We present an overview of recent static and time-resolved vibrational spectroscopic studies of liquid water from ambient conditions to the supercooled state, as well as of crystalline and amorphous ice forms. The structure and dynamics of the complex hydrogen-bond network formed by water molecules in the bulk and interphases are discussed, as well as the dissipation mechanism of vibrational energy throughout this network. A broad range of water investigations are addressed, from conventional infrared and Raman spectroscopy to femtosecond pump-probe, photon-echo, optical Kerr effect, sum-frequency generation, and two-dimensional infrared spectroscopic studies. Additionally, we discuss novel approaches, such as two-dimensional sum-frequency generation, three-dimensional infrared, and two-dimensional Raman terahertz spectroscopy. By comparison of the complementary aspects probed by various linear and nonlinear spectroscopic techniques, a coherent picture of water dynamics and energetics emerges. Furthermore, we outline future perspectives of vibrational spectroscopy for water researches.

The structural and dynamical properties of water are known to be affected by ion solvation. However, a consistent molecular picture that describes how and to what extent ions perturb the water structure is still missing. Here we apply 2D Raman-terahertz spectroscopy to investigate the impact of monatomic cations on the relaxation dynamics of the hydrogen-bond network in aqueous salt solutions. The inherent ability of multidimensional spectroscopy to deconvolute heterogeneous relaxation dynamics is used to reveal the correlation between the inhomogeneity of the collective intermolecular hydrogen-bond modes and the viscosity of a salt solution. Specifically, we demonstrate that the relaxation time along the echo direction t= t correlates with the capability of a given cation to 'structure' water. Moreover, we provide evidence that the echo originates from the water-water modes, and not the water-cation modes, which implies that cations can structure the hydrogen-bond network to a certain extent.

The 2D Raman-terahertz (THz) response of liquid water is studied in dependence of temperature and isotope substitution (H 2 O, D 2 O, and H 18 2 O). In either case, a very short-lived (i.e., between 75 and 95 fs) echo is observed that reports on the inhomogeneity of the low-frequency intermolecular modes and hence, on the heterogeneity of the hydrogen bond networks of water. The echo lifetime slows down by about 20% when cooling the liquid from room temperature to the freezing point. Furthermore, the echo lifetime of D 2 O is 6.5 ± 1% slower than that of H 2 O, and both can be mapped on each other by introducing an effective temperature shift of ∆T = 4.5 ± 1 K. In contrast, the temperature-dependent echo lifetimes of H 18 2 O and H 2 O are the same within error. D 2 O and H 18 2 O have identical masses, yet H 18 2 O is much closer to H 2 O in terms of nuclear quantum effects. It is, therefore, concluded that the echo is a measure of the structural inhomogeneity of liquid water induced by nuclear quantum effects.water | multidimensional spectroscopy | THz spectroscopy | nuclear quantum effects

Hybrid 2D Raman-THz spectroscopy with the Raman-THz-THz (RTT) pulse sequence is used to explore the ultrafast intra-and intermolecular degrees of freedom of liquid bromoform (CHBr3) in the frequency range of 1-8 THz. Cross peaks observed in these 2D spectra are assigned to the coupling between the narrow intramolecular modes of the molecules and the much broader intermolecular degrees of freedom of the liquid. This assignment is based on the frequency position of the crosspeaks, however, it is shown that these frequency positions can be deduced accurately only when properly taking into account the convolution of the molecular response with the instrument response function of the experimental setup, the latter of which distorts the 2D spectra considerably. The assignment is backed up with additional experiments on diiodomethane (CH2I2), which has only one intramolecular mode in the frequency range of the experiment, and hence excludes the possibility of intramolecular couplings.

Recently, various spectroscopic techniques have been developed, which can measure the 2D response of the inter-molecular degrees of freedom of liquids in the THz regime. By employing hybrid Raman-THz pulse sequences, the inherent experimental problems of 2D-Raman spectroscopy are circumvented completely, culminating in the recent measurement of the 2D-Raman-THz responses of water and aqueous salt solutions. This review article focuses on the possibility to observe echoes in such experiments, which would directly reveal the inhomogeneity of the typically extremely blurred THz bands of liquids, and hence the heterogeneity of local structures that are transiently formed, in particular, in a hydrogen-bonding liquid such as water. The generation mechanisms of echoes in 2D-Raman-THz spectroscopy are explained, which differ from those in "conventional" 2D-IR spectroscopy in a subtle but important manner. Subsequently, the circumstances are discussed, under which echoes are expected, revealing a physical picture of the information content of an echo. That is, the echo decay reflects the lifetime of local structures in the liquid on a length scale that equals the delocalization length of the intermolecular modes. Finally, recent experimental results are reviewed from an echo perspective.