscopy as an efficient tool for rapid time-resolved screening of pharmaceuticals for membrane binding. We illustrate the approach with molecules containing both fluorinated tails and an acrylate moiety, resolving the signals of molecules in solution from those bound to synthetic imyristoylphosphatidylcholine bilayers both with and without magic angle sample spinning. The potential in vitro and in vivo biomedical applications are outlined. The presented method is applicable with the conventional NMR equipment, magnetic fields of several Tesla, stationary samples, and natural abundance isotopes.https://resolver.caltech.edu/CaltechAUTHORS:20170717-151917105An optical NMR spectrometer for Larmor-beat detection and high-resolution POWER NMR
https://resolver.caltech.edu/CaltechAUTHORS:KEMrsi08
Year: 2008
DOI: 10.1063/1.2936257
Optical nuclear magnetic resonance (ONMR) is a powerful probe of electronic properties in III-V semiconductors. Larmor-beat detection (LBD) is a sensitivity optimized, time-domain NMR version of optical detection based on the Hanle effect. Combining LBD ONMR with the line-narrowing method of POWER (perturbations observed with enhanced resolution) NMR further enables atomically detailed views of local electronic features in III-Vs. POWER NMR spectra display the distribution of resonance shifts or line splittings introduced by a perturbation, such as optical excitation or application of an electric field, that is synchronized with a NMR multiple-pulse time-suspension sequence. Meanwhile, ONMR provides the requisite sensitivity and spatial selectivity to isolate local signals within macroscopic samples. Optical NMR, LBD, and the POWER method each introduce unique demands on instrumentation. Here, we detail the design and implementation of our system, including cryogenic, optical, and radio-frequency components. The result is a flexible, low-cost system with important applications in semiconductor electronics and spin physics. We also demonstrate the performance of our systems with high-resolution ONMR spectra of an epitaxial AlGaAs/GaAs heterojunction. NMR linewidths down to 4.1 Hz full width at half maximum were obtained, a 10^3-fold resolution enhancement relative any previous optically detected NMR experiment.https://resolver.caltech.edu/CaltechAUTHORS:KEMrsi08Imaging quantum confinement with optical and POWER (perturbations observed with enhanced resolution) NMR
https://resolver.caltech.edu/CaltechAUTHORS:KEMpnas08
Year: 2008
DOI: 10.1073/pnas.0806563106
PMCID: PMC2629273
The nanoscale distributions of electron density and electric fields in GaAs semiconductor devices are displayed with NMR experiments. The spectra are sensitive to the changes to the nuclear-spin Hamiltonian that are induced by perturbations delivered in synchrony with a line-narrowing pulse sequence. This POWER (perturbations observed with enhanced resolution) method enhanced resolution up to 103-fold, revealing the distribution of perturbations over nuclear sites. Combining this method with optical NMR, we imaged quantum-confined electron density in an individual AlGaAs/GaAs heterojunction via hyperfine shifts. Fits to the coherent evolution and relaxation of nuclei within a hydrogenic state established one-to-one correspondence of radial position to frequency. Further experiments displayed the distribution of photo-induced electric field within the same states via a quadrupolar Stark effect. These unprecedented high-resolution distributions discriminate between competing models for the luminescence and support an excitonic state, perturbed by the interface, as the dominant source of the magnetically modulated luminescence.https://resolver.caltech.edu/CaltechAUTHORS:KEMpnas08Hyperpolarized ^1H NMR employing low γ nucleus for spin polarization storage
https://resolver.caltech.edu/CaltechAUTHORS:20090814-132951758
Year: 2009
DOI: 10.1021/ja809634u
PMCID: PMC2662390
The PASADENA (parahydrogen and synthesis allow dramatically enhanced nuclear alignment)(1, 2) and DNP (Dynamic Nuclear Polarization)(3) methods efficiently hyperpolarize biologically relevant nuclei such as 1^H, (31)^P, (13)^C, (15)^N achieving signal enhancement by a factor of ~ 100000 on currently utilized MRI scanners. Recently, many groups have demonstrated the utility of hyperpolarized MR in biological systems using hyperpolarized (13)^C biomarkers with a relatively long spin lattice relaxation time T_1 on the order of tens of seconds.(4-7) Moreover, hyperpolarized (15)^N for biomedical MR has been proposed due to even longer spin lattice relaxations times.(8) An additional increase of up to tens of minutes in the lifetime of hyperpolarized agent in vivo could be achieved by using the singlet states of low gamma (γ) nuclei.(9) However, as NMR receptivity scales as γ^3 for spin 1/2 nuclei, direct NMR detection of low γ nuclei results in a lower signal-to-noise ratio compared to proton detection. While protons are better nuclei for detection, short spin lattice relaxation times prevent direct 1^H hyperpolarized MR in biomedical applications.https://resolver.caltech.edu/CaltechAUTHORS:20090814-132951758A selective ^(15)N-to-^1H polarization transfer sequence for more sensitive detection of ^(15)N-choline
https://resolver.caltech.edu/CaltechAUTHORS:20100715-093443660
Year: 2010
DOI: 10.1016/j.jmr.2010.04.010
The sensitivity and information content of heteronuclear nuclear magnetic resonance is frequently optimized by transferring spin order of spectroscopic interest to the isotope of highest detection sensitivity prior to observation. This strategy is extended to ^(15)N-choline using the scalar couplings to transfer polarization from ^(15)N to choline's nine methyl ^1H spins in high field. A theoretical analysis of a sequence using nonselective pulses shows that the optimal efficiency of this transfer is decreased by 62% as the result of competing ^(15)N–^1H couplings involving choline's four methylene protons. We have therefore incorporated a frequency-selective pulse to support evolution of only the ^(15)N–methyl ^1H coupling during the transfer period. This sequence provides a 52% sensitivity enhancement over the nonselective version in in vitro experiments on a sample of thermally polarized ^(15)N-choline in D_2O. Further, the ^(15)N T_1 of choline in D_2O was measured to be 217 ± 38 s, the ^(15)N–methyl ^1H coupling constant was found to be 0.817 ± 0.001 Hz, and the larger of choline's two ^(15)N–methylene ^1H coupling constants was found to be 3.64 ± 0.01 Hz. Possible improvements and applications to in vivo experiments using long-lived hyperpolarized heteronuclear spin order are discussed.https://resolver.caltech.edu/CaltechAUTHORS:20100715-093443660Nanoscale Torsional Resonator for Polarization and Spectroscopy of Nuclear Spins
https://resolver.caltech.edu/CaltechAUTHORS:20101110-095440773
Year: 2010
DOI: 10.1103/PhysRevLett.105.177601
We propose a torsional resonator that couples to the transverse spin dipole of an attached sample. The absence of relative motion eliminates a source of friction that would otherwise hinder nanoscale implementation. Enhanced spontaneous emission induced by the resonator relaxes the longitudinal spin dipole at a rate of ~1 s^(-1) in the low-temperature limit. With signal averaging, single-proton magnetic resonance spectroscopy appears feasible at ~10 mK and a high magnetic field, while single-shot sensitivity is practical for samples with at least tens of protons in a volume of ~5 nm^3.https://resolver.caltech.edu/CaltechAUTHORS:20101110-095440773Communication: Partial polarization transfer for single-scan
spectroscopy and imaging
https://resolver.caltech.edu/CaltechAUTHORS:20111108-072024737
Year: 2011
DOI: 10.1063/1.3652965
A method is presented to partially transfer nuclear spin polarization from one isotope S to another isotope I by the way of heteronuclear spin couplings, while minimizing the loss of spin order to other degrees of freedom. The desired I spin polarization to be detected is a design parameter, while the sequence of pulses at the two Larmor frequencies is optimized to store the greatest unused S spin longitudinal polarization for subsequent use. The unitary evolution for the case of I_NS spin systems illustrates the potentially ideal efficiency of this strategy, which is of particular interest when the spin-lattice relaxation time of S greatly exceeds that of I. Explicit timing and pulses are tabulated for the cases for which M ≤ 10 partial transfers each result in equal final polarization of 1/M or more compared to the final I polarization expected in a single transfer for N = 1, 2, or 3 I spins. Advantages for the ratiometric study of reacting molecules and hyperpolarized initial conditions are outlined.https://resolver.caltech.edu/CaltechAUTHORS:20111108-072024737Polarization of nuclear spins by a cold nanoscale resonator
https://resolver.caltech.edu/CaltechAUTHORS:20120105-140856686
Year: 2011
DOI: 10.1103/PhysRevA.84.063407
A cold nanoscale resonator coupled to a system of nuclear spins can induce spin relaxation. In the low-temperature limit where spin-lattice interactions are "frozen out," spontaneous emission by nuclear spins into a resonant mechanical mode can become the dominant mechanism for cooling the spins to thermal equilibrium with their environment. We provide a theoretical framework for the study of resonator-induced cooling of nuclear spins in this low-temperature regime. Relaxation equations are derived from first principles, in the limit where energy donated by the spins to the resonator is quickly dissipated into the cold bath that damps it. A physical interpretation of the processes contributing to spin polarization is given. For a system of spins that have identical couplings to the resonator, the interaction Hamiltonian conserves spin angular momentum, and the resonator cannot relax the spins to thermal equilibrium unless this symmetry is broken by the spin Hamiltonian. The mechanism by which such a spin system becomes "trapped" away from thermal equilibrium can be visualized using a semiclassical model, which shows how an indirect spin-spin interaction arises from the coupling of multiple spins to one resonator. The internal spin Hamiltonian can affect the polarization process in two ways: (1) By modifying the structure of the spin-spin correlations in the energy eigenstates, and (2) by splitting the degeneracy within a manifold of energy eigenstates, so that zero-frequency off-diagonal terms in the density matrix are converted to oscillating coherences. Shifting the frequencies of these coherences sufficiently far from zero suppresses the development of resonator-induced correlations within the manifold during polarization from a totally disordered state. Modification of the spin-spin correlations by means of either mechanism affects the strength of the fluctuating spin dipole that drives the resonator. In the case where product states can be chosen as energy eigenstates, spontaneous emission from eigenstate populations into the resonant mode can be interpreted as independent emission by individual spins, and the spins relax exponentially to thermal equilibrium if the development of resonator-induced correlations is suppressed. When the spin Hamiltonian includes a significant contribution from the homonuclear dipolar coupling, the energy eigenstates entail a correlation specific to the coupling network. Simulations of dipole-dipole coupled systems of up to five spins suggest that these systems contain weakly emitting eigenstates that can trap a fraction of the population for time periods ≫100/R_0, where R_0 is the rate constant for resonator-enhanced spontaneous emission by a single spin 1/2. Much of the polarization, however, relaxes with rates comparable to R_0. A distribution of characteristic high-field chemical shifts tends to increase the relaxation rates of weakly emitting states, enabling transitions to states that can quickly relax to thermal equilibrium. The theoretical framework presented in this paper is illustrated with discussions of spin polarization in the contexts of force-detected nuclear-magnetic-resonance spectroscopy and magnetic-resonance force microscopy.https://resolver.caltech.edu/CaltechAUTHORS:20120105-140856686Resonator-induced dissipation of transverse nuclear-spin signals in cold nanoscale samples
https://resolver.caltech.edu/CaltechAUTHORS:20120330-134535697
Year: 2012
DOI: 10.1103/PhysRevB.85.104405
The back action of typical macroscopic resonators used for detecting nuclear magnetic resonance can cause a reversible decay of the signal, known as radiation damping. A mechanical resonator that is strongly coupled to a microscopic sample can in addition induce an irreversible dissipation of the nuclear-spin signal, distinct from radiation damping. We provide a theoretical description of resonator-induced transverse relaxation that is valid for samples of a few nuclear spins in the low-temperature regime, where quantum fluctuations play a significant role in the relaxation process, as well as for larger samples and at higher temperatures. Transverse relaxation during free evolution and during spin locking are analyzed, and simulations of relaxation in example systems are presented. In the case where an isolated spin 1/2 interacts with the resonator, transverse relaxation is exponential during free evolution, and the time constant for the relaxation is T_2=2/R_h, where R_h is the rate constant governing the exchange of quanta between the resonator and the spin. For a system of multiple spins, the time scale of transverse relaxation during free evolution depends on the spin Hamiltonian, which can modify the relaxation process through the following effects: (1) changes in the structure of the spin-spin correlations present in the energy eigenstates, which affect the rates at which these states emit and absorb energy, (2) frequency shifts that modify emission and absorption rates within a degenerate manifold by splitting the energy degeneracy and thus suppressing the development of resonator-induced correlations within the manifold, and (3) frequency shifts that introduce a difference between the oscillation frequencies of single-quantum coherences ρ_(ab) and ρ_(cd) and average to zero the transfers between them. This averaging guarantees that the spin transitions responsible for the coupling between ρ_(ab) and ρ_(cd) cause irreversible loss of order rather than a reversible interconversion of the coherences. In systems of a few spins, transverse relaxation is accelerated by a dipolar Hamiltonian that is either the dominant term in the internal spin Hamiltonian or a weak perturbation to the chemical-shift Hamiltonian. A pure chemical-shift Hamiltonian yields exponential relaxation with T_2=2/R_h in the case where the Larmor frequencies of the spins are distinct and sufficiently widely spaced. During spin locking with a nutation frequency fast enough to average the evolution under the internal spin Hamiltonian but not the interactions occurring during the correlation time of the resonator, relaxation of the spin-locked component is exponential with time constant T_(1ρ)=2/R_h.https://resolver.caltech.edu/CaltechAUTHORS:20120330-134535697Sensitivity of force-detected NMR spectroscopy with resonator-induced polarization
https://resolver.caltech.edu/CaltechAUTHORS:20130314-111403151
Year: 2013
DOI: 10.1103/PhysRevB.87.064413
In the low-temperature regime where the thermal polarization P is of order unity and spin-lattice relaxation is "frozen out," resonator-induced relaxation can be used to polarize a nuclear-spin sample for optimal detection sensitivity. We characterize the potential of resonator-induced polarization for enhancing the sensitivity of nuclear-magnetic-resonance spectroscopy. The sensitivities of two detection schemes are compared, one involving detection of a polarized sample dipole and the other involving detection of spin-noise correlations in an unpolarized sample. In the case where the dominant noise source is instrument noise associated with resonator fluctuations and with detection of the mechanical motion, a simple criterion can be used to compare the two schemes. Polarizing the sample improves sensitivity when P is larger than the signal-to-noise ratio for detection of a fully-polarized spin during a single transient. Even if the instrument noise is decreased to a level near the quantum-mechanical limit, it is larger than spin noise for unpolarized samples containing up to a few tens of nuclei. Under these conditions, spin polarization of order unity would enhance spectroscopic detection sensitivity by an order of magnitude or more. In the limiting case where signal decay is due to resonator-induced dissipation during ideal spin locking, and where resonator fluctuations are the noise source, the only parameter of the spin-resonator system that affects the sensitivity per spin is the ratio of frequency to temperature. A balance between the coupling strength, the noise power, and the signal lifetime causes the cancellation of other parameters from the sensitivity formula. Partial cancellation of parameters, associated with a balance between the same three quantities, occurs more generally when the resonator is both the dominant noise source and the dominant source of signal decay. An intrinsic sensitivity limit exists for resonant detection of coherent spin evolution, due to the fact that the detector causes signal decay by enhancing the spins' spontaneous emission. For a single-spin sample, the quantum-limited signal-to-noise ratio for resonant detection is 1/3. In contrast to the sensitivity, the time required for sample polarization between transients depends strongly on resonator parameters. We discuss resonator design and show that for a torsional resonator, the coupling is optimal when the resonator's magnetization remains aligned with the applied field during the mechanical oscillations.https://resolver.caltech.edu/CaltechAUTHORS:20130314-111403151