Optical Kerr Effect Spectroscopy Research Articles

Lifting Hofmeister's Curse: Impact of Cations on Diffusion, Hydrogen Bonding, and Clustering of Water.

Water plays a role in the stability, reactivity, and dynamics of the solutes that it contains. The presence of ions alters this capacity by changing the dynamics and structure of water. However, our understanding of how and to what extent this occurs is still incomplete. Here, a study of the low-frequency Raman spectra of aqueous solutions of various cations by using optical Kerr-effect spectroscopy is presented. This technique allows for the measurement of the changes that ions cause in both the diffusive dynamics and the vibrations of the hydrogen-bond structure of water. It is found that when salts are added, some of the water molecules become part of the ion solvation layers, while the rest retain the same diffusional properties as those of pure water. The slowing of the dynamics of the water molecules in the solvation shell of each ion was found to depend on its charge density at infinite dilution conditions and on its position in the Hofmeister series at higher concentrations. It is also observed that all cations weaken the hydrogen-bond structure of the solution and that this weakening depends only on the size of the cation. Finally, evidence is found that ions tend to form amorphous aggregates, even at very dilute concentrations. This work provides a novel approach to water dynamics that can be used to better study the mechanisms of solute nucleation and crystallization, the structural stability of biomolecules, and the dynamic properties of complex solutions, such as water-in-salt electrolytes.

Time-Resolved Two-Photon Spectroscopy: Applications in Quantum Physics and Photonics

Time-resolved two-photon spectroscopy (TR2PS) is a powerful technique for investigating the dynamics of photo-excited states in materials with fem to second time resolution. TR2PS has been applied to a wide range of materials, including inorganic and organic semiconductors, perovskites, and magnetic materials. This technique can provide valuable insights into the underlying physics and chemistry of these materials, as well as their potential applications in optoelectronics, energy conversion, and quantum technologies. In this review, we compare TR2PS with other time-resolved spectroscopy techniques, including resolved fluorescence spectroscopy, transient absorption spectroscopy, time-resolved Raman spectroscopy, time-resolved X-ray diffraction, and time-resolved optical Kerr effect spectroscopy. We also discuss the different materials that have been studied using TR2PS, including the types of dynamics that have been observed in each material, potential applications, and challenges associated with studying these materials. Finally, we compare different data analysis and interpretation techniques for TR2PS data, considering factors such as the level of expertise required, computational resources needed, and types of information that can be obtained using each technique. Overall, this review highlights the versatility and importance of TR2PS in materials science and provides a comprehensive overview of the current state of research in this field.

Dynamics of Acrylamide Hydrogels, Polymers, and Monomers in Water Measured with Optical Heterodyne-Detected Optical Kerr Effect Spectroscopy.

The ultrafast dynamics of acrylamide monomers (AAm), polyacrylamide (PAAm), and polyacrylamide hydrogels (PAAm-HG) in water were studied using optical heterodyne-detected optical Kerr effect (OHD-OKE) spectroscopy. Previous ultrafast infrared (IR) measurements of the water dynamics showed that at the same concentration of the acrylamide moiety, AAm, PAAm, and PAAm-HG exhibited identical water dynamics and that these dynamics slowed with increasing concentration. In contrast to the IR measurements, OHD-OKE experiments measure the dynamics of both the water and the acrylamide species, which occur on different time scales. In this study, the dynamics of all the acrylamide systems slowed with increasing concentration. We found that AAm exhibits tetraexponential decays, the longest component of which followed Debye-Stokes-Einstein behavior except for the highest concentration, 40% (w/v). Low concentrations of PAAm followed a single power law decay, while high concentrations of PAAm and all concentrations of PAAm-HG decayed with two power laws. The highest concentrations, 25% and 40%, of PAAm and PAAm-HG showed nearly identical dynamics. We interpreted this result as reflecting a similar extent of chain-chain interactions. At low concentrations, PAAm displays non-Markovian, single-chain dynamics (single power law), but PAAm displays entangled chain-chain interactions at high concentrations (two power laws). PAAm-HG has chain-chain interactions at all concentrations that arise from the cross-linking. At high concentrations, the dynamics of the entangled of PAAm become identical within error as those of the cross-linked PAAm-HG.

Ultrafast Dynamics of Solute Molecules Probed by Resonant Optical Kerr Effect Spectroscopy.

Ultrafast molecular dynamics in fluids is of great importance in many biological and chemical systems. Although such dynamics in bulk liquids has been explored by various methods, experimental tools that unveil the dynamics of solvated solutes are limited. In this work, we have developed resonant optical Kerr effect spectroscopy (ROKE), which is an analogue of optical Kerr effect spectroscopy that measures the reorientational relaxation of a dilute solute in solution. By adjusting the pump and probe wavelengths at the resonant absorption band of a solute, the time response of the solute was distinguished easily from the negligible signal of the solvent. The heterodyne detection of ROKE enables the determination of reorientational relaxation time constants with an accuracy of 2.6%. The signal-to-noise ratio was high enough (average ∼26.7) to obtain an adequate signal from even a 10 μM solution. Thus, ROKE is a powerful tool to study solute dynamics with high sensitivity in a broad range of applications.

Optical Kerr Effect and Structural Tetrahedrality of Supercooled Water at Ambient Pressure

The correlation between the optical Kerr effect (OKE) spectroscopy of supercooled water at ambient pressure and its structural tetrahedrality was investigated by contrasting simulation results of two non-polarizable water models modified with the same collective polarizability, which involves intrinsic molecular polarizability and induced polarizability arising from interactions between molecular dipoles. The tetrahedrality of water structure was typified with the second-peak maximum in the pair distribution function of oxygens and the fraction of molecules, which and their neighbours up to the second hydration shell all have four H-bond coordinators. Our results indicate that the intermolecular vibrational band in the OKE spectrum of supercooled water is considerably correlated to its structural tetrahedrality.

Optical Kerr Effect of Liquid Acetonitrile Probed by Femtosecond Time-Resolved X-ray Liquidography.

Optical Kerr effect (OKE) spectroscopy is a method that measures the time-dependent change of the birefringence induced by an optical laser pulse using another optical laser pulse and has been used often to study the ultrafast dynamics of molecular liquids. Here we demonstrate an alternative method, femtosecond time-resolved X-ray liquidography (fs-TRXL), where the microscopic structural motions related to the OKE response can be monitored using a different type of probe, i.e., X-ray solution scattering. By applying fs-TRXL to acetonitrile and a dye solution in acetonitrile, we demonstrate that different types of molecular motions around photoaligned molecules can be resolved selectively, even without any theoretical modeling, based on the anisotropy of two-dimensional scattering patterns and extra structural information contained in the q-space scattering data. Specifically, the dynamics of reorientational (libration and orientational diffusion) and translational (interaction-induced motion) motions are captured separately by anisotropic and isotropic scattering signals, respectively. Furthermore, the two different types of reorientational motions are distinguished from each other by their own characteristic scattering patterns and time scales. The measured time-resolved scattering signals are in excellent agreement with the simulated scattering signals based on a molecular dynamics simulation for plausible molecular configurations, providing the detailed structural description of the OKE response in liquid acetonitrile.

Search for the origin of synergistic solvation in methanol/chloroform mixture using optical Kerr effect spectroscopy

The synergistic effect of methanol/chloroform mixture on solute solvation was earlier attributed to the formation of an extended hydrogen-bonding network in the mixture. Such a network was proposed to be weak through the solute dipole-moment dependent experiment. In this study, we search for signatures of such interactions using the femtosecond time-resolved optical Kerr effect spectroscopy, which is sensitive to the ultrafast intermolecular dynamics. We observed a 2-3-fold retardation of the orientational diffusion time of chloroform molecules that is attributed to the formation of hydrogen-bonds with methanol. Our frequency domain analysis in terms of the excess reduced spectral density and partial reduced spectral density allowed us to detect a hydrogen-bond stretching band around 90 cm−1 (with 107 cm−1 natural frequency) associated with methanol molecules simultaneously accepting a hydrogen-bond from chloroform and donating a hydrogen-bond to another methanol molecule. Additionally, in an auxiliary mixture used in this study, where chloroform is replaced with carbon tetrachloride, we found evidence of carbon tetrachloride-methanol halogen-bond formation; however, its signature in the spectra is much weaker than in the case of chloroform-methanol hydrogen-bonding.

Optical Anisotropy and Phase Transitions in Lead Halide Perovskites.

Optical anisotropy originates from crystalline structures with low symmetry and governs the polarization-dependent light propagation. Optical anisotropy is particularly important to lead halide perovskites that have been under intense investigation for optoelectronic and photonic applications, as this group of materials possesses rich structural phases that deviate from the high-symmetry cubic phase. Here we apply 2D optical Kerr effect spectroscopy to quantify the optical anisotropy in single-crystal methylammonium lead bromide (MAPbBr3). We determine the strong photon energy dependence of optical anisotropy near the band gap and show the dramatic change in optical anisotropy across phase transitions. We correlate the optical anisotropy with the structural anisotropy and demonstrate the tuning of optical anisotropy by alloyed CsxMA1-xPbBr3 perovskite crystals.

Orientational and low-frequency (0–450 cm−1) dynamics of methyl methacrylate: OHD-RIKES measurements and DFT calculations

This paper presents results of a study in which optical heterodyne-detected Raman-induced Kerr effect spectroscopy (OHD-RIKES) and density functional theory (DFT) were used to glean information about the low-frequency (0–450 cm−1) intramolecular and intermolecular dynamics and picosecond reorientational dynamics of methyl methacrylate (MMA) as a function of temperature. Based on DFT calculations, the two intramolecular bands at 220 and 380 cm−1 in this spectral region are assigned, respectively, to the (O)CH3 torsional and C=C out-of-plane modes of the cis-conformer of MMA. In the 2–40 ps time range, the OHD-RIKES response of MMA can be well fit by a biexponential decay function with a fast component (relaxation time τ1) associated with intermolecular dynamics, and a slow component (relaxation time τ2) associated with collective reorientation. The temperature dependence of τ2 can be understood in terms of a modified Debye-Stoke-Einstein (DSE) equation with the rotation of MMA molecules being governed by slip boundary conditions, which is consistent with MMA being a non‑hydrogen-bonded liquid. τ1 and τ2 are found to be linearly correlated with each other, which is attributed to τ1 being dominated by structural relaxation and therefore to motional narrowing. The low-frequency broad band in the 0–200 cm−1 region of the Kerr spectrum of MMA was analyzed in terms of a multicomponent line-shape model, with the sum of the first three components being primarily associated with the intermolecular modes of MMA. With increasing temperature, the intermolecular band shifts to lower frequency and narrows with increasing temperature. The shift to lower frequency is attributed to the softening of the effective intermolecular potential due to decreasing density with increasing temperature and the narrowing of the intermolecular band due to motional narrowing. Finally, it is shown that the Kerr spectrum of MMA provides support for the interpretation of the main band of the low-frequency reduced-Raman spectrum of poly(methyl methacrylate) (PMMA) being due to the librational motion of the monomer units in the polymer. This explanation is physically reasonably based on fits of the multicomponent line-shape model to the spectra of PMMA and MMA. It is proposed that this librational motion is associated with the torsional motion of the methoxycarbonyl pendant group on the polymer chain.

Low-Frequency (Gigahertz to Terahertz) Depolarized Raman Scattering Off n-Alkanes, Cycloalkanes, and Six-Membered Rings: A Physical Interpretation.

Molecular liquids have long been known to undergo various distinct intermolecular motions, from fast librations and cage-rattling oscillations to slow orientational and translational diffusion. However, their resultant gigahertz to terahertz spectra are far from simple, appearing as broad shapeless bands that span many orders of magnitude of frequency, making meaningful interpretation troublesome. Ad hoc spectral line shape fitting has become a notoriously fine art in the field; a unified approach to handling such spectra is long overdue. Here we apply ultrafast optical Kerr-effect (OKE) spectroscopy to study the intermolecular dynamics of room-temperature n-alkanes, cycloalkanes, and six-carbon rings, as well as liquid methane and propane. This work provides stress tests and converges upon an experimentally robust model across simple molecular series and range of temperatures, providing a blueprint for the interpretation of the dynamics of van der Waals liquids. This will enable the interpretation of low-frequency spectra of more complex liquids.

Temperature-Dependent Ultrafast Solvation Response and Solute Diffusion in Acetamide-Urea Deep Eutectic Solvent.

In the present paper, we have studied the temperature dependence of translational diffusion and solvation dynamics of a dissolved dipolar dye in the nonionic acetamide-urea deep eutectic solvent (DES), to characterize the viscosity coupling of the measured relaxation times and verify the dynamical heterogeneity aspect of this medium. Three different time-resolved experimental techniques have been employed for this purpose: fluorescence correlation spectroscopy, transient absorption (TA) spectroscopy, and optical Kerr effect (OKE) spectroscopy. The first method provides the proof that the translational diffusion time of a solute in acetamide-urea DES [fCH3CONH2 + (1 - f)CO(NH2)2, f = 0.6] exhibits a fractional viscosity dependence, with exponent 0.758, which, when compared with the viscosity-diffusion relationship for the same solute in common molecular solvents, suggests moderate deviation from the Stokes-Einstein relation. Stokes shift dynamics of a solvatochromic dye, 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran in this DES, followed via femtosecond TA measurements, have been found to be triexponential in nature and dominated by a ∼100 fs component. The other two components, which contribute to a total dynamic Stokes shift magnitude of ∼2500 cm-1, are characterized by time constants in the ∼5 and ∼50 ps regimes. Subsequent comparison with the femtosecond OKE measurements suggests that the relatively slower picosecond solvation components originate from the rapid reorientation of the solvent molecules, while the subpicosecond solvation response arises from the participation of the collective low-frequency solvent modes (such as intermolecular vibrations and librations). We find that the rotational diffusion lifetimes also exhibit fractional power dependence on medium viscosity and thus deviate from the Stokes-Einstein-Debye pprediction. All of these results therefore suggest that the nonionic acetamide-urea DES is a moderately heterogeneous medium.

Fine structures in Raman spectra of tetrahedral tetrachloride molecules in femtosecond coherent spectroscopy.

Herewith, we present fast Fourier transforms of time resolved signals, obtained by use of the femtosecond transient transmission (TT) spectroscopy, for three tetrachlorides, CCl4, SiCl4, and GeCl4, and chloroform, CHCl3. Due to coherent excitation of molecules, the isotopic splitting of their spectral bands in the range of symmetric stretching vibration can be observed with high resolution not available in spontaneous Raman scattering. The intensity distribution in the isotopic fine structure pattern appears to differ for various studied molecules, which is explained by the role of intermolecular interactions and the local order of molecules in the liquids. In particular, in SiCl4, the vibrational band exhibits anomalous ratios of the peak amplitudes, which do not agree with the natural abundance of the isotopologues. Using the simple oscillatory model of the liquid and fitting theoretical curves to the experimental results, we have been able to find the intermolecular force constants for all three liquids and to formulate the conclusion that the anomalous spectral pattern in SiCl4 results from strong interactions between the closest Cl atoms belonging to adjacent molecules. Application of the windowed Fourier transform enables us to study the dynamics of intermolecular interactions. The strength of intermolecular interactions in CCl4, SiCl4, and GeCl4, found by the TT technique, is compared with the results obtained by means of the femtosecond optical Kerr effect spectroscopy.

Dynamics and Microstructures of Nicotine/Water Binary Mixtures near the Lower Critical Solution Temperature.

The orientational dynamics and microscopic structures of nicotine/water binary mixtures near the system's lower critical solution temperature (LCST) were elucidated using optical heterodyne-detected optical Kerr effect (OHD-OKE) spectroscopy, nuclear magnetic resonance correlation spectroscopy (NMR COSY), first-principles calculations, and molecular dynamics simulations. Water concentrations were investigated from zero to close to pure water. At temperatures below the LCST, OHD-OKE experiments measured an anomalous slowing as the phase transition concentration was approached. At moderate concentrations and low temperatures, intermolecular cross-peaks between nicotine and water molecules were observed in the COSY spectra, demonstrating the formation of structures that persist for milliseconds. These results suggest that pair correlations contribute to the slowdown in the OHD-OKE data at moderate water concentrations. First-principles calculations revealed that intermolecular hydrogen bonding coordination between nitrogen atoms in pyridine moieties and water lowers the energy barriers for the reorientations of the two nicotine rings. Atomistic simulations demonstrate that with increasing water concentration, hydrogen bonding interactions between pyridine moieties and water molecules first increase and then decrease with a maximum at moderate water concentrations. These experimental and computational characterizations of the dynamics of nicotine molecules are attributed to the distinct configurations of water molecules around the pyridine ring moieties in nicotine molecules.

Molecular dynamics simulations for optical Kerr effect of TIP4P/2005 water in liquid and supercooled states

The optical Kerr effect (OKE) spectroscopy measured with heterodyne detection (HD) is a useful tool to provide information regarding intermolecular vibrations and structural relaxations in liquid water. Recently, the measurements of the OKE spectroscopy have been extended to the supercooled regime of water. Though the measured results can be well described by using a phenomenological model, the time-resolved OKE spectroscopy of liquid and supercooled water still need a comprehensive understanding. In this paper, we investigated the OKE nuclear response functions of this peculiar liquid and their reduced spectral densities by performing molecular dynamics simulations with the TIP4P/2005 water model. The collective polarizability of water was computed via a dipolar induction scheme, which involves the intrinsic polarizability and the first-order hyperpolarizability tensor of water molecule. Our simulation results were qualitatively consistent with the HD-OKE experimental observations for displaying that the polarizability anisotropy relaxation of supercooled water in the high-density liquid phase was fragile-like by following a stretching exponential decay with an exponent βs insensitive to temperature and the temperature dependence of the relaxation time exhibited a power-law divergence at a singular temperature Ts with a critical exponent γs. Indicated by our quantitative results, Ts was predominately determined by the structural arrest, but βs and γs were not only related to the structural relaxation but also influenced by the collective polarizability of the liquid. For all investigations, the effects due to the first-order hyperpolarizability tensor were examined.

The influence of hydrophilicity on the orientational dynamics and structures of imidazolium-based ionic liquid/water binary mixtures.

The orientational dynamics and microscopic structures of imidazolium-based ionic liquids of varying hydrophilicity were investigated using optical heterodyne-detected optical Kerr effect (OHD-OKE) spectroscopy and atomistic simulations. Hydrophilicity was tuned via anion selection, cation alkyl chain length, and the addition of a strong hydrogen bond donor on the cation (protic ionic liquid). In the hydrophobic samples, which saturate at relatively low water concentration, OHD-OKE data display Debye Stokes Einstein (DSE) behavior as a function of water concentration. The DSE behavior indicates that the microstructures of the hydrophobic ionic liquid/water mixtures do not fundamentally change as a function of water concentration. The hydrophilic samples have two regimes of different DSE behaviors demonstrating the presence of two structural regimes depending on water concentration. These experimental results indicate that in hydrophilic ionic liquid/water samples, significant structural changes occur to accommodate high water concentrations, while hydrophobic samples become water saturated because the restructuring of local ionic structures is unfavorable. Atomistic simulations show that the local ionic microstructures experience distinct changes in these hydrophilic ionic liquid/water binary samples because of the delicate interplay of intermolecular interactions among imidazolium cations, hydrophilic anions, and water molecules.

Frustration vs Prenucleation: Understanding the Surprising Stability of Supersaturated Sodium Thiosulfate Solutions.

Gibbs classical nucleation theory predicts that a supersaturated solution will have transient nuclei that flitter in and out of existence. Only when one of these nuclei becomes larger than a critical size, will the solution crystalize. Recently, nonclassical nucleation theories have invoked the presence of prenuclei possibly associated with a liquid-liquid phase separation. However, there are few experimental observations of such prenuclei. Here, we use ultrafast optical Kerr-effect spectroscopy to measure the temperature-dependent low-frequency (sub-gigahertz to terahertz) anisotropic Raman spectra of supersaturated aqueous sodium thiosulfate solutions. Clear evidence of clusters is obtained in the spectra. However, on the basis of the inferred stability of these clusters, it appears that they frustrate rather than promote the formation of crystals. This would explain the surprising stability of supersaturated sodium thiosulfate and similar solutions.

A simulation study of CS2 solutions in two related ionic liquids with dications and monocations.

Atomistic simulations of solutions of CS2 in an ionic liquid, , with a divalent cation and in the corresponding ionic liquid with a monovalent cation, [C4C1im][NTf2], were carried out. The low-frequency librational density of states of the CS2 was of particular interest in view of recent optical heterodyne-detected Raman-induced Kerr effect spectroscopy (OHD-RIKES). Compared to the monocation ionic liquid, the maximum shifts to higher frequencies in the dication ionic liquid under ambient conditions, but was found to be significantly pressure-dependent. CS2 molecules lie above and below the plane of the imidazolium rings and found to be close to the butyl tails of the monocation. The diffusion rates and embedding energies of solvent ions and CS2 in the two ionic liquids were measured.

The influence of interactions between isotopoloques on coherent, ultrafast vibrational dynamics of liquid C2Cl4.

The dynamics of intramolecular and intermolecular vibrations in liquid tetrachloroethylene are studied for the first time by use of femtosecond time-resolved techniques, such as transient transmission spectroscopy and optical Kerr effect spectroscopy. Fourier transforms of time signals are compared with spontaneous Raman spectra for both isotropic and anisotropic components. The isotopic effect resulting from natural abundance of chlorine isotopes manifests itself as splitting of the isotropic spectra of intramolecular symmetric vibrations. Application of windowed Fourier transform enables us to study the dynamics of both spectral responses in real time and to analyze the role of intermolecular interactions on the coherence in the system. In order to describe the dynamics of molecules in a liquid and to explain the experimental results, we use a simple theoretical model taking into account intermolecular interactions, which allowed us to find vibrational and rotational life times.

Optical Kerr effect spectroscopy of CS2 in monocationic and dicationic ionic liquids: insights into the intermolecular interactions in ionic liquids.

A comparative study of the intermolecular dynamics of CS2 in monocationic and dicationic ionic liquids (ILs) was performed using optical heterodyne-detected Raman-induced Kerr effect spectroscopy (OHD-RIKES). The reduced spectral densities (RSDs) of mixtures of CS2 in 1-alkyl-3-methylimidazolium bis[(trifluoromethane)sulfonyl]amide ([CnC1im][NTf2] for n = 3-5) and 1,2n-bis(3-methylimidazolium-1-yl) alkane bis[(trifluoromethane)sulfonyl]amide ([(C1im)2C2n][NTf2]2 for n = 3-5) were investigated as a function of concentration at 295 K. An additivity model was used to obtain the CS2 contribution to the RSD of a mixture in the 0-200 cm-1 region. One of the aims of this study is to show how CS2 can be used as a probe of intermolecular/interionic interactions in ILs. The concentrations were chosen such that the CS2-to-imidazolium ring mole fraction of a mixture with [(C1im)2C2n][NTf2]2 (DIL(2n)) is the same as that of a mixture with [CnC1im][NTf2] (MIL(n)). As found previously for CS2 in monocationic ILs, the intermolecular spectrum of CS2 in dicationic ILs is lower in frequency and narrower than that of neat CS2. The new result is that the intermolecular spectrum of CS2 is higher in frequency in DIL(2n) than in the corresponding MIL(n), indicating that CS2 molecules experience a stiffer potential in dicationic ILs than in monocationic ILs. The intermolecular dynamics of CS2 being higher in frequency in DIL(2n) than in MIL(n) is consistent with recent molecular dynamics simulations (Lynden-Bell and Quitevis, J. Chem. Phys., 2018, 148, 193844) that show the stiffer potential is the result of greater confinement of CS2 in DIL(2n) than in MIL(n). We also show in this study how effects due to dilution and the intermolecular potential seen by a solute molecule in solution are unraveled.

Empirical Analysis of Optical Kerr Effect Spectra: A Case for Constraint.

Ultrafast optical Kerr effect (OKE) spectroscopy is a widely used method for studying the depolarized, Raman-active intermolecular dynamics of liquids. Through appropriate manipulation of OKE data, it is possible to determine the reduced spectral density (RSD), which is the Bose-Einstein-corrected, low-frequency Raman spectrum with the contribution of diffusive reorientation removed. OKE RSDs for van der Waals liquids can often be fit well to an empirical function that is the sum of a Bucaro-Litovitz function and an antisymmetrized Gaussian (AG). Although these functions are not directly representative of specific intermolecular dynamics, the AG fit parameters can provide useful insights into the microscopic properties of liquids. Here we show that fits using the AG function are typically not well-determined, and that equally good results can be obtained with a wide range of fitting parameters. We propose the use of a physically motivated constraint on the amplitude of the AG function, and demonstrate that this constraint leads to more intuitive trends in the fit parameters for temperature-dependent RSDs in 1,3,5-trifluorobenzene and hexafluorobenzene.