Coherence Transfer Processes Research Articles

A Comprehensive Emission Model for Layered Irregular and Inhomogeneous Medium

Reported radiometric measurements indicate that incoherent and coherent radiative transfer processes characterize the microwave emission features of layered irregular and inhomogeneous medium. In order to develop a forward emission model to be capable of considering the medium and boundary scattering and coherent boundary interaction that may exist in general layered medium, in this paper, we present a comprehensive layer emission model (CLEM) based on the scattering operator formulation. We introduce wave-based coherent multiple reflection operators to account for coherent boundary interaction and first integrate them with the intensity-based multiple scattering processes, allowing the comprehensive description of rough boundary scattering, volume scattering, medium/boundary interaction, and coherent boundary interaction in the framework of CLEM. Simulations and analyses for ice- and snow-covered ground cases are conducted based on CLEM to evaluate the coherent boundary interaction and different impacting factors. Validation on CLEM is conducted with emission observations of snow-covered terrain in campaign Nordic Snow Radar Experiment (NoSREx) 2010-2013 during dry snow period and time series emission observations of frozen soil during freezing and thawing processes. CLEM simulation results show good agreement with measurements. For NoSREx, RMSEs at L-to Ka-band are below 5.5 K for both polarizations, which are of different levels of promotion compared with the incoherent model simulations, especially for horizontal polarization at L- and X-band. Application to frozen soil case illustrates the capability of CLEM to explain coherent oscillation feature with impact from incoherent scattering effect.

Coherent Energy and Charge Transport Processes in Oligothiophene Dendrimers Probed in Solution and in the Solid State with Time-Resolved Spectroscopy and Microscopy Methods

Dendritic molecules demonstrate a broad variety of transfer processes under different geometric and coupling regimes, which can be utilized in photovoltaic applications. Coherent energy and charge transfer processes were investigated in different oligothiophene dendritic systems functionalized with diketopyrrolopyrrole (DPP) groups in solution and in the solid-phase. For the first time, high-resolution time-resolved spectroscopy and interferometric microscopy techniques have been compared to investigate ultrafast coherent dynamics. Investigation of coherent dynamics in photovoltaic organic materials is important as related to longer exciton diffusion length in organic macromolecules. Differences in the excited state coherent dynamics have been found in films compared to the solution phase. Interestingly, the dendron structure has shown higher charge delocalization in the solid-phase compared to the dendrimer system due to stronger intra- and inter-molecular couplings occurring in dendrons. These findings ...

Vibrational coherence transfer illuminates dark modes in models of the FeFe hydrogenase active site

Within the conceptual framework of Redfield theory, the optical response function arises from the dynamical evolution of the system’s density operator, where nonunitary relaxation is encoded in the Redfield relaxation superoperator. In the conventional approach, the so-called secular approximation neglects terms that induce transitions between distinct coherences and among coherences and populations. The rationale is that these nonsecular terms are small in comparison to the far more dominant population relaxation and coherence dephasing contributions. Since two-dimensional infrared (2D-IR) spectroscopy has significant contributions arising from population relaxation and transfer pathways, it can be challenging to isolate signatures of the nonsecular relaxation. We report here that in three diiron dithiolate hexacarbonyl complexes that serve as small-molecule models of the [FeFe] hydrogenase H-cluster subsite, a fortuitous vibrational energy structure enables direct and clear signatures of vibrational coherence transfer in alkane solution. This finding holds promise towards developing a molecularly detailed understanding of the mechanism of vibrational coherence transfer processes, thanks to the ease of synthesizing derivatives based on the chemical modularity of these well studied diiron compounds. In addition to the fundamental need to characterize coherence transfer in molecular spectroscopy, we find in this set of molecules a practical utility for the nonsecular dynamics: the ability to determine the frequency of an IR-inactive mode. A coherence generated during the waiting time of the 2D-IR measurement transfers to a coherence involving the single dark CO stretching mode, which modulates some peak amplitudes in the 2D spectrum, revealing its transient excitation.

Vibrational coherence and energy transfer in two-dimensional spectra with the optimized mean-trajectory approximation.

The optimized mean-trajectory (OMT) approximation is a semiclassical method for computing vibrational response functions from action-quantized classical trajectories connected by discrete transitions that represent radiation-matter interactions. Here, we extend the OMT to include additional vibrational coherence and energy transfer processes. This generalized approximation is applied to a pair of anharmonic chromophores coupled to a bath. The resulting 2D spectra are shown to reflect coherence transfer between normal modes.

Vibronic couplings and coherent electron transfer in bridged systems.

A computational strategy to analyze the dynamics of coherent electron transfer processes in bridged systems, involving three or more electronic states, is presented. The approach is based on partitioning of the Hilbert space of the time independent basis functions in subspaces of increasing dimensionality, which allows us to easily check the convergence of the time dependent wave function. Vibronic couplings are determined by Duschinsky's analysis of the equilibrium position displacements, carried out using the equilibrium geometries and normal modes of the redox partners obtained at the DFT computational level.

Nonlocal thermoelectric effects and nonlocal Onsager relations in a three-terminal proximity-coupled superconductor-ferromagnet device.

We study thermal and charge transport in a three-terminal setup consisting of one superconducting and two ferromagnetic contacts. We predict that the simultaneous presence of spin filtering and of spin-dependent scattering phase shifts at each of the two interfaces will lead to very large nonlocal thermoelectric effects both in clean and in disordered systems. The symmetries of thermal and electric transport coefficients are related to fundamental thermodynamic principles by the Onsager reciprocity. Our results show that a nonlocal version of the Onsager relations for thermoelectric currents holds in a three-terminal quantum coherent ferromagnet-superconductor heterostructure including a spin-dependent crossed Andreev reflection and coherent electron transfer processes.

Ultrafast Multidimensional Spectroscopy: Principles and Applications to Photosynthetic Systems

We present the utility of 2-D electronic spectroscopy for the investigation of energy transfer dynamics in photosynthetic light-harvesting systems. Elucidating ultrafast energy transfer within photosynthetic systems is difficult due to the large number of molecules and complex environments involved in the process. In many spectroscopic methods, these systems appear as overlapping peaks with broad linewidths, obscuring the details of the dynamics. 2-D spectroscopy is a nonlinear, ultrafast method that yields a correlation map between excitation and emission energies, and can track incoherent and coherent energy transfer processes with femtosecond resolution. A 2-D spectrum can provide important insight into the structure and the mechanisms behind the excited state dynamics. We review the principles behind 2-D spectroscopy and describe the content of a 2-D electronic spectrum. Several recent applications of this technique to the major light-harvesting complex of Photosystem II are presented, including monitoring the time scales of energy transfer processes, investigation of the excited state energies, and determination of the relative orientations of the excited state transition dipole moments.

Contributions of Exchange and Dipole–Dipole Interactions to the Shape of EPR Spectra of Free Radicals in Diluted Solutions

This report presents a comprehensive analysis of the contributions of the Heisenberg exchange and dipole-dipole interactions in diluted solutions of nitroxide radicals to the shape of their electron paramagnetic resonance (EPR) spectra taking into account all coherence transfer processes. It is shown that these contributions interfere. The approaches to obtain the molecular-kinetic and exchange integral parameters by analyzing the EPR spectrum dependence on the radical concentration and the solvent viscosity are discussed.

Probing the Dynamics of Intraband Electronic Coherences in Cylindrical Molecular Aggregates

Electronic coherence transfer has been detected in only a small number of systems despite the potential impact of these dynamics on natural and artificial light harvesting. Nonlinear spectroscopies designed to probe the dynamics of electronic coherences are challenged by signal emission associated with electronic populations. This paper presents a newly developed nonlinear laser spectroscopy capable of measuring intraband electronic coherences (i.e., for pairs of single exciton states) in molecular aggregates with full suppression of undesired signal components. In comparison with methods applying all-femtosecond laser pulses, the present experiment uses both narrowband and broadband pulses to obtain similar information with a greater than 360-fold faster data acquisition rate. In addition, the technique enhances spectral resolution with experimental control of the measured line widths. High instrument throughput facilitates the comparison of measurements for a wide variety of materials. As the first application of this technique, we investigate the dynamics of intraband electronic coherences in double-walled cylindrical molecular aggregates possessing five slightly different morphologies controlled by varying the solvent conditions. Interfering coherences associated with pairs of exciton states give rise to well-resolved quantum beats in the measured signal fields. In addition, coherence transfer processes are investigated using a superposition of tensor elements (i.e., an analogue of probing population transfer with pump-probe anisotropy). The comparison of experimental measurements and calculations based on a theoretical model supports the finding of coherence transfer processes terminating in an electronic coherence between the inner and outer cylinder excitons.

Investigation of coherence transfer between “CH 3” and “CH 2” groups in ethanol with time-resolved multiplex CARS technique

The coherence transfer between stretching vibration modes of C–H bonds in the ethanol is detected by time-resolved multiplex CARS technique and it occurs via “through-bond transfer” pathway. The time scale and velocity of coherence transfer are estimated at 90 fs and 1670 m/s. Moreover, coherence transfer process requires vibrational modes of acceptor and donor are different.

A two-frequency coherent pulse NQR spectrometer

A two-frequency coherent pulse spectrometer has been designed for seeking and recording nuclear quadrupole resonance spectra and studying coherence transfer processes, quasi-stationary states, and transient processes in multipulse sequences. The functional diagram of this spectrometer can be used in production of equipment for remote detection of explosives and drugs, which will require only an increase in the output power of the transmitter.

Direct observation of coherent spin transfer processes in an InGaAs/GaAs quantum well via two-color time-resolved Kerr rotation measurements

A two-color time-resolved Kerr rotation spectroscopy system was built, with a femtosecond Ti:sapphire laser and a photonic crystal fiber, to study coherent spin transfer processes in an InGaAs/GaAs quantum well sample. The femtosecond Ti:sapphire laser plays two roles: besides providing a pump beam with a tunable wavelength, it also excites the photonic crystal fiber to generate supercontinuum light ranging from 500 nm to 1600 nm, from which a probe beam with a desirable wavelength is selected with a suitable interference filter. With such a system, we studied spin transfer processes between two semiconductors of different gaps in an InGaAs/GaAs quantum well sample. We found that electron spins generated in the GaAs barrier were transferred coherently into the InGaAs quantum well. A model based on rate equations and Bloch–Torrey equations is used to describe the coherent spin transfer processes quantitatively. With this model, we obtain an effective electron spin accumulation time of 21 ps in the InGaAs quantum well.

Ultrafast exciton transfers in DNA and its nonlinear optical spectroscopy

We have calculated the nonlinear response function of a DNA duplex helix including the contributions from the exciton population and coherence transfers by developing an appropriate exciton theory as well as by utilizing a projector operator technique. As a representative example of DNA double helices, the B-form (dA)10-(dT)10 is considered in detail. The Green functions of the exciton population and coherence transfer processes were obtained by developing the DNA exciton Hamiltonian. This enables us to study the dynamic properties of the solvent relaxation and exciton transfers. The spectral density describing the DNA base-solvent interactions was obtained by adjusting the solvent reorganization energy to reproduce the absorption and steady-state fluorescence spectra. The time-dependent fluorescence shift of the model DNA system is found to be ultrafast and it is largely determined by the exciton population transfer processes. It is further shown that the nonlinear optical spectroscopic techniques such as photon echo peak shift and two-dimensional photon echo can provide important information on the exciton dynamics of the DNA double helix. We have found that the exciton-exciton coherence transfer plays critical roles in the peculiar energy transfer and ultrafast memory loss of the initially created excitonic state in the DNA duplex helix.

Proton-detected nitrogen-14 NMR by recoupling of heteronuclear dipolar interactions using symmetry-based sequences

The recoupling of heteronuclear dipolar interactions between 14N and 1H with symmetry-based RN sequences of the R 20 5 9 type applied to protons in samples spinning at the magic angle allows one to excite 14N single-quantum (SQ) or double-quantum (DQ) coherences and reconvert them into observable proton coherences. This offers an alternative to the recoupling of heteronuclear dipolar interactions by rotary resonance. The experimental efficiency of the two-way coherence transfer process is on the order of 9 and 4% for SQ and DQ spectroscopy. Spectra were obtained at 400 and 800 MHz for protons. The parameters of the 14N quadrupolar tensors can be determined in peptides, both in rotating 14 N 1 H 3 + groups in protonated amines and in rigid 14N 1H pairs in amide groups.

Heterodyned fifth-order two-dimensional IR spectroscopy: Third-quantum states and polarization selectivity

A heterodyned fifth-order two-dimensional (2D) IR spectrum of a model coupled oscillator system, Ir(CO)(2)(C(5)H(7)O(2)), is reported. The spectrum is generated by a pulse sequence that probes the eigenstate energies up to the second overtone and combination bands, providing a more rigorous potential-energy surface of the coupled carbonyl local modes than can be obtained with third-order spectroscopy. Furthermore, the pulse sequence is designed to generate and then rephase a two-quantum coherence so that the spectrum is line narrowed and the resolution improved for inhomogeneously broadened systems. Features arising from coherence transfer processes are identified, which are more pronounced than in third-order 2D IR spectroscopy because the transition dipoles of the second overtone and combination states are not rigorously orthogonal, relaxing the polarization constraints on the signal intensity for these features. The spectrum provides a stringent test of cascading signals caused by third-order emitted fields and no cascading is observed. In the Appendix, formulas for calculating the signal intensities for resonant fifth-order spectroscopies with arbitrarily polarized pulses and transition dipoles are reported. These relationships are useful for interpreting and designing polarization conditions to enhance specific spectral features.

Vibrational Coherence Transfer and Trapping as Sources for Long-Lived Quantum Beats in Polarized Emission from Energy Transfer Complexes

We entertain vibrational coherence transfer and related processes as possible sources for certain long-lived quantum beats observed in time-resolved polarized emission signals from photosynthetic l...

Spin-locking of half-integer quadrupolar nuclei in nuclear magnetic resonance of solids: creation and evolution of coherences.

Spin-locking of half-integer quadrupolar nuclei, such as 23Na (I=3/2) and 27Al (I=5/2), is of renewed interest owing to the development of variants of the multiple-quantum and satellite-transition magic angle spinning (MAS) nuclear magnetic resonance experiments that either utilize spin-locking directly or offer the possibility that spin-locked states may arise. However, the large magnitude and, under MAS, the time dependence of the quadrupolar interaction often result in complex spin-locking phenomena that are not widely understood. Here we show that, following the application of a spin-locking pulse, a variety of coherence transfer processes occur on a time scale of approximately 1/omegaQ before the spin system settles down into a spin-locked state which may itself be time dependent if MAS is performed. We show theoretically for both spin I=3/2 and 5/2 nuclei that the spin-locked state created by this initial rapid dephasing typically consists of a variety of single- and multiple-quantum coherences and nonequilibrium population states and we discuss the subsequent evolution of these under MAS. In contrast to previous work, we consider spin-locking using a wide range of radio frequency field strengths, i.e., a range that covers both the "strong-field" (omega1 >> omegaQPAS and "weak-field" (omega1 << omegaQPAS limits. Single- and multiple-quantum filtered spin-locking experiments on NaNO2, NaNO3, and Al(acac)3, under both static and MAS conditions, are used to illustrate and confirm the results of the theoretical discussion.

Ultrafast fluorescence investigation of excitation energy transfer in different dendritic core branched structures.

The mechanism of energy transport in branching structures is suggestively related to the geometry of the multichromophore architecture. In organic conjugated dendrimers, both incoherent (hopping) and coherent energy transfer processes have been observed from different dendritic architectures with different building blocks. In this communication, we report the investigation of three fundamental dendritic architectures (G0) with the same attached chromophores, but with different core atoms, C, N, and P. The synthesis of a phosphorus-containing G0 system with distyrylbenzene chromophores is provided. These three systems provide a comparison by which the relative interaction of branching chromophores can be compared on the basis of their different branching centers. Ultrafast fluorescence anisotropy measurements provide a dual measure of the geometry of the chromophores around the different central units as well as the strength of the interactions among chromophores. The nitrogen-cored system appeared to have both the strongest coupling of chromophore excitation as well as the most planar geometry of the three. Interestingly, the phosphorus system appeared to have the least planar geometry, and its interaction strength was found to be stronger than that observed for the carbon system. These results provide a comparison of the energy migration dynamics of the most common and new dendritic architectures with applications for light emission and light harvesting.

Coherence transfer selectivity in two-dimensional solid-state NMR

In solid-state NMR, recoupling sequences are often used as the mixing elements of two-dimensional pulse sequences. We show that in many cases, the commutation properties of the mixing Hamiltonian leads to selectivity in the allowed coherence transfer processes. This property may lead to enhanced resolution in the cross-peaks of two-dimensional solid-state NMR spectra.

Quasi-isotropic single-transition cross-polarization in nuclear magnetic resonance

The theory of single-transition cross-polarization in nuclear magnetic resonance is presented and verified by experimental evidence. In comparison to conventional cross-polarization a qualitative change in the mechanism is observed. Under the influence of matched radio-frequency fields with amplitudes that are smaller than the scalar coupling constant JIS for a two-spin system with I=12 and S=12 in isotropic solution, two simultaneous coherence transfer processes are observed between single-transition coherences which have phases that are parallel to those of the radio-frequency fields, an on-resonance transfer from SxIα to SαIx and an off-resonance transfer from SxIβ to SβIx, without mixing between the two pathways. Coherence transfer is also observed between single-transition coherences with phases that are perpendicular to the radio-frequency fields, from SyIα to SαIy and from SyIβ to SβIy, as well as between longitudinal components, from SzIα to SαIz and from SzIβ to SβIz. The transfer may therefore be considered quasi-isotropic. We consider the conditions under which such transfer processes can be observed. Coherence transfer is affected by differential relaxation due to cross-correlation effects.