Session Q37: Focus Session: Structure and Dynamics of Interfacial Water II

Surveying the potential landscapes controlling the accommodation of excess electrons by water networks

We present recent results of a new experimental approach where we use ir-ir pump-probe methods to measure the transition states and relative energies of isomers associated with the negatively charged water clusters. First, the vibrational spectra of various isomers are systematically disentangled using hole-burning Ar predissociation spectroscopy in a triple-stage time-of-flight mass spectrometer. We then monitor the spectra of fragment ions that are created by photoevaporation of Ar atoms through the various vibrational levels identified in the spectroscopic step. The major conclusion is the weak binding isomers are readily transformed into more strongly bound forms, while the reverse process is very inefficient. We describe progress on using this strategy to identify the large amplitude motions available in the finite systems using trace isotopes.   

X-ray Studies of Hydrogen Bonding of Water; the Liquid Phase and on Surfaces

Many of the unique properties of water have been interpreted as due either to a continuum of distorted mainly near-tetrahedral hydrogen bond or a mixture of differently coordinated distinct species. Here we report high-resolution measurements of small-angle x-ray scattering and x-ray emission spectroscopy from liquid water at ambient conditions. A model based on 10-20 {\AA} diameter size density fluctuations with energetically strongly tetrahedral bonded water in a soup of disordered entropic structures with large hydrogen bond distortion can explain the spectroscopy and scattering data. I will also address fundamental questions regarding geometric structure, electronic structure, nature of surface chemical and hydrogen bonding and reactivity of water on surfaces. The connection between studies performed at both UHV and ambient conditions will be emphasized. Several examples of different water adsorption system will be illustrated such as Pt(111), Ru(001), Cu(110), Cu(111), TiO$_{2}$, Fe$_{2}$O$_{3}$ and MgO.   

Femtosecond Sum-Frequency Generation Studies of the Structure and Dynamics of Interfacial Water

Water interfaces play a central role in a wide variety of disciplines including electrochemistry, (photo-) catalysis and biophysics. Knowledge of the details of water interfacial structure is thus essential both for a fundamental understanding of this ubiquitous liquid and for a basic understanding of the many systems in which aqueous interfaces play a key role. Although considerable progress has been made in understanding of bulk water, substantially less progress has been made at the interface. Here we report a series of surface specific studies of various water interfaces using surface-specific vibrational spectroscopies, both in equilibrium and on ultrafast (femtosecond) time scales. Our approach allows us to selectively investigate the one monolayer of water molecules at the different water interfaces. Water is characterized through its O-H stretch vibration. We find that interfacial hydrogen bonding depends strongly in the type of interface. Remarkably, for the water-air interface, interfacial hydrogen bonding is very similar to that occurring in bulk.   

Ultrafast response of water near hydrophobic and hydrophilic surfaces

Ultrafast response of water near hydrophobic and hydrophilic surfaces is investigated by femtosecond pump-probe ellipsometry. Pump and probe pulses are from a dual Ti:sapphire laser with stable difference repetition rate. Every difference repetition rate, time delay is swept the whole pulse-to-pulse interval without an optical delay stage. Pump pulses induce heating and acoustic vibration to a Pd surface. The Pd surface is modified by thiol chemistry. Thiols with --OH and -CH3 end groups generate uniform hydrophilic and hydrophobic surfaces, and these mixtures modulate hydrophobicity two-dimensionally. Probe pulses with circular polarization impinge at the Brewster's angle and are analyzed by a polarizer. The transient ellipticity shows a refractive index change of water by thermal conductance and novel insight into the peculiar qualities of interfacial water.   

Dynamics of Water Confined in Partially Hydrophobic Nanosized Cylindrical Sieves

Using three high resolution quasielastic neutron scattering spectrometers we have investigated the single particle dynamics of water confined in a hydrophobically modified MCM-41-S sample. This latter is a silica matrix containing cylindrical sieves with diameter $<$ 20 {\AA} arranged in a hexagonal geometry. In the hydrophobically modified sample some of the sylanol groups in the pores' wall have been substituted with methanol groups resulting in a partially hydrophobic confining surface, which could be used as a model system. We have been able to analyze the data in the temperature range from 300 K to 210 K using a single consistent model, the Relaxing Cage Model (RCM) for the dynamics of supercooled water. Because of the heterogenous environment experienced by the water molecules in the pores, the relaxational dynamics show a broad distribution of relaxation times. However, the Fickian diffusive behaviour is retained. The obtained results help clarify the role that the chemical interaction between the water molecules and the walls of the confining host plays in determining the characteristics of the water dynamics, as compared to purely geometric constraints such as the size and shape of the pores.   

Reconstructing the dynamic hydration around hydrophobic molecular solutes using inelastic x-ray scattering

We combine inelastic x-ray scattering (IXS) data and liquid-state theory to image the dynamical hydration structure of water solvating molecular, hydrophobic solutes. Using `linear response imaging', we computationally reconstruct the {\AA}-scale spatial and fs-scale temporal evolution of density fluctuations in water using IXS. The imaginary part of density propagator $\chi $(q,$\omega )$ is directly extracted from the IXS data, and the real part recovered using Kramers-Kronig relations. The resultant complex-valued $\chi $(q,$\omega )$ is the Fourier transform of the density-density response function $\chi $(r,t) which measures the dynamical density fluctuations of water due to a point-like instantaneous pulse. We have shown that this propagator can be used to reconstruct the dynamical hydration around prototypical charge distributions. To extend this technique to more realistic solutes, Ornstein-Zernike integral equations from liquid state theory are used to study hydration around small excluded volumes. We will present results for simple geometries and discuss the implications of combining exclusion and charge.