Two-dimensional Raman-terahertz (THz) spectroscopy is presented as a multidimensional spectroscopy directly in the far-IR regime. The method is used to explore the dynamics of the collective intermolecular modes of liquid water at ambient temperatures that emerge from the hydrogen-bond networks water forming. Twodimensional Raman-THz spectroscopy interrogates these modes twice and as such can elucidate couplings and inhomogeneities of the various degrees of freedoms. An echo in the 2D RamanTHz response is indeed identified, indicating that a heterogeneous distribution of hydrogen-bond networks exists, albeit only on a very short 100-fs timescale. This timescale appears to be too short to be compatible with more extended, persistent structures assumed within a two-state model of water.W ater forms local structures due to the directionality of hydrogen bonding, and it is generally accepted that the emerging hydrogen-bond network is ultimately responsible for the many anomalies in the macroscopic thermodynamic properties of water (1). The consequences of that statement are however still discussed in an extremely controversial manner (2-11). For example, an increasing scattering amplitude for small angles in X-ray scattering experiments of room-temperature water has been interpreted in terms of the so-called "two-state model" of water with patches of a low-density liquid (LDL) and a highdensity (HDL) liquid (5), whereas other researchers have interpreted essentially the same experimental result as continuous--in contrast with bimodal--density fluctuations, as they are expected from the isothermal compressibility (6). From a theoretical point of view, the hypothesized liquid-to-liquid phase transition in the deeply supercooled regime is discussed as either separating two metastable thermodynamic states of the liquid (2,3,8,10) or as an artifact from nonequilibrated molecular dynamics simulations (7,11). These two problems are related because two-state behavior at ambient temperatures is thought to be a reminiscence of the liquid-to-liquid phase transition in the deeply supercooled regime. It is currently not clear whether a liquid-to-liquid phase transition exists [neither for computer models (2,3,7,8,10,11) nor for real water], and even if it does exist, it is not clear whether two-state behavior survives into ambient conditions. To provide unique experimental input to these types of questions, we present a 2D spectroscopy directly in the low-frequency terahertz (THz) regime, where the intermolecular motions of water are detected. The method can elucidate the heterogeneity and lifetimes of the hydrogen-bond networks of liquid water.In contrast with any X-ray experiment performed so far (4-6), all of which take essentially an instantaneous snapshot (10), multidimensional IR spectroscopies (12-17), as well as related holeburning or photon echo experiments (18-21), interrogate the molecular system at least twice and thus can deduce time correlations of interconverting structures. These experiments have been performed o...

It is generally assumed that the hydrated electron occupies a quasi-spherical cavity surrounded by only a few water molecules in its equilibrated state. However, in the very moment of its generation, before water has had time to respond to the extra charge, it is expected to be significantly larger in size. According to a particle-in-a-box picture, the frequency of its absorption spectrum is a sensitive measure of the initial size of the electronic wavefunction. Here, using transient terahertz spectroscopy, we show that the excess electron initially absorbs in the far-infrared at a frequency for which accompanying ab initio molecular dynamics simulations estimate an initial delocalization length of 40 Å. The electron subsequently shrinks due to solvation and thereby leaves the terahertz observation window very quickly, within 200 fs.

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.

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We present high resolution measurements of the ultrafast temporal dynamics of the critical surface in moderately overdense, hot plasma by using two-color, pump-probe Doppler spectrometry. Our measurements clearly capture the initial inward motion of the plasma inside the critical surface of the pump laser which is followed by outward expansion. The measured instantaneous velocity and acceleration profiles are very well reproduced by a hybrid simulation that uses a 1D electromagnetic particle-in-cell simulation for the initial evolution and a hydrodynamics simulation for the later times. The combination of high temporal resolution and dynamic range in our measurements clearly provides quantitative unraveling of the dynamics in this important region, enabling this as a powerful technique to obtain ultrafast snapshots of plasma density and temperature profiles for providing benchmarks for simulations.