Vibrational spectroscopy of materials under extreme pressure and temperature

Abstract

We have obtained vibrational spectra of molecular materials at simultaneous high pressure and high temperature using the combined techniques of shock wave compression and coherent Raman spectroscopy. The high pressure/high temperature states produced and investigated are similar to those found in the interior of planets and in detonating high explosives. The molecular structures of simple molecular fluids and fluid mixtures, such as of diatomic and triatomic molecules, are inferred from the spectral parameters — Raman frequencies, linewidths, and peak Raman susceptibilities — of the transitions, which are extracted from the spectral data using a standard semi-classical treatment. For a simple diatomic molecule such as N 2, CO or O 2, the measured vibrational frequencies increase with increasing shock pressure. In N 2, above 17 GPa single-shock pressure the vibrational frequency ceases to rise further and appears to begin to decrease. Observation of a further decrease at higher pressures is prevented by the onset of optical opacity. CO and O 2 become optically opaque at pressures insufficient to observe a similar maximum. The vibrational frequencies of CO and N 2 exhibit distinctly different shock pressure dependencies. The data show a factor of 5 smaller vibrational frequency versus pressure slope in CO as in N 2. Under shock compression, mixtures of CO and N 2 show a highly non linear dependence of their vibrational frequencies with mixture mole fraction, while at ambient pressure this dependence is linear. We have also estimated vibrational relaxation times for these simple molecules under shock-compression, using a simple population filling argument for the longitudinal relaxation time (T 1), and from the measured linewidth for the transverse relaxation time (T 2). T 2 shows a strong temperature dependence but little density (pressure) dependence. The intensities of the vibrational hot bands were those expected from a Boltzmann vibrational population distribution, and they were used to estimate vibrational temperatures. These temperatures have been also used to refine the equations of state for nitrogen and argon as well as to understand the effects of non-ideal mixing at these extreme conditions.