X-ray absorption spectroscopy and x-ray Raman scattering were used to probe the molecular arrangement in the first coordination shell of liquid water. The local structure is characterized by comparison with bulk and surface of ordinary hexagonal ice Ih and with calculated spectra. Most molecules in liquid water are in two hydrogen– bonded configurations with one strong donor and one strong acceptor hydrogen bond in contrast to the four hydrogen– bonded tetrahedral structure in ice. Upon heating from 25°C to 90°C, 5 to 10% of the molecules change from tetrahedral environments to two hydrogen– bonded configurations. Our findings are consistent with neutron and x-ray diffraction data, and combining the results sets a strong limit for possible local structure distributions in liquid water. Serious discrepancies with structures based on current molecular dynamics simulations are observed.

/pdf/the-structure-of-the-first-coordination-shell-in-liquid-4zzqdpmjkc.pdf

The Structure of the First Coordination Shell in Liquid Water

Despite long study, a molecular picture of the mechanism of water reorientation is still lacking Using numerical simulations, we find support for a pathway in which the rotating water molecule breaks a hydrogen bond (H-bond) with an overcoordinated first-shell neighbor to form an H-bond with an undercoordinated second-shell neighbor The H-bond cleavage and the molecular reorientation occur concertedly and not successively as usually considered This water reorientation mechanism involves large-amplitude angular jumps, rather than the commonly accepted sequence of small diffusive steps, and therefore calls for reinterpretation of many experimental data wherein water rotational relaxation is assumed to be diffusive

https://science.sciencemag.org/content/sci/311/5762/832.full.pdf

A molecular jump mechanism of water reorientation.

Extensive terahertz laser vibration-rotation-tunneling spectra and mid-IR laser spectra have been compiled for several isotopomers of small (dimer through hexamer) water clusters These data, in conjunction with new theoretical advances, quantify the structures, force fields, dipole moments, and hydrogen bond rearrangement dynamics in these clusters This new information permits us to systematically untangle the intricacies associated with cooperative hydrogen bonding and promises to lead to a more complete molecular description of the liquid and solid phases of water, including an accurate universal force field

/pdf/water-clusters-untangling-the-mysteries-of-the-liquid-one-1b5uj00qz0.pdf

Water clusters: Untangling the mysteries of the liquid, one molecule at a time

It's challenging to measure non-repetitive events in real time in the field of instrumentation and measurement. Dispersive Fourier transformation is an emerging method that permits capture of rare events, such as optical rogue waves and rare cancer cells in blood. This Review article covers the principle of dispersive Fourier transformation and its implementation in diverse applications.

Dispersive Fourier transformation for fast continuous single-shot measurements

Theoretical descriptions of two-dimensional (2D) vibrational and electronic spectroscopy are presented By using a coupled multi-chromophore model, some examples of 2D spectroscopic studies of peptide solution structure determination and excitation transfer process in electronically coupled multi-chromophore system are discussed A few remarks on perspectives of this research area are given

Coherent Two-Dimensional Optical Spectroscopy

A femtosecond mid-infrared pump-probe study of the vibrational and orientational dynamics of the OH-stretching mode of HDO dissolved in D2O is presented. The orientational relaxation of the HDO molecules was observed to occur on either a very slow or a very fast time scale, with associated time constants of τ R = 13 picoseconds and τ R = 0.7 picosecond. It was observed that strongly hydrogen-bonded water molecules only relax through the slow orientational relaxation process, whereas the fast process dominates for weakly hydrogen-bonded molecules. This suggests that, with respect to orientional dynamics, two distinct molecular species exist in liquid water.

https://science.sciencemag.org/content/sci/278/5338/658.full.pdf

Femtosecond Mid-IR Pump-Probe Spectroscopy of Liquid Water: Evidence for a Two-Component Structure

We have used femtosecond two-color midinfrared spectroscopy to determine the temperature dependence of the OH-stretching lifetime in dilute $\mathrm{HDO}:{\mathrm{D}}_{2}\mathrm{O}$ solution, both in the liquid and solid (ice Ih) state. Like many other properties of water, the vibrational lifetime shows a remarkable temperature dependence: In liquid water the vibrational relaxation of the OH-stretching mode is twice as slow as in ice, and becomes even slower with increasing temperature.

/pdf/anomalous-temperature-dependence-of-vibrational-lifetimes-in-4tn24vyc06.pdf

Anomalous Temperature Dependence of Vibrational Lifetimes in Water and Ice

We present a femtosecond midinfrared pump–probe study of hydrogen bonding. It is shown that upon excitation of the OH-stretching vibration of hydrogen-bonded ethanol dissolved in CCl4, the hydrogen bonds are predissociated on a femtosecond time scale. The measured predissociation time constant depends strongly on the excitation frequency, and ranged from ∼250 fs at 3330 cm−1 to ∼900 fs at 3450 cm−1. The time constant of the subsequent reassociation of the hydrogen bonds was found to be 15 ps, in accordance with previous picosecond studies. Furthermore, polarization-resolved measurements show that orientational relaxation takes place on a time scale much shorter than the pulse length of ∼200 fs. This rapid orientational relaxation can be explained from the fast delocalization of the O–H stretching excitation over the hydrogen-bonded ethanol oligomers. The orientational anisotropy R reaches a value of 0.15 instantaneously, and remains constant for all delays.

A femtosecond midinfrared pump–probe study of hydrogen-bonding in ethanol

A Femtosecond Mid-infrared Pumpprobe Study Of Hydrogen-bonding In Ethanol

A simple extension of a commercial laser system permits the generation of intense femtosecond infrared pulses near 3 µm by optical parametric generation and amplification at kilohertz repetition rates. Pulse durations of a few hundred femtoseconds and pulse energies of tens of microjoules can be routinely produced, thus permitting nonlinear optical studies of molecular vibrations.

Uli Emmerichs

Papers

Femtosecond Mid-IR Pump-Probe Spectroscopy of Liquid Water: Evidence for a Two-Component Structure

Anomalous Temperature Dependence of Vibrational Lifetimes in Water and Ice

A femtosecond midinfrared pump–probe study of hydrogen-bonding in ethanol

A Femtosecond Mid-infrared Pumpprobe Study Of Hydrogen-bonding In Ethanol

Generation of intense femtosecond optical pulses near 3 µm with a kilohertz repetition rate