Theory of three-pulse photon echo spectroscopy with dual frequency combs

A theoretical analysis is carried out for the recently developed three-pulse photon echo spectroscopy employing dual frequency combs (DFC) as the light sources. In this method, the molecular sample interacts with three pulse trains derived from the DFC and the generated third-order signal is displayed as a two-dimensional (2D) spectrum that depends on the waiting time introduced by employing asynchronous optical sampling method. Through the analysis of the heterodyne-detected signal interferogram using a local oscillator derived from one of the optical frequency combs, we show that the 2D spectrum closely matches the spectrum expected from a conventional approach with four pulses derived from a single femtosecond laser pulse and the waiting time between the second and third field-matter interactions is given by the down-converted detection time of the interferogram. The theoretical result is applied to a two-level model system with solvation effect described by solvatochromic spectral density. The model 2D spectrum reproduces spectral features such as the loss of frequency correlation, dephasing, and spectral shift as a function of the population time. We anticipate that the present theory will be the general framework for quantitative descriptions of DFC-based nonlinear optical spectroscopy.Comment: 20 pages, 2 figures are included in the PDF fil

Theory of coherent two-dimensional vibrational spectroscopy

Two-dimensional (2D) vibrational spectroscopy has emerged as one of the most important experimental techniques useful to study the molecular structure and dynamics in condensed phases. Theory and computation have also played essential and integral roles in its development through the nonlinear optical response theory and computational methods such as molecular dynamics (MD) simulations and electronic structure calculations. In this article, we present the fundamental theory of coherent 2D vibrational spectroscopy and describe computational approaches to simulate the 2D vibrational spectra. The classical approximation to the quantum mechanical nonlinear response function is invoked from the outset. It is shown that the third-order response function can be evaluated in that classical limit by using equilibrium or non-equilibrium MD simulation trajectories. Another simulation method is based on the assumptions that the molecular vibrations can still be described quantum mechanically and that the relevant molecular response functions are evaluated by the numerical integration of the Schrodinger equation. A few application examples are presented to help the researchers in this and related areas to understand the fundamental principles and to use these methods for their studies with 2D vibrational spectroscopic techniques. In summary, this exposition provides an overview of current theoretical efforts to understand the 2D vibrational spectra and an outlook for future developments. c.Published under license by AIP Publishing

Ion Transport in Super-Concentrated Aqueous Electrolytes for Lithium-Ion Batteries

© 2021 American Chemical Society.Water-in-salt electrolytes (WiSEs) are a safer alternative to conventional organic electrolytes in battery applications because of their nonflammable nature. The electrochemical performance of these concentrated aqueous solutions of Li electrolytes critically depends on their high electrical conductivity at saturation. Still, the underlying molecular mechanism of Li-ion transport has not yet been clearly elucidated. To better understand this, we investigate four types of predominant atomic interactions and dynamics involving Li ions, anionic oxygen atoms (OT), and atoms of water molecules (OW) in the WiSE made of lithium bis(trifluoromethanesulfonyl)imide with molecular dynamics simulation and theoretical analysis. We present the distribution of atomic composition in the first solvation shells of these atoms and water molecules, thermodynamic stabilities of the contact atom pairs, their lifetimes based on the reactive flux time correlation function and the transition state theory analyses, and the correlation of the Li-ion mobility with the local solvation environment and its dynamics. We find that Li ions follow heterogeneous trajectories on the sub-nanosecond time scale consisting of distinctive water-rich and anion-rich segments, switching between a vehicle-type and a hopping-type mechanism in respective regions. The Li···OW contact pair is slightly more stable than Li···OT at saturation, and this subtle balance appears responsible for the fast Li-ion transport in this class of WiSE.11Nsciescopu

Ab initio Modeling of the Vibrational Sum-Frequency Generation Spectrum of Interfacial Water

Understanding the structural and dynamical features of interfacial water is of greatest interest in physics, chemistry, biology, and materials science. Vibrational sum-frequency generation (SFG) spectroscopy, which is sensitive to the molecular orientation and dynamics on the surfaces or at the interfaces, allows one to study a wide variety of interfacial systems. The structural and dynamical features of interfacial water at the air/water interface have been extensively investigated by SFG spectroscopy. However, the interpretations of the spectroscopic features have been under intense debate. Here, we report a simulated SFG spectrum of the air/water interface based on ab initio molecular dynamics simulations, which covers the OH stretching, bending, and libration modes of interfacial water. Quantitative agreement between our present simulations and the most recent experimental studies ensures that ab initio simulations predict unbiased structural features and electrical properties of interfacial systems. By utilizing the kinetic energy spectral density (KESD) analysis to decompose the simulated spectra, the spectroscopic features can then be assigned to specific hydrogen-bonding configurations of interfacial water molecules. © 2019 American Chemical Societ

An Improved Polarflex Water Model

The three-site polarizable and flexible water potential employing the multistate empirical valence bond (MS-EVB) description for the electronic polarizability [A. E. Lefohn, M. Ovchinnikov, and G. A. Voth, J. Phys. Chem. 105, 6628 (2001)] has been modified for better reproduction of liquid water properties under ambient conditions. The improvement of the potential model was accomplished by (i) replacing the point charge distribution associated with the atomic interaction sites in the original model with a diffuse Gaussian charge distribution and (ii) reparameterizing the molecular geometry, components of electronic polarizability tensor, the Lennard-Jones parameters, and the widths of the Gaussian charge distribution. Static and dynamic properties, such as the intermolecular interaction energy, radial distribution function, diffusion constant, and dielectric constant, have been used in the model parameterization and the resulting model well reproduces the experimental data. A closely related rigid version of the model is also developed and compared with the flexible one. For computational efficiency, the extended Lagrangian algorithm for the electronic degrees of freedom has been implemented in the MS-EVB molecular dynamics simulation and utilized in the calculations. Relations between the new features of the potential model, such as the Gaussian charge distribution and the anisotropy in the electronic polarizability, and the liquid properties are established and discussed

Modeling and Simulation of Concentrated Aqueous Solutions of LiTFSI for Battery Applications

© 2020 American Chemical Society. We propose a new nonpolarizable molecular mechanics force field for concentrated aqueous solutions of lithium bistriflylimide (LiTFSI), a promising candidate for battery applications. The model describes the TFSI anion by GAFF2-based Lennard-Jones parameters and new MP2-optimized intramolecular parameters. They are combined with existing models of Li+ and water (TIP4P-Ew). The charge transfer and electronic polarization effects between oppositely charged ions, depicted with ionic charge scaling by 0.8 in the present model, turn out to be crucial for the correct prediction of solution density and diffusivity of ions and water molecules over the concentration range from 1 to 21 m. Molecular dynamics simulations using this new model reveal that TFSI- interacts with Li+ predominantly through its sulfonyl oxygens (O-T) and that O-T can readily form hydrogen bonds (H-bonds) with water molecules. Moreover, a single Li+ is, on average, coordinated by approximately four oxygen atoms, either O-T or O-W, at all concentrations studied. These observations indicate that the extended and heterogeneous H-bond network formed by water and O-T facilitates the solvation and ion conduction of Li+ in concentrated aqueous solutions of LiTFSI. The present modeling approach is applicable to a wide range of electrolyte solutions11sci

Theory of coherent two-dimensional vibrational spectroscopy

Two-dimensional (2D) vibrational spectroscopy has emerged as one of the most important experimental techniques useful to study the molecular structure and dynamics in condensed phases. Theory and computation have also played essential and integral roles in its development through the nonlinear optical response theory and computational methods such as molecular dynamics (MD) simulations and electronic structure calculations. In this article, we present the fundamental theory of coherent 2D vibrational spectroscopy and describe computational approaches to simulate the 2D vibrational spectra. The classical approximation to the quantum mechanical nonlinear response function is invoked from the outset. It is shown that the third-order response function can be evaluated in that classical limit by using equilibrium or non-equilibrium MD simulation trajectories. Another simulation method is based on the assumptions that the molecular vibrations can still be described quantum mechanically and that the relevant molecular response functions are evaluated by the numerical integration of the Schrodinger equation. A few application examples are presented to help the researchers in this and related areas to understand the fundamental principles and to use these methods for their studies with 2D vibrational spectroscopic techniques. In summary, this exposition provides an overview of current theoretical efforts to understand the 2D vibrational spectra and an outlook for future developments. c.Published under license by AIP Publishing

Dual frequency-comb spectroscopy of chromophores in condensed phases

Femtosecond time-resolved spectroscopy and frequency-comb spectroscopy have been individually developed to achieve better time and frequency resolutions, respectively. The two spectroscopic techniques have been developed for different systems, even though they use mode-locked laser in common. Recently, there was an interesting merge of the two techniques into a dual frequency-comb (DFC) spectroscopy, resulting in a new femtosecond spectroscopy with simple instrumentation and high data acquisition speed compared to conventional femtosecond spectroscopic techniques. By slightly detuning the repetition rates of two phase-locked frequency-comb lasers, both automatic time-delay scanning and parallel data recording with single point detectors are possible. Thus, we anticipate that the DFC spectroscopy would allow one to expand the application limits of the conventional femtosecond spectroscopic methods. In this Perspective article, we provide reviews of linear and nonlinear DFC spectroscopy theory and applications with a perspective on the development of coherent multidimensional frequency-comb spectroscopy. © 2018 Elsevier B.V. All rights reserved.11sciescopu