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Shaul Mukamel - One of the best experts on this subject based on the ideXlab platform.
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Ultrafast nonlinear spectroscopy with spatially confined fields
2020Co-Authors: Felix Schlosser, Mario Schoth, Sven Burger, Frank Schmidt, Andreas Knorr, Shaul Mukamel, Marten RichterAbstract:Abstract. The combination of pulse shaping techniques for ultrashort laser pulses with nanoplasmonics leads to applications in the spatiotemporal control of electronic excitations with subwavelength precision. First, a method of finding shaped pulses for spatial selective excitation of individual nanostructures is presented. Second, double quantum Coherence, a twodimensional spectroscopic technique, yields the energies of single excitons and biexcitons and shows correlations between them. Through combining the spectroscopy with localization of optical fields we obtain more detailed information about the excitonic energies and couplings
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study of double core hole excitations in molecules by x ray double quantum Coherence signals a multi configuration simulation
Chemical Science, 2016Co-Authors: Kochise Bennett, Weijie Hua, Yu Zhang, Yi Luo, Shaul MukamelAbstract:The multi-configurational self-consistent field method is employed to simulate the two-dimensional all-X-ray Double-Quantum-Coherence (XDQC) spectroscopy, a four-wave mixing signal that provides direct signatures of double core hole (DCH) states. The valence electronic structure is probed by capturing the correlation between the single (SCH) and double core hole states. The state-averaged restricted-active-space self-consistent field (SA-RASSCF) approach is used which can treat the valence, SCH, and DCH states at the same theoretical level, and applies to all types of DCHs (located on one or two atoms, K-edge or L-edge), with both accuracy and efficiency. Orbital relaxation introduced by the core hole(s) and the static electron correlation is properly accounted for. The XDQC process can take place via different intermediate DCH state channels by tuning the pulse frequencies. We simulate the XDQC signals for the three isomers of aminophenol at 8 pulse frequency configurations, covering all DCH pathways involving the N1s and O1s core hole (N1sN1s, O1sO1s and N1sO1s), which reveal different patterns of valence excitations.
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dissecting biexciton wave functions of self assembled quantum dots by double quantum Coherence optical spectroscopy
Physical Review B, 2012Co-Authors: Benjamin P Fingerhut, Marten Richter, Junwei Luo, Alex Zunger, Shaul MukamelAbstract:Biexcitons feature prominently in various scenarios for utilization of quantum dots (QDs) for enhancing the efficiencies of solar cells, and for the generation of entangled photon pairs in single QD sources. Two-dimensional double quantum Coherence (2D-DQC) nonlinear optical spectra provide novel spectroscopic signatures of such states beyond global intensity and lifetime characteristics which are available by more conventional techniques. We report the simulation of a prototype 2D-DQC optical experiment of a self-assembled InAs/GaAs dot. The simulations consider the QD in different charged states and are based on a state-of-the-art atomistic many-body pseudopotential method for the calculation of the electronic structure and transition dipole matrix elements. Comparison of the spectra of negatively charged, neutral, and positively charged QD reveals optical signatures of their electronic excitations. This technique directly accesses the biexciton ($XX$) energies as well as the projections of their wave functions on the single-exciton manifold. These signals also provide a unique tool for probing the charged state of the QD and thus the occupation of the quantum state. Signatures of Pauli blockade of the creation of certain single and two excitons due to charges on the particles are observed. For all quantum states of the QD, the spectra reveal a strong multiconfiguration character of the biexciton wave functions. Peak intensities can be explained by interference of the contributing Liouville space pathways.
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two dimensional double quantum spectra reveal collective resonances in an atomic vapor
Physical Review Letters, 2012Co-Authors: Xingcan Dai, Shaul Mukamel, Marten Richter, Alan D Bristow, Cyril Falvo, Steven T CundiffAbstract:We report the observation of Double-Quantum Coherence signals in a gas of potassium atoms at twice the frequency of the one-quantum Coherences. Since a single atom does not have a state at the corresponding energy, this observation must be attributed to a collective resonance involving multiple atoms. These resonances are induced by weak interatomic dipole-dipole interactions, which means that the atoms cannot be treated in isolation, even at a low density of ${10}^{12}\text{ }\text{ }{\mathrm{cm}}^{\ensuremath{-}3}$.
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coherent multidimensional optical probes for electron correlations and exciton dynamics from nmr to x rays
Accounts of Chemical Research, 2009Co-Authors: Shaul Mukamel, Darius Abramavicius, Lijun Yang, Wei Zhuang, Igor V Schweigert, Dmitri V VoronineAbstract:Linear-spectroscopy is one-dimensional (1D); the absorption spectrum provides information about excitation energies and transition dipoles as projected into a single frequency axis. In contrast, multidimensional optical spectroscopy uses sequences of laser pulses to perturb or label the electronic degrees of freedom and watch for correlated events taking place during several controlled time intervals. The resulting correlation plots can be interpreted in terms of multipoint correlation functions that carry considerably more detailed information on dynamical events than the two-point functions provided by 1D techniques1–7. Correlations between spins have been routinely used in NMR to study complex molecules. The Nobel prize was awarded to Richard Ernst8 for inventing the technique and to Kurt Wuthrich9 for developing pulse sequences suitable for large proteins. Optical analogues of 2D NMR techniques first designed to study vibrational dynamics by Raman or infrared pulses1 and later extended to resonant electronic excitations in chromophore aggregates10 have been made possible thanks to the development of stable femtosecond laser sources with controlled phases11. In an ideal heterodyne-detected 2D experiment (Fig. 1) 3 laser pulses with wavevectors k1, k2, k3 interact sequentially with the molecules in the sample to create a polarization with wavevector k4 given by one of the linear combinations ±k1 ±k2 ±k3. In all other directors the polarization vanishes due to the random phases of contributions from different molecules. The coherent signal is generated in directions close to the various possible k4. The missmatch caused by frequency variation of the index of refraction is optimized (“phase matched”) to generate an intense signal detected by interference with a 4th pulse at the desired wavevector k4. When the radiation field is described quantum mechanically the entire process can be viewed as a concerted 4 photon process. The signal S(t3,t2,t1) depends parametrically on the time intervals between pulses which constitute the primary control-parameters. Other parameters include the direction k4, pulse polarizations, envelope shapes, and even the phases. Figure 1 Scheme of the time-resolved four-wave-mixing experiment. All calculations are given for the three-band scheme shown on the bottom left. We shall illustrate the power of 2D techniques and how they work using the three-band model system shown in Fig. 1 which has a ground state (g), a singly excited manifold (e) and a doubly excited manifold (f ). The dipole operator can induce transitions between g to e and e to f . All transitions in the system are stimulated: spontaneous emission is neglected. This three-band model represents electronic excitations in the various physical systems covered in the this article. Multidimensional signals monitor the dynamics of the system’s density matrix during the time intervals between pulses. Diagonal elements of this matrix ρnn represent populations of various states, while the off diagonal elements ρnm (n ≠ m), known as Coherences, carry additional valuable phase information. These signals can be described intuitively using the Feynman diagrams shown in Fig. 2 which display the Liouville space pathways: sequences of interactions with the various fields and the relevant elements of the density matrix during the controlled intervals between interactions6. The two vertical lines represent the ket (left) and the bra (right) of the density-matrix. Figure 2 Feynman diagrams for two 2D techniques with wavevectors kI and kIII. Incoming and outgoing arrows represent the interaction events, labels indicate states of the system during various intervals between interactions. ESA - excited state absorption, GSB ... Time runs from bottom to top and the labels mark the density matrix elements during the evolution periods between interactions. The arrows represent interactions with photons and are labelled by their wavevectors. Photon absorption is accompanied by a molecular excitation (g to e or e to f transition) whereas photon emission induces deexcitation (e to g or f to e). Our discussion will focus on two signals: the photon-echo SkI with kI = −k1+k2+k3 and the Double-Quantum-Coherence SkIII with kIII =+k1+k2−k3. We first present the Feynman diagrams and the quantum pathways relevant for the two techniques for the generic exciton model of Fig. 1. Simulated signals are then presented for three different physical systems: Wannier excitons in semiconductor quantum wells12–15, Frenkel excitons in photosynthetic complexes6,7, and soft x-ray core excitons in molecules16–19. We demonstrate that both techniques provide new insights into the structure and exciton dynamics in semiconductor nanostructures and molecular aggregates and are highly sensitive to the separation between core-shells and the localization of the core-excited states. The three contributions to the SkI signal depicted in Fig. 2 are known as ground state bleaching (GSB), excited state stimulated emission (ESE) and excited state absorption (ESA)6. In the GSB pathway the system returns to the ground state (and described by the density matrix element ρgg), during the second interval t2, after interacting with the first two pulses. The third interaction is affected by the decrease of the ground state population which reduces (bleaches) the subsequent photon absorption. In the ESE pathway, the system resides in the singly-excited (e) manifold during t2 and the third interaction brings it back to the ground state by stimulated emission. The ESA pathway shares the same t1 and t2 history of the ESE, however the third interaction now creates a doubly-excited state f . The SkI signal is usually displayed as a frequency/frequency correlation plot SkI (Ω3,t2,Ω1) obtained by a double Fourier transform with respect to the time delays t1 and t3, holding t2 fixed. Ω3 and Ω1 reveal the various resonance transitions, as can be anticipated from the diagrams. Only single-exciton ωeg resonances corresponding to optical Coherences ρeg show up during t1 and are projected onto the Ω1 axis. The Ω3 axis shows either ωe′g resonances (ESE, GSB) or ωfe (ESA). The t2 evolution reflects exciton populations ρee and intra-band single-exciton Coherences ρee′. Population transport, Coherence oscillations and spectral diffusion dominate this interval in the ESE and ESA paths6. Since the molecular frequencies during t1 (ωge) are negative and during t3 (ωeg and ωfe) are positive, the Ω1 frequency axis is reversed in the 2D plots. With this convention uncoupled excitons only show diagonal peaks. Off diagonal cross-peaks are markers of some kind of communication between various excitations which causes their resonance frequency to be different during t1 and t3. This can be attributed either to exciton delocalization or to population transport. A simple interpretation of the signals is possible by using a basis of states localized on the various chromophores. Since the dipole is localized on each chromophore and can only excite one chromophore at a time, cross peaks only appear when the chromophores are coupled. NMR spectra are similarly interpreted in terms of the couplings of localized spin states8. The couplings of chromophores can always be formally eliminated by diagonalizing the single-exciton Hamiltonian and switching to the delocalized exciton basis. However in this representation the dipole operator matrix elements will depend on the details of the eigenstates, which prevents the simple intuitive interpretation of the signal. The SkIII technique has two ESA-type contributions (Fig. 2). 2D spectra is obtained by either correlating t1 → Ω1 with t2 → Ω2, SkIII (Ω1, Ω2, t3), or t2 → Ω2 with t3 → Ω3, SkIII (t1,Ω2,Ω3). The density matrix evolution during t1 and t2 is identical for the ESA1 and ESA2 diagrams: single-exciton resonances corresponding to ρeg show up during t1. During t2 the system is in a coherent superposition (Coherence) ρfg between the doubly-excited state f and the ground-state g. Two-exciton Double-Quantum-Coherence resonances corresponding to the different doubly excited states f are then projected onto Ω2. The t3 evolution is very different: In ESA1 the system is in a Coherence between f and e′ (ρfe′) which results in resonances at Ω3 = ωfe′, corresponding to all possible transitions between doubly- and singly- excited states. For ESA2 the system is in a Coherence between e′ and g (ρe′g) and reveals single- exciton resonances at Ω3 = ωe′g as t3 is scanned. When the single-exciton states e and e′ do not interact (e.g. when they belong to two uncoupled chromophores), the corresponding two-exciton state is given by a direct product |f〉 = |ee′〉 and the double-excitation energy is the sum ef = ee + ee. In that case ωeg =ωfe′ =ee, the two diagrams exactly cancel and the signal vanishes! The entire SkIII signal is thus induced by correlations and its peak pattern provides a characteristic fingerprint for the correlated doubly excited wavefunctions. This conclusion goes beyond the present simple model. SkIII vanishes for uncorrelated many electron systems described by the Hartree Fock wavefunction and thus provides an excellent background-free probe for electron correlations12,20. The (Ω2,Ω3) correlation plots spread the two-exciton (f state) information along both axes, thus improving the resolution of the two-exciton manifold.
Marten Richter - One of the best experts on this subject based on the ideXlab platform.
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Ultrafast nonlinear spectroscopy with spatially confined fields
2020Co-Authors: Felix Schlosser, Mario Schoth, Sven Burger, Frank Schmidt, Andreas Knorr, Shaul Mukamel, Marten RichterAbstract:Abstract. The combination of pulse shaping techniques for ultrashort laser pulses with nanoplasmonics leads to applications in the spatiotemporal control of electronic excitations with subwavelength precision. First, a method of finding shaped pulses for spatial selective excitation of individual nanostructures is presented. Second, double quantum Coherence, a twodimensional spectroscopic technique, yields the energies of single excitons and biexcitons and shows correlations between them. Through combining the spectroscopy with localization of optical fields we obtain more detailed information about the excitonic energies and couplings
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coherent coupling of individual quantum dots measured with phase referenced two dimensional spectroscopy photon echo versus double quantum Coherence
Physical Review B, 2017Co-Authors: V Delmonte, Judith F Specht, Tomasz Jakubczyk, Sven Hofling, Martin Kamp, Christian Schneider, Wolfgang Werner Langbein, Gilles Nogues, Marten RichterAbstract:We employ two-dimensional (2D) coherent, nonlinear spectroscopy to investigate couplings within individual InAs quantum dots (QD) and QD molecules. Swapping pulse ordering in a two-beam sequence permits to distinguish between rephasing and non-rephasing four-wave mixing (FWM) configurations. We emphasize the non-rephasing case, allowing to monitor two-photon Coherence dynamics. Respective Fourier transform yields a double quantum 2D FWM map, which is corroborated with its single quantum counterpart, originating from the rephasing sequence. We introduce referencing of the FWM phase with the one carried by the driving pulses, overcoming the necessity of its active-stabilization, as required in 2D spectroscopy. Combining single and double quantum 2D FWM, provides a pertinent tool in detecting and ascertaining coherent coupling mechanisms between individual quantum systems, as exemplified experimentally.
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dissecting biexciton wave functions of self assembled quantum dots by double quantum Coherence optical spectroscopy
Physical Review B, 2012Co-Authors: Benjamin P Fingerhut, Marten Richter, Junwei Luo, Alex Zunger, Shaul MukamelAbstract:Biexcitons feature prominently in various scenarios for utilization of quantum dots (QDs) for enhancing the efficiencies of solar cells, and for the generation of entangled photon pairs in single QD sources. Two-dimensional double quantum Coherence (2D-DQC) nonlinear optical spectra provide novel spectroscopic signatures of such states beyond global intensity and lifetime characteristics which are available by more conventional techniques. We report the simulation of a prototype 2D-DQC optical experiment of a self-assembled InAs/GaAs dot. The simulations consider the QD in different charged states and are based on a state-of-the-art atomistic many-body pseudopotential method for the calculation of the electronic structure and transition dipole matrix elements. Comparison of the spectra of negatively charged, neutral, and positively charged QD reveals optical signatures of their electronic excitations. This technique directly accesses the biexciton ($XX$) energies as well as the projections of their wave functions on the single-exciton manifold. These signals also provide a unique tool for probing the charged state of the QD and thus the occupation of the quantum state. Signatures of Pauli blockade of the creation of certain single and two excitons due to charges on the particles are observed. For all quantum states of the QD, the spectra reveal a strong multiconfiguration character of the biexciton wave functions. Peak intensities can be explained by interference of the contributing Liouville space pathways.
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two dimensional double quantum spectra reveal collective resonances in an atomic vapor
Physical Review Letters, 2012Co-Authors: Xingcan Dai, Shaul Mukamel, Marten Richter, Alan D Bristow, Cyril Falvo, Steven T CundiffAbstract:We report the observation of Double-Quantum Coherence signals in a gas of potassium atoms at twice the frequency of the one-quantum Coherences. Since a single atom does not have a state at the corresponding energy, this observation must be attributed to a collective resonance involving multiple atoms. These resonances are induced by weak interatomic dipole-dipole interactions, which means that the atoms cannot be treated in isolation, even at a low density of ${10}^{12}\text{ }\text{ }{\mathrm{cm}}^{\ensuremath{-}3}$.
Jack H Freed - One of the best experts on this subject based on the ideXlab platform.
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open and closed form of maltose binding protein in its native and molten globule state as studied by electron paramagnetic resonance spectroscopy
Biochemistry, 2018Co-Authors: Benjamin Selmke, Peter P. Borbat, Jack H Freed, Raghavan Varadarajan, Chen Nickolaus, Wolfgang E TrommerAbstract:An intensively investigated intermediate state of protein folding is the molten globule (MG) state, which contains secondary but hardly any tertiary structure. In previous work, we have determined the distances between interacting spins within maltose binding protein (MBP) in its native state using continuous wave and double electron-electron resonance (DEER) electron paramagnetic resonance (EPR) spectroscopy. Seven double mutants had been employed to investigate the structure within the two domains of MBP. DEER data nicely corroborated the previously available X-ray data. Even in its MG state, MBP is known to still bind its ligand maltose. We therefore hypothesized that there must be a defined structure around the binding pocket of MBP already in the absence of tertiary structure. Here we have investigated the functional and structural difference between native and MG state in the open and closed form with a new set of MBP mutants. In these, the spin-label positions were placed near the active site. Binding of its ligands leads to a conformational change from open to closed state, where the two domains are more closely together. The complete set of MBP mutants was analyzed at pH 3.2 (MG) and pH 7.4 (native state) using Double-Quantum Coherence EPR. The values were compared with theoretical predictions of distances between the labels in biradicals constructed by molecular modeling from the crystal structures of MBP in open and closed form and were found to be in excellent agreement. Measurements show a defined structure around the binding pocket of MBP in MG, which explains maltose binding. A new and important finding is that in both states ligand-free MBP can be found in open and closed form, while ligand-bound MBP appears only in closed form because of maltose binding.
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the determination of pair distance distributions by pulsed esr using tikhonov regularization
Journal of Magnetic Resonance, 2005Co-Authors: Yunwei Chiang, Peter P. Borbat, Jack H FreedAbstract:Pulsed ESR techniques with the aid of site-directed spin labeling have proven useful in providing unique structural information about proteins. The determination of distance distributions in electron spin pairs directly from the dipolar time evolution of the pulsed ESR signals by means of the Tikhonov regularization method is reported. The difficulties connected with numerically inverting this ill-posed mathematical problem are clearly illustrated. The Tikhonov regularization with the regularization parameter determined by the L-curve criterion is then described and tested to confirm its accuracy and reliability. The method is applied to recent experimental results on doubly labeled proteins that have been studied using two pulsed ESR techniques, double quantum Coherence (DQC) ESR and double electron–electron resonance (DEER). The extracted distance distributions are able to provide valuable information about the conformational constraints in various partially folded states of proteins. This study supplies a mathematically reliable method for extracting pair distributions from pulsed ESR experimental data and has extended the use of pulsed ESR to provide results of greater value for structural biology.
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spin labeled gramicidin a channel formation and dissociation
Biophysical Journal, 2004Co-Authors: Boris Dzikovski, Peter P. Borbat, Jack H FreedAbstract:Gramicidin A was studied by continuous wave electron spin resonance (CW-ESR) and by Double-Quantum Coherence electron spin resonance (DQC-ESR) in several lipid membranes (using samples that were macroscopically aligned by isopotential spin-dry ultracentrifugation) and vesicles. As a reporter group, the nitroxide spin-label was attached at the C-terminus yielding the spin-labeled product (GAsl). ESR spectra of aligned membranes containing GAsl show strong orientation dependence. In DPPC and DSPC membranes at room temperature the spectral shape is consistent with high ordering, which, in conjunction with the observed high polarity of the environment of the nitroxide, is interpreted in terms of the nitroxide moiety being close to the membrane surface. In contrast, spectra of GAsl in DMPC membranes indicate deeper embedding and tilt of the NO group. The GAsl spectrum in the DPPC membrane at 35°C (the gel to Pβ phase transition) exhibits sharp changes, and above this temperature becomes similar to that of DMPC. The dipolar spectrum from DQC-ESR clearly indicates the presence of pairs in DMPC membranes. This is not the case for DPPC, rapidly frozen from the gel phase; however, there are hints of aggregation. The interspin distance in the pairs is 30.9 A, in good agreement with estimates for the head-to-head GAsl dimer (the channel-forming conformation), which matches the hydrophobic thickness of the DMPC bilayer. Both DPPC and DSPC, apparently as a result of hydrophobic mismatch between the dimer length and bilayer thickness, do not favor the channel formation in the gel phase. In the Pβ and Lα phases of DPPC (above 35°C) the channel dimer forms, as evidenced by the DQC-ESR dipolar spectrum after rapid freezing. It is associated with a lateral expansion of lipid molecules and a concomitant decrease in bilayer thickness, which reduces the hydrophobic mismatch. A comparison with studies of dimer formation by other physical techniques indicates the desirability of using low concentrations of GA (∼0.4–1 mol %) accessible to the ESR methods employed in the study, since this yields non-interacting dimer channels.
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protein structure determination using long distance constraints from double quantum Coherence esr study of t4 lysozyme
Journal of the American Chemical Society, 2002Co-Authors: Peter P. Borbat, Hassane S Mchaourab, Jack H FreedAbstract:We report the use of a novel pulsed ESR technique for distance measurement, based on the detection of double quantum Coherence (DQC), which yields high quality dipolar spectra, to significantly extend the range of measurable distances in proteins using nitroxide spin-labels. Eight T4 lysozyme (T4L) mutants, doubly labeled with methanethiosulfonate spin-label (MTSSL), have been studied using DQC-ESR at 9 and 17 GHz. The distances span the range from 20 A for the 65/76 mutant to 47 A for the 61/135 mutant. The high quality of the dipolar spectra also allows the determination of the distance distributions, the width of which can be used to set upper and lower bounds in future computational strategy. It is also demonstrated that the shape of these distributions can reveal the presence of multiple conformations of the spin-label, an issue of critical relevance to the structural interpretation of the distances. The distances and distributions found in this study are readily rationalized in terms of the known cr...
Zhong Chen - One of the best experts on this subject based on the ideXlab platform.
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Signal-to-Noise Ratio Enhancement of Intermolecular Double-Quantum Coherence MR Spectroscopy in Inhomogeneous Fields With Phased Array Coils on a 3 Tesla Whole-Body Scanner
Journal of Magnetic Resonance Imaging, 2011Co-Authors: Zhong Chen, Jianhui ZhongAbstract:Purpose To improve signal-to-noise ratio (SNR) of intermolecular Double-Quantum Coherence (iDQC) MRS on a 3 Tesla (T) whole-body scanner. Materials and Methods A 32-channel phased array coil was used to acquire iDQC signal of a MRS phantom in the presence of large field inhomogeneity. The obtained individual spectra from the array elements were combined together in the time domain using a multichannel nonparametric singular value decomposition algorithm. The results were compared quantitatively with those acquired with a circularly polarized (CP) head coil. Results The achieved gain in SNR ranges from 1.63 to 2.10 relative to the CP coil, mainly depending on the relative position between the surface of the phased array coil and the voxel of acquisition. Conclusion SNR enhancement of iDQC MRS in inhomogeneous fields on a 3T whole-body scanner is feasible with phased array coils. This can facilitate iDQC applications of high-resolution in vivo spectroscopy in the presence of field inhomogeneity for potential disease diagnosis in humans. J. Magn. Reson. Imaging 2011;33:698–703. © 2011 Wiley-Liss, Inc.
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rotating frame intermolecular double quantum spin lattice relaxation t1ρ dqc weighted magnetic resonance imaging
Magnetic Resonance in Medicine, 2005Co-Authors: Zhong Chen, Bingwen Zheng, Dennis W Hwang, Lianpin HwangAbstract:In this study, spin-locking techniques were added as a part of intermolecular multiple-quantum experiments, thereby introducing the concept of rotating-frame intermolecular Double-Quantum spin-lattice relaxation, T1ρ, DQC. A novel magnetic resonance imaging methodology based on intermolecular multiple-quantum Coherences is demonstrated on a 7.05-T microimaging scanner. The results clearly reveal that the intermolecular Double-Quantum Coherence T1ρ, DQC-weighted imaging technique provides an alternative contrast mechanism to conventional imaging. Magn Reson Med 53:930–936, 2005. © 2005 Wiley-Liss, Inc.
M Engelsberg - One of the best experts on this subject based on the ideXlab platform.
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low field intermolecular double quantum Coherence imaging via the overhauser effect
Journal of Magnetic Resonance, 2003Co-Authors: Wilson Barros, Paulo Loureiro De Sousa, M EngelsbergAbstract:Abstract Intermolecular Double-Quantum Coherence (i-DQC) signals in liquids are usually associated with high magnetic fields. We demonstrate that, in a magnetic field of only 16 mT, i-DQC imaging of water protons is feasible thanks to the nuclear magnetization enhancement produced by the Overhauser effect. i-DQC images of a phantom containing an aqueous solution of a trityl free radical, with phase encoding in the DQC evolution period or in the acquisition period, are presented. Possible applications of low field i-MQC images are proposed.