Two-dimensional Raman Spectra Research Articles

Femtosecond Laser Induced Lattice Deformation in KTN Crystal.

In recent years, many novel optical phenomena have been discovered based on perovskite materials, but the practical applications are limited because of the difficulties of device fabrication. Here, we propose a method to directly induce localized lattice modification inside the potassium tantalate niobate crystal by using the femtosecond laser. This selective modification at the processed regions and the surrounding areas is characterized by two-dimensional Raman spectrum mapping. The spectrum variations corresponding to specific lattice vibration modes demonstrate the lattice structure deformation. In this way, the lattice expansion at the femtosecond laser irradiated regions and the lattice compression at the surrounding areas are revealed. Furthermore, surface morphology measurement confirms this lattice expansion and suggests the extension of lattice structure along the space diagonal direction. Moreover, the existence of an amorphization core is revealed. These modifications on the sample lattice can induce localized changes in physicochemical properties; therefore, this method can realize the fabrication of both linear diffraction and nonlinear frequency conversion devices by utilizing the novel optical responses of perovskite materials.

Sintering- and coking-resistant nickel catalysts embedded in boron nitride supported nickel aluminate spinels for dry reforming of methane

Dry reforming of methane (DRM) showed prospective to reduce greenhouse gas emissions if the sintering- and coking-resistant Ni-based catalysts were successfully developed. In this work, emergent Ni nanoparticles embedded in boron nitride supported nickel aluminate spinels were originally developed for DRM. The acidity and basicity derived from nickel aluminate spinels could enhance the decomposition of CH4 and adsorption of CO2, respectively, which provides an ensemble effect with emergent Ni nanoparticles and cooperatively enhances the intrinsic activity. Moreover, the strong metal-support interaction derived from exsolution effects as well as the increased contact interface between nickel aluminate spinels and boron nitride would suppress the metal sintering while the boron nitride could inhibit the carbon deposition. The two-dimensional Raman spectra demonstrated the deposited carbon featured with low graphitic degree in the catalysts. This work provides a novel approach for the development of sintering- and coking-resistant Ni-based catalysts for carbon dioxide reforming of methane.

An antifatigue and self-healable ionic polyurethane/ionic liquid composite as the channel layer for a low energy cost synaptic transistor

The developing trends of the traditional transistor include being synaptic and flexible, which could down low the energy cost and realize the daily application of flexible electronics, respectively. However, only bendable synaptic transistor has been developed, which is mainly restricted by the rigid components. Among which, the channel layer has not been replaced by any stretchable material yet. In this work, we synthesized an ionic polyurethane (iPU), and blend it with ionic liquid (IL) to construct a flexible channel layer with high transparency, stability, antifatigue and stretchability. The self-healing process can be completed at ambient and operating temperatures, and the self-healing speed was accelerated by the elevated temperature. The recovery ratio of the iPU/IL film was 95% after cyclic stretch to 50% strain for 100 cycles, which proved good antifatigue property. The stable combination between iPU and IL was verified by the two-dimensional Raman spectra, and the interconnected paths of IL around hard domains were discovered, which was probably due to the strong interaction between ions and hard segments. Spin coating technology was applied to construct the iPU/IL channel layer on a Si matrix, and an ionic conductive transistor was manufactured subsequently, in which the conductive mechanism is similar with a true synapse. The resulted transistor showed an on-off ratio of 2.1 × 102, and its energy dissipation was only 64 nJ when simulating the signal transmission of a nerve synapse, which was close to the power cost in other synaptic transistors.

Nonlinear Electromagnetic Interactions in Energetic Materials

We study the scattering of electromagnetic waves in anisotropic energetic materials. Nonlinear light-matter interactions in molecular crystals result in frequency-conversion and polarization changes. Applied electromagnetic fields of moderate intensity can induce these nonlinear effects without triggering chemical decomposition, offering a mechanism for non-ionizing identification of explosives. We use molecular dynamics simulations to compute such two-dimensional Raman spectra in the terahertz range for planar slabs made of PETN and ammonium nitrate. We discuss third-harmonic generation and polarization-conversion processes in such materials. These observed far-field spectral features of the reflected or transmitted light may serve as an alternative tool for stand-off explosive detection.

REDISTRIBUTION OF ALKALINE ELEMENTS IN ASSOCIATION WITH AQUEOUS ACTIVITY IN THE EARLY SOLAR SYSTEM

It is known that the Sayama meteorite (CM2) shows an extensive signature for aqueous alteration on the meteorite parent body, and that most of the primary minerals in the chondrules are replaced with phyllosilicates as the result of the aqueous alteration. In this paper, it is confirmed from the observation of two-dimensional Raman spectra that a part of olivine in a chondrule collected from the Sayama chondrite is serperntinized. Ion microprobe analysis of the chondrule showed that alkaline elements such as Rb and Cs are heterogeneously redistributed in the chondrule. The result of higher Rb and Cs contents in serpentinized phases in the chondrule rather than in other parts suggested the selective adsorption of alkaline elements into the serpentine in association with early aqueous activity on the meteorite parent body. Furthermore Ba isotopic analysis provided variations of 135Ba/138Ba and 137Ba/138Ba in the chondrule. This result was consistent with our previous isotopic data suggesting isotopic evidence for the existence of the presently extinct nuclide 135Cs in the Sayama meteorite, but the abundance of 135Cs in the solar system remains unclear because of large analytical uncertainties.

Analyzing atomic liquids and solids by means of two-dimensional Raman spectra in frequency domain

A practical method to evaluate the contributions of the nonlinear polarizability and anharmonicity of potentials from the experimental and simulation data by using double Fourier transformation is presented. In a Lennard-Jones potential system, an approximated expression of the fifth-order response function using the ratio between nonlinear polarizability and anharmonicity exhibits a good agreement with the results of the molecular dynamics simulation. In a soft-core case, the fifth-order Raman signal indicates that the system consists of the delocalized and localized modes, and only the delocalized mode affects the dramatic change of the fifth-order Raman response functions between solid and liquid phases through nonlinear polarizability.

Two-dimensional Raman spectra of atomic solids and liquids

We calculate third- and fifth-order Raman spectra of simple atoms interacting through a soft-core potential by means of molecular-dynamics (MD) simulations. The total polarizability of molecules is treated by the dipole-induced dipole model. Two- and three-body correlation functions of the polarizability at various temperatures are evaluated from equilibrium MD simulations based on a stability matrix formulation. To analyze the processes involved in the spectroscopic measurements, we divide the fifth-order response functions into symmetric and antisymmetric integrated response functions; the symmetric one is written as a simple three-body correlation function, while the antisymmetric one depends on a stability matrix. This analysis leads to a better understanding of the time scales and molecular motions that govern the two-dimensional (2D) signal. The 2D Raman spectra show novel differences between the solid and liquid phases, which are associated with the decay rates of coherent motions. On the other hand, these differences are not observed in the linear Raman spectra.

Assessment of composition and anisotropic elastic properties of secondary osteon lamellae

Measurement of the elastic properties of single osteon lamellae is still one of the most demanding tasks in bone mechanics to be solved. By means of site-matched Raman microspectroscopy, acoustic microscopy and nanoindentation the structure, chemical composition and anisotropic elasticity of individual lamellae in secondary osteons were investigated. Acoustic impedance images (911-MHz) and two-dimensional Raman spectra were acquired in sections of human femoral bone. The samples were prepared with orientations at various observation angles θ relative to the femoral long axis. Nanoindentations provided local estimations of the elastic modulus and landmarks necessary for spatial fusion of the acoustic and spectral Raman images. Phosphate ν 1 (961 cm −1) and amide I (1665 cm −1) band images representing spatial distributions of mineral and collagen were fused with the acoustic images. Acoustic impedance was correlated with the indentation elastic modulus E IT ( R 2 = 0.61 ). Both parameters are sensitive to elastic tissue anisotropy. The lowest values were obtained in the direction perpendicular to the femoral long axis. Acoustic images exhibit a characteristic bimodal lamellar pattern of alternating high and low impedance values. Since this undulation was not associated with a variation of the phosphate ν 1-band intensity in the Raman images, it was attributed to variations of the lamellar orientation. After threshold segmentation and conversion to elastic modulus the orientation and transverse isotropic elastic constants were derived for individual ensembles of apparent thin and thick lamellae. Our results suggest that this model represents the effective anisotropic properties of an asymmetric twisted plywood structure made of transverse isotropic fibrils. This is the first report that proves experimentally the ability of acoustic microscopy to map tissue elasticity in two dimensions with micrometer resolution. The combination with Raman microspectroscopy provides a unique way to study bone and mineral metabolism and the relation with mechanical function at the ultrastructural tissue level.

Qualitative features of the two-dimensional Raman spectrum in liquids

The theory presented earlier [J. Kim and T. Keyes, Phys. Rev. E 66, 051110 (2002)] is analyzed to determine the information available from the two-dimensional Raman spectrum R((5))(t(2),t(1)) in liquids. The known spectra are well represented by the sum of two products of ordinary time correlations predicted by the theory. The shape of R((5)) is related in general to the values of simple same-time averages and concepts amenable to physical intuition. Using standard models for the time correlations entering the theory, specific analytic expressions for the spectrum are obtained depending on two parameters and a time scale, and the behavior of the spectrum is mapped out in the parameter space.

Selecting the information content of two-dimensional Raman spectra in liquids

The wealth of information carried by the two-dimensional Raman spectra of liquids comes with a price. The signal arises from a mixture of two entirely different mechanisms, each of which reveals its own perspective on intermolecular dynamics. In this paper we analyze the dynamical origins and consequences of both of these mechanisms. By applying an instantaneous-normal-mode formalism to the two-dimensional Raman spectrum of a solution of CS2 dissolved in Xe, we find discernable differences in the specific molecular degrees of freedom and basic symmetries that contribute to each mechanism. We then show how these differences can be exploited to separate these mechanisms experimentally. In particular, we point out how it should be possible to use two-dimensional Raman spectroscopy to measure the explicitly anharmonic contributions to liquid dynamics without obscuring the results by simultaneously measuring the nonlinear coupling of the polarizability to that dynamics.

What Do We Learn from Two-Dimensional Raman Spectra by Varying the Polarization Conditions?

The signals obtained from the <TEX>$5^{th}$</TEX>-order (two-dimensional) Raman spectrum of a liquid can depend dramatically on the polarizations of the various light beams, but to date there has been no evidence presented that different polarization conditions probe any fundamentally different aspects of liquid dynamics. In order to explore the molecular significance of polarization we have carried out a molecular dynamics simulation of the <TEX>$5^{th}$</TEX>-order spectrum of a dilute solution of CS₂ in liquid Xe, perhaps the simplest system capable of displaying a full range of polarization dependencies. By focusing on the 5 distinct rotational invariants revealed by the different polarizations and by comparing our results with those from liquid Xe, a liquid whose spectrum has no significant polarization dependence, we discovered that the polarization experiments do, in fact, yield valuable microscopic information. With different linear combinations of the experimental response functions one can separate the part of the signal derived from the purely interaction-induced part of the many-body polarizability from the portion with the largest contributions from single-molecule polarizabilities. This division does not directly address the underlying liquid dynamics, but it significantly simplifies the interpretation of the theoretical calculations which do address this issue. We find that the different linear combinations differ as well in whether they exhibit nodal lines. Despite the absence of nodes with the atomic liquid Xe, observing the resilience of our solution's nodes when we artificially remove the anisotropy of our solute leads us to conclude that there is no direct connection between nodes and specifically molecular degrees of freedom.

The molecular origins of the two-dimensional Raman spectrum of an atomic liquid. I. Molecular dynamics simulation

As complex as it may seem, a two-dimensional (fifth-order) nonresonant Raman spectrum may provide one of the simplest ways to get at the character of intermolecular dynamics in liquids. Its status as an echo spectroscopy means that it should not only permit us to survey the intermolecular vibrations, it should allow us to ascertain the extent of their coherence. Arriving at a microscopic interpretation of those spectra, however, poses some genuine theoretical challenges. We describe here the first complete molecular dynamics simulation of such a spectrum. By using classical dynamics and focusing on liquid Xe, we find that we are able to produce a spectrum strikingly similar to the experimentally reported (nonmagic-angle) spectra of liquid CS2. We observe, in particular, that there is no discernable echo, suggesting that the dynamics is strongly homogeneously broadened. We turn, in a companion paper, to the implications of these results for instantaneousnormal-mode models of liquids.

Mode-coupling theory of the fifth-order Raman spectrum of an atomic liquid

A fully microscopic molecular hydrodynamic theory for the two-dimensional (fifth-order) Raman spectrum of an atomic liquid (Xe) is presented. The spectrum is obtained from a simple mode-coupling theory by projecting the dynamics onto bilinear pairs of fluctuating density variables. Good agreement is obtained in comparison with recently reported molecular dynamics simulation results. The microscopic theory provides an understanding of the timescales and molecular motions that govern the two-dimensional signal. Predictions are made for the behavior of the spectrum as a function of temperature and density. The theory shows that novel signatures in the two-dimensional Raman spectrum of supercritical and supercooled liquids are expected.

The third- and fifth-order nonlinear Raman response of liquid CS2 calculated using a finite field nonequilibrium molecular dynamics method

A finite field molecular dynamics (MD) method has been developed to calculate the off-resonant Raman response of liquids. The method has been used to calculate the third- and fifth-order optical responses of CS2. From the third-order response, the intensity of third-order cascading processes has been estimated. The calculated ratio between the fifth-order intensity and the intensity of the third-order cascading processes supports experimental observations, claiming that two-dimensional Raman spectra are dominated by third-order cascading processes. © 2000 American Institute of Physics.

Fifth-order two-dimensional Raman spectra of CS2 are dominated by third-order cascades

We have demonstrated that fifth-order stimulated Raman spectra of the intermolecular modes in CS2 are dominated by cascading third-order processes. Previous studies have successfully discriminated against a sequential cascading process, but did not account for parallel third-order cascades. All of our measured spectra were successfully simulated considering only cascades built directly from our measured third-order spectra. Using an appropriately chosen phase matching geometry we also measured the sequential cascade, which should exist with equal probability to the parallel cascade. When employing a phase matching geometry that provided substantial discrimination against all of the third-order cascades we were not able to measure any signal. We assign an upper limit for the true fifth-order signal of 2% of the cascaded signal.

Nanosecond Two-Dimensional Resonance Raman Correlation Spectroscopy of Benzil Radical Anion

Nanosecond two-dimensional resonance Raman spectroscopy was used to investigate the photochemistry of the production and decay of the radical anion of benzil in various solvents. A newly developed correlation formalism was applied to a set of time-resolved resonance Raman spectra of the benzil radical anion to generate two-dimensional Raman spectra. Unlike the 2D correlation method previously developed for IR spectroscopy, which was based on signals induced by a sinusoidally varying external perturbation, the new correlation formalism is generally applicable to the studies of any transient spectroscopic signals having an arbitrary waveform. This makes it ideally suited for the analysis of time-resolved spectroscopic signals following photoexcitation. 2D Raman spectra effectively accentuate certain useful information which is sometimes obscured in the original time-resolved spectra. Spectral intensity changes and peak shifts arising from the photochemical reaction processes were clearly observed by the synchronous and asynchronous correlation.