Terahertz echoes reveal the inhomogeneity of aqueous salt solutions

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 t1 = t2 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

Low-frequency anharmonic couplings in bromoform revealed from 2D Raman-THz spectroscopy: From the liquid to the crystalline phase

Two-dimensional (2D) Raman-THz spectroscopy in the frequency of up to 7 THz has been applied to study the crystalline β-phase of bromoform (CHBr3). As for liquid CHBr3, cross peaks are observed, which, however, sharpen up in the crystalline sample and split into assignable sub-contributions. In the Raman dimension, the frequency positions of these cross peaks coincide with the intramolecular bending modes of the CHBr3 molecules and in the THz dimension with the IR-active lattice modes of the crystal. This work expands the applicability of this new 2D spectroscopic technique to solid samples at cryogenic temperatures. Furthermore, it provides new experimental evidence that the cross peaks, indeed, originate from the coupling between intra- and intermolecular vibrational modes

Four Wave Mixing Spectroscopy at the Interface Between the Time and Frequency Domains

Combined spectrally and time resolved measurements provide information not otherwise available if performed in either domain alone. We demonstrate a new approach to Four Wave Mixing spectroscopy, where spectral selectivity is achieved by phase matching filtering without a spectrometer, and the time resolved signal is obtained within a single pulse and without mechanically scanning any delays. We analyze the Degenerate Four Wave Mixing signal, and show that a counter-rotating Feynman diagram not previously considered is necessary in order to understand the measured frequency and time resolved spectrograms.Comment: 24 page article with 4 figure

2D-Raman-THz Spectroscopy with Single-Shot THz Detection

We present a 2D-Raman-THz setup with multichannel (single-shot) THz detection, utilizing two crossed echelons, in order to reduce the acquisition time of typical 2D-Raman-THz experiments from days to a few hours. This speed-up is obtained in combination with a high repetition rate (100 kHz) Yb-based femtosecond laser system and a correspondingly fast array detector. The wavelength of the Yb-laser (1030 nm) is advantageous, since it assures almost perfect phase matching in GaP for THz generation and detection, and since dispersion in the transmissive echelons is minimal. 2D-Raman-THz test measurements on liquid bromoform (CHBr3) are reported. An enhancement of ∼5.8 times in signal-to-noise ratio is obtained for single-shot detection when comparing to conventional step scanning measurements in the THz time-domain, corresponding to speed up of acquisition time of 34

Signatures of Intra- and Intermolecular Vibrational Coupling in Halogenated Liquids Revealed by Two-Dimensional Raman-Terahertz Spectroscopy

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

Dielectric response of light, heavy and heavy-oxygen water: isotope effects on the hydrogen-bonding network's collective relaxation dynamics

Isotopic substitutions largely affect the dielectric relaxation dynamics of hydrogen-bonded liquid water; yet, the role of the altered molecular masses and nuclear quantum effects has not been fully established. To disentangle these two effects we study the dielectric relaxation of light (H216O), heavy (D216O) and heavy-oxygen (H218O) water at temperatures ranging from 278 to 338 K. Upon 16O/18O exchange, we find that the relaxation time of the collective orientational relaxation mode of water increases by 4–5%, in quantitative agreement with the enhancement of viscosity. Despite the rotational character of dielectric relaxation, the increase is consistent with a translational mass factor. For H/D substitution, the slow-down of the relaxation time is more pronounced and also shows a strong temperature dependence. In addition to the classical mass factor, the enhancement of the relaxation time for D216O can be described by an apparent temperature shift of 7.2 K relative to H216O, which is higher than the 6.5 K shift reported for viscosity. As this shift accounts for altered zero-point energies, the comparison suggests that the underlying thermally populated states relevant to the activation of viscous flow and dielectric relaxation differ

2D Raman–THz Spectroscopy of Binary CHBr3–MeOH Solvent Mixture

Hybrid 2D Raman–terahertz (THz) spectroscopy is used to measure the interactions between two solvents paired in the binary CHBr3–MeOH mixture in the frequency range of 1–7 THz. Changes in the cross peak signature are monitored, originating from the coupling of an intramolecular bending mode of CHBr3 to the collective intermolecular degrees of freedom of the mixture. The appearance of a new cross peak in the 2D spectrum measured for solvent mixture with MeOH molar fraction of 0.3 indicates a coupling to a new set of low-frequency modes formed due to the hydrogen bond interactions between the two solvents. This interpretation is supported by the measurement of the CHBr3–CS2 binary solvent mixture as well as by 1D absorption measurements of neat MeOH