The Experts below are selected from a list of 31266 Experts worldwide ranked by ideXlab platform
John M Lupton - One of the best experts on this subject based on the ideXlab platform.
-
homo fret in π conjugated polygons intermediate strength dipole dipole Coupling makes energy transfer reversible
Nano Letters, 2019Co-Authors: Philipp Wilhelm, Jan Vogelsang, Sigurd Hoger, John M LuptonAbstract:The concept of homo-FRET is often used to describe energy transfer between like chromophores of molecular aggregates such as in π-conjugated polymers. Homo-FRET is revealed by a dynamic depolarization in fluorescence but strictly only applies to the limit of weak dipole–dipole Coupling, where energy transfer occurs on time scales much longer than those of nuclear relaxation. By considering the polarization anisotropy of photoluminescence emission and excitation of model multichromophoric aggregates on the single-molecule level, we demonstrate the transition of energy-transfer dynamics from the case of weak Coupling to that of strong Coupling, revealing the elusive regime of intermediate-strength Coupling where energy transfer between degenerate donor and acceptor chromophores becomes reversible so that information on the excitation route of the emitting chromophore is lost.
-
efficient dipole dipole Coupling of mott wannier and frenkel excitons in ga in n quantum well polyfluorene semiconductor heterostructures
Physical Review B, 2007Co-Authors: Grigorios Itskos, John M Lupton, G Heliotis, Pavlos G Lagoudakis, N P Barradas, E Alves, S Pereira, Ian Watson, M D Dawson, Jochen FeldmannAbstract:We investigate interactions between Mott-Wannier (MW) and Frenkel excitons in a family of hybrid structures consisting of thin organic (polyfluorene) films placed in close proximity (systematically adjusted by GaN cap layer thickness) to single inorganic [(Ga,In)N/GaN] quantum wells (QWs). Characterization of the QW structures using Rutherford backscattering spectrometry and atomic force microscopy allows direct measurement of the thickness and the morphology of the GaN cap layers. Time-resolved photoluminescence experiments in the 8–75 K temperature range confirm our earlier demonstration that nonradiative energy transfer can occur between inorganic and organic semiconductors. We assign the transfer mechanism to resonant Forster (Dipole-Dipole) Coupling between MW exciton energy donors and Frenkel exciton energy acceptors and at 15 K we find transfer efficiencies of up to 43%. The dependence of the energy transfer rate on the distance R between the inorganic QW donor dipole and organic film acceptor dipole indicates that a plane-plane interaction, characterized by a 1/R2 variation, best describes the situation found in our structures.
Niels Chr Nielsen - One of the best experts on this subject based on the ideXlab platform.
-
resolution enhanced solid state nmr 13c 13c correlation spectroscopy by optimal control dipolar driven spin state selective coherence transfer
Journal of Physical Chemistry Letters, 2011Co-Authors: Cindie Kehlet, Jakob Nielsen, Zdenek Tosner, Niels Chr NielsenAbstract:Using optimal control, we have designed spin-state-selective coherence transfer experiments for biological solid-state NMR based on transfer via dipole−dipole Coupling interactions. This enables co...
-
reCoupling of native homonuclear dipolar Couplings in magic angle spinning solid state nmr by the double oscillating field technique
Journal of Chemical Physics, 2010Co-Authors: Lasse Arnt Straaso, Niels Chr NielsenAbstract:A new solid-state NMR method, the double-oscillating field technique (DUO), that under magic-angle-spinning conditions produces an effective Hamiltonian proportional to the native high-field homonuclear Dipole-Dipole Coupling operator is presented. The method exploits one part of the radio frequency (rf) field to recouple the dipolar Coupling interaction with a relatively high scaling factor and to eliminate offset effects over a reasonable bandwidth while in the reCoupling frame, the other part gives rise to a sufficiently large longitudinal component of the residual rf field that averages nonsecular terms and in addition ensures stability toward rf inhomogeneity and rf miscalibration. The capability of the DUO experiment to mediate transfer of polarization is described theoretically and compared numerically and experimentally with finite pulse rf driven reCoupling and experimentally with dipolar-assisted rotational resonance. Two-dimensional reCoupling experiments were performed on antiparallel amyloid ...
-
composite dipolar reCoupling anisotropy compensated coherence transfer in solid state nuclear magnetic resonance
Journal of Chemical Physics, 2006Co-Authors: Navin Khaneja, Cindie Kehlet, Steffen J Glaser, Niels Chr NielsenAbstract:The efficiency of Dipole-Dipole Coupling driven coherence transfer experiments in solid-state nuclear magnetic resonance (NMR) spectroscopy of powder samples is limited by dispersion of the orientation of the internuclear vectors relative to the external magnetic field. Here we introduce general design principles and resulting pulse sequences that approach full polarization transfer efficiency for all crystallite orientations in a powder in magic-angle-spinning experiments. The methods compensate for the defocusing of coherence due to orientation dependent dipolar Coupling interactions and inhomogeneous radio-frequency fields. The compensation scheme is very simple to implement as a scaffold (comb) of compensating pulses in which the pulse sequence to be improved may be inserted. The degree of compensation can be adjusted and should be balanced as a compromise between efficiency and length of the overall pulse sequence. We show by numerical and experimental data that the presented compensation protocol si...
Danuta Kruk - One of the best experts on this subject based on the ideXlab platform.
-
field dependent nuclear relaxation of spins 1 2 induced by dipole dipole Couplings to quadrupole spins laf3 crystals as an example
Journal of Magnetic Resonance, 2006Co-Authors: Danuta Kruk, O LipsAbstract:A general theory of spin-lattice nuclear relaxation of spins I=1/2 caused by Dipole-Dipole Couplings to quadrupole spins S1, characterized by a non-zero averaged (static) quadrupole Coupling, is presented. In multispin systems containing quadrupolar and dipolar nuclei, transitions of spins 1/2 leading to their relaxation are associated through Dipole-Dipole Couplings with certain transitions of quadrupole spins. The averaged quadrupole Coupling attributes to the energy level structure of the quadrupole spin and influences in this manner relaxation processes of the spin 1/2. Typically, quadrupole spins exhibit also a complex multiexponential relaxation sensed by the dipolar spin as an additional modulation of the mutual Dipole-Dipole Coupling. The proposed model includes both effects and is valid for an arbitrary magnetic field and an arbitrary quadrupole spin quantum number. The theory is applied to interpret fluorine relaxation profiles in LaF3 ionic crystals. The obtained results are compared with predictions of the 'classical' Solomon relaxation theory.
-
nuclear spin relaxation in paramagnetic systems with zero field splitting and arbitrary electron spin
Physical Chemistry Chemical Physics, 2001Co-Authors: Danuta Kruk, Tomas Nilsson, Jozef KowalewskiAbstract:A complete theory for paramagnetic relaxation enhancement (PRE) and its dependence on the magnetic field is developed for systems with electron spin S = 1, 3/2, 2, 5/2, 3 and 7/2, characterised by the presence of zero-field splitting (ZFS). The electron spin interacts through dipole–dipole Coupling with the nuclear spin residing in the paramagnetic complex (the inner-sphere case) as well as outside of it (the outer-sphere case). The earlier theory for S = 1 and inner-sphere interaction only is included as a special case of the present, more general approach. The theory assumes a slow reorientation of the paramagnetic complex and a lack of correlation between the rotation and translation of the complex and the electron spin dynamics. The electron spin energy level structure is determined by a combination of the Zeeman interaction and the static ZFS, and depends thus on the orientation of the complex in the magnetic field. The electron spin relaxation is described by a Redfield formulation, using the pseudorotation model for the modulation of the transient zero-field splitting. Illustrative calculations are presented, showing that the typical field dependences of the inner- and outer-sphere relaxation enhancements are in general different. The static ZFS is demonstrated to influence the magnetic field dependences by affecting the frequencies occurring in the expressions for spectral densities. The model is applied to interpret the PRE of a slowly-rotating gadolinium(III) complex, a potential magnetic resonance imaging (MRI) contrast agent.
John W Blanchard - One of the best experts on this subject based on the ideXlab platform.
-
measurement of untruncated nuclear spin interactions via zero to ultralow field nuclear magnetic resonance
Physical Review B, 2015Co-Authors: John W Blanchard, Tobias F Sjolander, Jonathan King, M P Ledbetter, E H Levine, Vikram S BajajAbstract:Author(s): Blanchard, JW; Sjolander, TF; King, JP; Ledbetter, MP; Levine, EH; Bajaj, VS; Budker, D; Pines, A | Abstract: © 2015 American Physical Society. Zero- to ultralow-field nuclear magnetic resonance (ZULF NMR) provides a new regime for the measurement of nuclear spin-spin interactions free from the effects of large magnetic fields, such as truncation of terms that do not commute with the Zeeman Hamiltonian. One such interaction, the magnetic Dipole-Dipole Coupling, is a valuable source of spatial information in NMR, though many terms are unobservable in high-field NMR, and the Coupling averages to zero under isotropic molecular tumbling. Under partial alignment, this information is retained in the form of so-called residual dipolar Couplings. We report zero- to ultralow-field NMR measurements of residual dipolar Couplings in acetonitrile-2-C13 aligned in stretched polyvinyl acetate gels. This permits the investigation of dipolar Couplings as a perturbation on the indirect spin-spin J Coupling in the absence of an applied magnetic field. As a consequence of working at zero magnetic field, we observe terms of the Dipole-Dipole Coupling Hamiltonian that are invisible in conventional high-field NMR. This technique expands the capabilities of zero- to ultralow-field NMR and has potential applications in precision measurement of subtle physical interactions, chemical analysis, and characterization of local mesoscale structure in materials.
O Lips - One of the best experts on this subject based on the ideXlab platform.
-
field dependent nuclear relaxation of spins 1 2 induced by dipole dipole Couplings to quadrupole spins laf3 crystals as an example
Journal of Magnetic Resonance, 2006Co-Authors: Danuta Kruk, O LipsAbstract:A general theory of spin-lattice nuclear relaxation of spins I=1/2 caused by Dipole-Dipole Couplings to quadrupole spins S1, characterized by a non-zero averaged (static) quadrupole Coupling, is presented. In multispin systems containing quadrupolar and dipolar nuclei, transitions of spins 1/2 leading to their relaxation are associated through Dipole-Dipole Couplings with certain transitions of quadrupole spins. The averaged quadrupole Coupling attributes to the energy level structure of the quadrupole spin and influences in this manner relaxation processes of the spin 1/2. Typically, quadrupole spins exhibit also a complex multiexponential relaxation sensed by the dipolar spin as an additional modulation of the mutual Dipole-Dipole Coupling. The proposed model includes both effects and is valid for an arbitrary magnetic field and an arbitrary quadrupole spin quantum number. The theory is applied to interpret fluorine relaxation profiles in LaF3 ionic crystals. The obtained results are compared with predictions of the 'classical' Solomon relaxation theory.