Coherent Two-dimensional Spectroscopy Research Articles

Static and Dynamic Disorder in Bacterial Light-Harvesting Complex LH2: A 2DES Simulation Study.

Two-dimensional coherent electronic spectroscopy (2DES) is a powerful technique in distinguishing homogeneous and inhomogeneous broadening contributions to the spectral line shapes of molecular transitions induced by environment fluctuations. Using an excitonic model of a double-ring LH2 aggregate, we perform simulations of its 2DES spectra and find that the model of a harmonic environment cannot provide a consistent set of parameters for two temperatures: 77 K and room temperature. This indicates the highly anharmonic nature of protein fluctuations for the pigments of the B850 ring. However, the fluctuations of B800 ring pigments can be assumed as harmonic in this temperature range.

Generalized magic angle for time-resolved spectroscopy with laser pulses of arbitrary ellipticity

We present a general approach for calculating the magic-angle condition for time-resolved spectroscopy of isotropic samples. Allowing for arbitrary static polarizations and propagation directions of each pulse enables us to retrieve not only the known magic-angle conditions for linear and circular polarization, but also analogous conditions for anisotropy-free spectroscopy using elliptically polarized laser pulses. The results are exemplified on transient absorption and transferred to coherent two-dimensional spectroscopy. Furthermore we derive for transient absorption spectroscopy the relation between the measurable anisotropy and the molecular structure, i.e., the angle between the pumped and probed transition dipole moments (TDMs). The impact of multiple spectrally overlapping signals is considered and discussed on the example of a molecule with a two-fold degenerate TDM.

Two-dimensional fluorescence-detected coherent spectroscopy with absolute phasing by confocal imaging of a dynamic grating and 27-step phase-cycling.

We present a novel experimental scheme for two-dimensional fluorescence-detected coherent spectroscopy (2D-FDCS) using a non-collinear beam geometry with the aid of "confocal imaging" of dynamic (population) grating and 27-step phase-cycling to extract the signal. This arrangement obviates the need for distinct experimental designs for previously developed transmission detected non-collinear two-dimensional coherent spectroscopy (2D-CS) and collinear 2D-FDCS. We also describe a novel method for absolute phasing of the 2D spectrum. We apply this method to record 2D spectra of a fluorescent dye in solution at room temperature and observe "spectral diffusion."

Unraveling the nature of coherent beatings in chlorosomes

Coherent two-dimensional (2D) spectroscopy at 80 K was used to study chlorosomes isolated from green sulfur bacterium Chlorobaculum tepidum. Two distinct processes in the evolution of the 2D spectrum are observed. The first being exciton diffusion, seen in the change of the spectral shape occurring on a 100-fs timescale, and the second being vibrational coherences, realized through coherent beatings with frequencies of 91 and 145 cm(-1) that are dephased during the first 1.2 ps. The distribution of the oscillation amplitude in the 2D spectra is independent of the evolution of the 2D spectral shape. This implies that the diffusion energy transfer process does not transfer coherences within the chlorosome. Remarkably, the oscillatory pattern observed in the negative regions of the 2D spectrum (dominated by the excited state absorption) is a mirror image of the oscillations found in the positive part (originating from the stimulated emission and ground state bleach). This observation is surprising since it is expected that coherences in the electronic ground and excited states are generated with the same probability and the latter dephase faster in the presence of fast diffusion. Moreover, the relative amplitude of coherent beatings is rather high compared to non-oscillatory signal despite the reported low values of the Huang-Rhys factors. The origin of these effects is discussed in terms of the vibronic and Herzberg-Teller couplings.

Coherent Excitonic Coupling in an Asymmetric Double InGaAs Quantum Well Arises from Many-Body Effects

We study an asymmetric double InGaAs quantum well using optical two-dimensional coherent spectroscopy. The collection of zero-quantum, one-quantum, and two-quantum two-dimensional spectra provides a unique and comprehensive picture of the double well coherent optical response. Coherent and incoherent contributions to the coupling between the two quantum well excitons are clearly separated. An excellent agreement with density matrix calculations reveals that coherent interwell coupling originates from many-body interactions.

Publisher's Note: “Communication: Two-dimensional gas-phase coherent anti-Stokes Raman spectroscopy (2D-CARS): Simultaneous planar imaging and multiplex spectroscopy in a single laser shot” [J. Chem. Phys. 138, 221101 (2013)

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Communication: Two-dimensional gas-phase coherent anti-Stokes Raman spectroscopy (2D-CARS): Simultaneous planar imaging and multiplex spectroscopy in a single laser shot

Coherent anti-Stokes Raman spectroscopy (CARS) has been widely used as a powerful tool for chemical sensing, molecular dynamics measurements, and rovibrational spectroscopy since its development over 30 years ago, finding use in fields of study as diverse as combustion diagnostics, cell biology, plasma physics, and the standoff detection of explosives. The capability for acquiring resolved CARS spectra in multiple spatial dimensions within a single laser shot has been a long-standing goal for the study of dynamical processes, but has proven elusive because of both phase-matching and detection considerations. Here, by combining new phase matching and detection schemes with the high efficiency of femtosecond excitation of Raman coherences, we introduce a technique for single-shot two-dimensional (2D) spatial measurements of gas phase CARS spectra. We demonstrate a spectrometer enabling both 2D plane imaging and spectroscopy simultaneously, and present the instantaneous measurement of 15,000 spatially correlated rotational CARS spectra in N2 and air over a 2D field of 40 mm(2).

Photophysics of Delocalized Excitons in Carbazole Dendrimers

The photophysical properties in solution of three generations of carbazole-based dendrons and dendrimers with fluorenyl surface groups were studied using steady-state, time-resolved femtosecond transient absorption and anisotropy, and coherent two-dimensional ultraviolet spectroscopy. It was found that increasing the generation caused a switch in the nature of the emissive state between the first-generation compounds and the second- and third-generation dendrimers. Time-resolved anisotropy measurements revealed low initial anisotropies that decreased with increasing dendrimer generation consistent with increasing intradendrimer interchromophore coupling. Two-dimensional UV spectroscopy showed that the signal from the second- and third-generation dendrimers is the product of multiple chromophores interacting. The maximum number of interacting chromophores is reached by the second generation.

High-Resolution Coherent Three-Dimensional Spectroscopy of Br2

In the past, high-resolution spectroscopy has been limited to small, simple molecules that yield relatively uncongested spectra. Larger and more complex molecules have a higher density of peaks and are susceptible to complications (e.g., effects from conical intersections) that can obscure the patterns needed to resolve and assign peaks. Recently, high-resolution coherent two-dimensional (2D) spectroscopy has been used to resolve and sort peaks into easily identifiable patterns for molecules where pattern-recognition has been difficult. For very highly congested spectra, however, the ability to resolve peaks using coherent 2D spectroscopy is limited by the bandwidth of instrumentation. In this article, we introduce and investigate high-resolution coherent three-dimensional spectroscopy (HRC3D) as a method for dealing with heavily congested systems. The resulting patterns are unlike those in high-resolution coherent 2D spectra. Analysis of HRC3D spectra could provide a means for exploring the spectroscopy of large and complex molecules that have previously been considered too difficult to study.

Correlated driving and dissipation in two-dimensional spectroscopy

The correlation between coherent driving and non-Markovian dissipation plays a vital role in optical processes. To exhibit its effect on the simulation of optical spectroscopy, we explore the correlated driving-dissipation equation (CODDE) [R. X. Xu and Y. J. Yan, J. Chem. Phys. 116, 9196 (2002)], which modifies the conventional Redfield theory with the inclusion of correlated driving-dissipation effect at the second-order system-bath coupling level. With an exciton model mimicking the Fenna-Matthews-Olson pigment-protein complex, we compare between the Redfield theory, CODDE, and exact hierarchical dynamics, for their results on linear absorption and coherent two-dimensional spectroscopy. We clarify that the failure of Redfield approach originates mainly from the neglect of driving-dissipation correlation, rather than its second-order nature. We further propose a dynamical inhomogeneity parameter to quantify the applicable range of CODDE. Our results indicate that CODDE is an efficient and quantifiable theory for many light-harvesting complexes of interest. To facilitate the evaluation of multi-dimensional spectroscopy, we also develop the mixed Heisenberg-Schrödinger picture scheme that is valid for any dynamics implementation on nonlinear response functions.

Novel coherent two-dimensional optical spectroscopy probes of chirality exchange and fluctuations in molecules

We demonstrate how stochastic transitions between molecular configurations with opposite senses of chirality may be probed by 2D optical signals with specific pulse polarization configurations. The third-order optical response of molecular dimers (such as biphenyls) with dynamical axial chirality is calculated to order of k(2) in the wavevector of light. Spectroscopic signatures of equilibrium chirality fluctuations are predicted for three dynamical models (Ornstein-Uhlenbeck, two-state jump, and diffusion in double well) of the dihedral angle that controls the chirality.

Optimized hierarchical equations of motion theory for Drude dissipation and efficient implementation to nonlinear spectroscopies

Hierarchical equations of motion theory for Drude dissipation is optimized, with a convenient convergence criterion proposed in advance of numerical propagations. The theoretical construction is on the basis of a Padé spectrum decomposition that has been qualified to be the best sum-over-poles scheme for quantum distribution function. The resulting hierarchical dynamics under the a priori convergence criterion are exemplified with a benchmark spin-boson system, and also the transient absorption and related coherent two-dimensional spectroscopy of a model exciton dimer system. We combine the present theory with several advanced techniques such as the block hierarchical dynamics in mixed Heisenberg-Schrödinger picture and the on-the-fly filtering algorithm for the efficient evaluation of third-order optical response functions.

Advancing Hierarchical Equations of Motion for Efficient Evaluation of Coherent Two-dimensional Spectroscopy

To advance hierarchical equations of motion as a standard theory for quantum dissipative dynamics, we put forward a mixed Heisenberg-Schrödinger scheme with block-matrix implementation on efficient evaluation of nonlinear optical response function. The new approach is also integrated with optimized hierarchical theory and numerical filtering algorithm. Different configurations of coherent two-dimensional spectroscopy of model excitonic dimer systems are investigated, with focusing on the effects of intermolecular transfer coupling and bi-exciton interaction.

Communication: Polarization-angle-scanning two-dimensional infrared spectroscopy of antiparallel β-sheet polypeptide: Additional dimensions in two-dimensional optical spectroscopy

A theoretical description of polarization-angle-scanning (PAS) two-dimensional infrared (2DIR) spectroscopy, where the incident beam polarization directions are considered to be novel dimensions in coherent two-dimensional (2D) optical spectroscopy, is presented. To shed light on the underlying principles and to illustrate a potential use of this measurement method, we investigate the PAS 2DIR spectroscopy of an alanine-based antiparallel β-sheet, using the relationships between cross-peak amplitudes and incident beam polarization directions and carrying out numerical simulations. The numerically simulated PAS 2DIR spectra of the antiparallel β-sheet show that the magnitude and sign of the cross peak reflecting the correlation between the two characteristic amide I vibrations change for varying beam polarization directions. This suggests that each individual cross peak in a given 2D spectrum can be selectively eliminated with an appropriate beam polarization configuration, which in turn provides information on the transition dipole angle and possibly on the structure of coupled multichromophoric systems. This novel measurement method combining the polarization-angle-scanning technique with 2D vibrational or electronic spectroscopy would be a useful tool for probing structural changes of nonequilibrium molecular systems and to investigate transfers of population and coherence by monitoring the time-dependent changes of angles between transition dipoles.

Coherent two-dimensional ultraviolet spectroscopy in fully noncollinear geometry

We introduce fully noncollinear coherent two-dimensional (2D) spectroscopy in the UV domain with an all-reflective and miniaturized setup design. Phase stability is achieved via pairwise beam manipulation, and the concept can be transferred to all wavelength regimes. Here we present results from an implementation that has been optimized for wavelengths between 250 and 375 nm. Interferometric measurements prove phase stability over several hours. We obtained 2D spectra of the nonpolar UV chromophore p-terphenyl in ethanol, excited with 50 fs pulses at 287 nm.

Polarization-Angle-Scanning Two-Dimensional Spectroscopy: Application to Dipeptide Structure Determination

Coherent two-dimensional optical spectroscopy based on a heterodyne-detected stimulated photon echo measurement technique requires four ultrashort pulses whose pulse-to-pulse delay times, wavevectors, and frequencies are experimentally controllable variables. In addition, the polarization directions of the four radiations can also be arbitrarily adjusted. We show that the polarization-angle-scanning two-dimensional spectroscopy can be of effective use to selectively suppress either all the diagonal peaks or a cross-peak in a given two-dimensional spectrum. Theoretical relationships between the transition dipole vectors of a given pair of coupled modes or quantum transitions and the polarization angle configuration making the corresponding cross-peak vanish are established. Here, to shed light into the underlying principles of the polarization-angle-scanning two-dimensional spectroscopy, we considered the amide I vibrations of various isotope-labeled dipeptide conformers and show that one can selectively suppress a cross-peak by properly controlling the polarization angle of a chosen beam among them. Once the relative directions of the amide I transition dipole vectors are determined using the polarization-angle-scanning technique theoretically proposed here, they can serve as a set of constraints for determining structures of model peptides. The present work demonstrates that the polarization-controlled two-dimensional vibrational or electronic spectroscopy can provide invaluable information on intricate details of molecular structures.

Energy-transfer and charge-separation pathways in the reaction center of photosystem II revealed by coherent two-dimensional optical spectroscopy

The excited state dynamics and relaxation of electrons and holes in the photosynthetic reaction center of photosystem II are simulated using a two-band tight-binding model. The dissipative exciton and charge carrier motions are calculated using a transport theory, which includes a strong coupling to a harmonic bath with experimentally determined spectral density, and reduces to the Redfield, the Förster, and the Marcus expressions in the proper parameter regimes. The simulated third order two-dimensional signals, generated in the directions -k(1)+k(2)+k(3), k(1)-k(2)+k(3), and k(1)+k(2)-k(3), clearly reveal the exciton migration and the charge-separation processes.

High Resolution Coherent 2D Spectroscopy

The purpose of this article is to describe recent progress on the use of coherent two-dimensional spectroscopy for investigating the electronic spectroscopy of gas phase molecules. Unlike conventional high resolution spectra where peaks are distributed along a single axis, high resolution coherent 2D spectra reveal informative patterns along two orthogonal frequency domains. The technique can successfully produce these patterns in situations where one-dimensional spectra appear patternless due to complexity and congestion. Molecular spectra that are difficult to analyze because of strongly perturbing effects (e.g., conical intersections) may be studied using this new technique. Several innovations, such as the ability to graphically separate rotational and vibrational information by clustering peaks and the ability to use multiple clusters to overcome spectral congestion help provide high resolution coherent 2D spectroscopy with the ability to analyze spectra that have previously resisted analysis.

Electronic coherence dephasing in excitonic molecular complexes: Role of Markov and secular approximations

We compare four different types of equations of motion for reduced density matrix of a system of molecular excitons interacting with thermodynamic bath. All four equations are of second order in the linear system-bath interaction Hamiltonian, with different approximations applied in their derivation. In particular we compare time-nonlocal equations obtained from so-called Nakajima-Zwanzig identity and the time-local equations resulting from the partial ordering prescription of the cummulant expansion. In each of these equations we alternatively apply secular approximation to decouple population and coherence dynamics from each other. We focus on the dynamics of intraband electronic coherences of the excitonic system which can be traced by coherent two-dimensional spectroscopy. We discuss the applicability of the four relaxation theories to simulations of population and coherence dynamics, and identify features of the two-dimensional coherent spectrum that allow us to distinguish time-nonlocal effects.

Probing the folding of mini-protein Beta3s by two-dimensional infrared spectroscopy; simulation study

We propose to use infrared coherent two-dimensional correlation spectroscopy (2DCS) to characterize the folding mechanism of the mini-protein Beta3s. In this study Beta3s was folded by molecular dynamics (MD) simulation and intermediate conformational ensembles were identified. The one and two-dimensional correlation spectrum was calculated for the intermediate and native states of the mini-protein. A direct structure-spectra relationship was determined by analysis of conformational properties and specific residue contributions. We identified the structural origin of diagonal and off-diagonal peaks in the 2DCS spectra for the native and intermediate conformational ensembles in the folding mechanism. This work supports the implementation of computational techniques in conjunction with experimental 2DCS to study the folding mechanism of proteins. In addition to exploring the folding mechanism the work presented here can be applied in combination with experiment to refine and validate current molecular dynamics force fields.PACS Codes: 87.15.Cc, 87.15.hm, 87.15.hp