The Equation of State and Acoustic Dynamics of High Temperature, High Pressure Fluids, Solids, and Surfaces by Transient Optical Grating Measurements

Transient acoustic grating measurements are performed on fluids and solids under simultaneous high temperature and high pressure conditions as a probe of the equation of state of fluids and solids relevant to the planetary interiors of the giant planets and some of their satellites. We have established our experimental capability to obtain high quality transient grating data on a variety of fluids and solids at high pressure. Our current effort focuses on utilizing high pressure high temperature transient grating measurements on fluids and solids. Several systems are of interest: (a) fluid ammonia, methane, water, and their mixtures, due to the planetary significance of these chemical systems (fluid helium and hydrogen at high temperature and pressure present some unique experimental requirements); (b) high pressure solids of ammonia and methane which may be significant constituents of the interiors of Uranus and Neptune (Consoimagno, 1983); and (c) high pressure water-ice, which may be important for our understanding of the planetary interiors of Ganymede, Callisto, and Titan (Poirier, 1982).

We obtain acoustic velocities under variable high pressure conditions in a diamond anvil cell by generating and probing the time evolution of laser induced acoustic gratings. Experimental measurements of acoustic dynamics are used to determine the elastic constants and the equation of state of materials under extreme temperature and pressure, which will be useful for developing physical models of planetary interiors.

Transient grating spectroscopy is a versatile technique for examining both acoustic velocities and thermal diffusion. We utilize this spectroscopic technique with diamond anvil high pressure and high temperature techniques as a unique probe with which to investigate both the equation of state for solids and dynamic relaxation processes under extreme pressure and temperature. The acoustic velocity as a function of applied pressure is obtained by measuring the frequency of the oscillations in an acoustic grating formed in a diamond anvil cell and the attenuation time of the transient grating measurement can yield information about the decay of a thermal grating due to diffusion. We study the equation of state studies over a pressure and temperature range up to 20 GPa at 900 K.

The focused lasers and directional emission utilized in transient grating measurements is perfectly suited to the optical transmission of diamonds and the small spatial and angular aperture of the high pressure cell. The high temperature measurements will be facilitated using diamond anvil cell vacuum ovens. The temperature of the cell will be measured using thermocouple probes whereas the sample pressure will be measured using either a Sm:YAG fluorescence or 13C/12C diamond Raman pressure scale, since the Ruby fluorescence method is not useful at very high temperatures (e.g. 900 K).

This interdisciplinary research project involves merging the technologies of high pressure, high temperature, research with nonlinear time-resolved spectroscopy. Results from this project demonstrate that scattering of acoustic waves in high pressure fluids and solids can yield useful information on the equation of state and dynamic relaxation mechanisms.