Water’s weird and anomalous properties near ambient conditions have been explored in the past; however, its behavior under extreme pressure and temperature still remains a mystery. A rough outline of the equation of state for water is known, but there is little to no information on the chemistry or dynamics of water in the extreme ranges. This project seeks to generate uniaxial shock-loading in liquids using our laser-launched flyer plate apparatus while probing liquid samples spectroscopically. Initially this work will be done on water and nitromethane with a focus on infrared (IR) absorption and various fluorescence techniques.

When a shock wave is applied to liquid water, a fast adiabatic compression results in an instantaneous pressure and temperature jump, and the dissociation of water into its ions increases. Currently, there are two methods for shocking water. The first is a single-stage shock, which sweeps through the water and maximizes the temperature (1500 K). During this process, water is heated and pressurized far beyond the supercritical state.  If the shock is large enough, 25-50 GPa, water dissociation becomes complete; and forms a superionic water state, which would behave as an extremely reactive fluid salt.  In this phase, properties such as the pH and hydrogen bonding are unknown. If the new state is created, the IR spectrum should drastically change due to the rearrangement of hydrogen bonding caused by increased temperature and density. When water dissociates into ions the pH will change significantly. If pH indicator dyes are dissolved in the water, then the pH and ionic nature of water can be probed.

In the second method, or ring up method, liquid water is placed between two windows with a high impedance, such as sapphire. When the shock transmits through the water it will repeatedly bounce off the sapphire windows and drive up the temperature more gradually than the single stage method, resulting in lower overall temperatures. This method has been used to produce ices that mimic the interiors of Uranus and Neptune.   The lower temperatures produced in the ring up method cause liquid water to freeze into the Ice VI or Ice VII region.  Water is one of the only liquids which freezes under shock conditions (most substances melt). This results in the ideal circumstances to study the kinetic solidification of water on a nanosecond timescale. Nucleation at the windows can be altered by attaching hydrophobic or hydrophilic self-assembled monolayers and the resulting kinetics of freezing water can be investigated for each of the monolayers.

 

 

A schematic representation of the experimental apparatus is shown in Fig. 1. Laser-driven flyer plates are launched with speeds of 0-5 km/s in 10-30 ns via laser ablation of a thin aluminum or copper foil.   [4]. The sample, HOD in H2O or D2O, will be inserted between sapphire or CaF2 optical windows. The IR spectrum of the OH/OD stretch (3100-3700 cm^(-1)) is sensitive to hydrogen bonding strengths, and the IR bending mode (1640 cm^(-1)) develops a prominent sideband with increasing H+ concentration.  [5]. The IR bending mode (1640 cm^(-1)) develops a prominent sideband with increasing H+ concentration [5]. Raman spectroscopy is used to determine the temperature from Stokes/anti-Stokes intensities, while the pressure can be obtained through shock velocity measurements. Oxidation chemistry, and the pH can be studied by dissolving probe molecules in the liquid, such as emissive dyes. Understanding the fundamental behavior of water under extreme conditions is vital for understanding and improving upon our current knowledge of geoscience and astrochemistry.

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