Vibrational spectroscopy is an essential tool in chemical analyses, biological assays, and studies of functional materials. Over the past decade, various coherent nonlinear vibrational spectroscopic techniques have been developed and enabled researchers to study time-correlations of the fluctuating frequencies that are directly related to solute-solvent dynamics, dynamical changes in molecular conformations and local electrostatic environments, chemical and biochemical reactions, protein structural dynamics and functions, characteristic processes of functional materials, and so on. In order to gain incisive and quantitative information on the local electrostatic environment, molecular conformation, protein structure and inter-protein contacts, ligand binding kinetics, and electric and optical properties of functional materials, a variety of vibrational probes have been developed and site-specifically incorporated into molecular, biological, and material systems for time-resolved vibrational spectroscopic investigation. However, still, an allencompassing theory that describes the vibrational solvatochromism, electrochromism, and dynamic fluctuation of vibrational frequencies has not been completely established mainly due to the intrinsic complexity of intermolecular interactions in condensed phases. In particular, the amount of data obtained from the linear and nonlinear vibrational spectroscopic experiments has been rapidly increasing, but the lack of a quantitative method to interpret these measurements has been one major obstacle in broadening the applications of these methods. Among various theoretical models, one of the most successful approaches is a semi-empirical model generally referred to as the vibrational spectroscopic map that is based on a rigorous theory of intermolecular interactions. Recently, genetic algorithm, neural network, and machine learning approaches have been applied to the development of vibrational solvatochromism theory. In this review, we provide comprehensive descriptions of the theoretical foundation and various examples showing its extraordinary successes in the interpretations of experimental observations. In addition, a brief introduction to a newly created repository website (http://frequencymap.org) for vibrational spectroscopic maps is presented. We anticipate that a combination of the vibrational frequency map approach and state-of-theart multidimensional vibrational spectroscopy will be one of the most fruitful ways to study the structure and dynamics of chemical, biological, and functional molecular systems in the future.

The CO2ν3 asymmetric stretching mode is established as a vibrational chromophore for ultrafast two-dimensional infrared (2D-IR) spectroscopic studies of local structure and dynamics in ionic liquids, which are of interest for carbon capture applications. CO2 is dissolved in a series of 1-butyl-3-methylimidazolium-based ionic liquids ([C4C1im][X], where [X](-) is the anion from the series hexafluorophosphate (PF6 (-)), tetrafluoroborate (BF4 (-)), bis-(trifluoromethyl)sulfonylimide (Tf2N(-)), triflate (TfO(-)), trifluoroacetate (TFA(-)), dicyanamide (DCA(-)), and thiocyanate (SCN(-))). In the ionic liquids studied, the ν3 center frequency is sensitive to the local solvation environment and reports on the timescales for local structural relaxation. Density functional theory calculations predict charge transfer from the anion to the CO2 and from CO2 to the cation. The charge transfer drives geometrical distortion of CO2, which in turn changes the ν3 frequency. The observed structural relaxation timescales vary by up to an order of magnitude between ionic liquids. Shoulders in the 2D-IR spectra arise from anharmonic coupling of the ν2 and ν3 normal modes of CO2. Thermal fluctuations in the ν2 population stochastically modulate the ν3 frequency and generate dynamic cross-peaks. These timescales are attributed to the breakup of ion cages that create a well-defined local environment for CO2. The results suggest that the picosecond dynamics of CO2 are gated by local diffusion of anions and cations.

Excitons, electron-hole pairs bound by the Coulomb potential, are fundamental quasiparticles of coherent light-matter interaction energizing processes from photosynthesis to optoelectronics 1−5 . Excitons are observed in semiconductors, and their existence is implicit in the quantum theory of metals, yet their appearance is tenuous due to the screening of the Coulomb interaction on few femtosecond timescale 6−8 .Here we present direct evidence for the dominant transient excitonic response at a Ag(111) surface, which precedes the full screening of the Coulomb interaction, in the course of a three-photon photoemission process with <15 femtosecond laser pulses. Electron-hole pair interaction through the excitonic response introduces coherent quasiparticle correlations beyond the single-particle description of the optics of metals, which dominate the multi-photon photoemission process.Reflection of light has made metal mirrors valued optical instruments since the bronze age 9 . At the macroscopic level the coherent optical response of a metallic surface is well described by the classical Maxwell's equations. At the quantum level, a photon interacting with a metal surface polarizes an electron-hole (e-h) pair to create an exciton-polariton, the arXiv:1405.0977v2 [cond-mat.mes-hall]

Ultrafast two-photon photoemission has been used to study electron solvation at two-dimensional metal/polar-adsorbate interfaces. The molecular motion that causes the excess electron solvation is manifested as a dynamic shift in the electronic energy. Although the initially excited electron is delocalized in the plane of the interface, interactions with the adsorbate can lead to its localization. A method for determining the spatial extent of the localized electron in the plane of the interface has been developed. This spatial extent was measured to be on the order of a single adsorbate molecule.

We demonstrate a method to collect purely absorptive three-dimensional ͑3D͒ fifth-order vibrational spectra on the model system CO 2 in H 2 O. The six beam interferometer is described, as well as a method to experimentally determine the phase of the 3D spectrum. The measured spectra agree very well with simulations of the data based on the cumulant expansion. There are five peaks corresponding to different paths up and down the vibrational ladder. The positions, signs, and amplitudes of the peaks agree with theoretical predictions, and the intensities of the peaks scale linearly with concentration. Based on the concentration dependence and agreement between the simulations and measurements, we conclude that cascaded lower order signals contribute negligibly to the observed signal.

The peak absorbance of the H2O OH stretching-band was used as a vibrational marker of water contamination. In all experiments for all samples the water concentration was determined to be no greater than 150 ppm.

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Ultrafast two-dimensional infrared spectroscopy (2D-IR) of thiocyanate ([SCN]−) in 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C4C1im][NTf2]) and 1-butyl-2,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide ([C4C1C1(2)im][NTf2]) ionic liquids probes local structure and dynamics as a function of the water content, solute counterion, and solute concentration. The 2D-IR spectra of the water-saturated ionic liquids resolve two distinct kinds of dynamics. This dynamical heterogeneity is explained as two subensembles, one with and one without a water molecule in the first solvation shell. When the countercation is K+, ion pairs between K+ and [SCN]− that persist for >100 ps are detected by long-lasting vibrational frequency correlations. The observed dynamics are invariant to [SCN]− concentration, which indicates that the [SCN]− does not cluster in ionic liquid solution. Taken together, these results are consistent with a picture of thiocyanate as a local probe that can interrogate ultrafast structure and dynamics at a small spatial scale in ionic liquids.