Laser Ultrasonics in a Diamond Anvil Cell for Investigation of Simple Molecular Compounds at Ultrahigh Pressures – LUDACism

Laser ultrasonics (LU) is an experimental technique providing opportunity to generate and detect acoustic waves by lasers in samples under extreme conditions and in hostile environment without contacting the materials. It is possible to measure the acoustic velocities and the dimensions of material sample from a distance by monitoring the laser-generated acoustic echoes in the sample. We propose to significantly improve the functionality of the laser ultrasonics for diagnostics of materials in diamond anvil cell (DAC) at pressures to 70-100 GPa, where the thickness of the samples is typically between 5 and 50 micrometers, by applying both for the generation and the detection of the bulk and interface acoustic waves the picosecond (ps) laser. The application of the laser with the pulse duration of 5-10 ps will provide opportunity to create the acoustic spectrometer with the frequency band up to 10 GHz for the evaluation of the acoustic velocity dispersion and acoustic absorption of the materials. The amplitude of the acoustic waves in DAC in this frequency band will be increased up to two orders in magnitude in comparison with the existing experimental setups. The accuracy of the measurements of acoustic velocity, accomplished by this LU spectrometer at high pressures, will be tested through the comparison with available data on both shear and longitudinal acoustic velocities in argon up to 70 GPa and cross-correlated with specially conducted measurements of the elastic moduli by X-ray diffraction.
The measurements of the acoustic velocities and elastic moduli in transparent samples of Ar, N2 and H2O, also with stress-induced anisotropy, will be documented for the first time up to the pressures of 70-100 GPa. The laser-based wide-frequency-band shear ultrasonic spectroscopy will be applied for the first time to monitor the fundamental phenomenon of shear rigidity evolution across the liquid-solid and solid-solid phase transitions. It will be also used to search acoustic manifestations of the existence/non-existence of the pressure-induced transition from low density to high density water, from cubic to hexagonal structure in solid Ar, and of the water ice X. It is worth noting here that singularities in sound velocities or their pressure derivatives, especially for shear waves, could be much better (and independent) indicators of phase transitions at ultrahigh pressures than the discontinuous changes of the densities, especially at pressures exceeding 40 GPa when the relative volume change in phase transition strongly decreases.
The constructed acoustic spectrometer will become a powerful tool of the diagnostics of thin films of solids and thin layers of liquids. The knowledge of the pressure-dependences of the sound velocities and elastic moduli of liquids and solids is of extreme importance for a few branches of natural sciences such as condensed matter physics, physics of the Earth, seismology, and planetology, as well as for monitoring of nuclear weapons tests.