Coherent anti-Stokes Raman spectroscopy of shock-compressed liquid carbon monoxide–oxygen and nitrogen–oxygen mixtures

Abstract

A two-stage light gas gun and single-pulse multiplex coherent anti-Stokes Raman scattering (CARS) have been used to obtain carbon monoxide, nitrogen, and oxygen vibrational spectra for several high-pressure/high-temperature, dense fluid, carbon monoxide–oxygen, and nitrogen–oxygen mixtures. The experimental spectra were compared to synthetic spectra calculated with a semiclassical model for CARS intensities and using best fit vibrational frequencies, peak Raman susceptibilities, and Raman linewidths for each mixture component. Up to a maximum shock pressure of 6.75 GPa for carbon monoxide–oxygen mixtures, the CO and O2 vibrational frequencies were found to increase monotonically with pressure and depended on the carbon monoxide–oxygen mixture ratio. For the nitrogen–oxygen mixtures, the N2 vibrational frequency increased monotonically with pressure to a maximum experimental pressure of 12.9 GPa, however the O2 vibrational frequency increased with pressure to about 11 GPa and then appeared to decrease slightly as the pressure increased to the experiment maximum of 12.9 GPa. Empirical fits of the measured Raman frequencies incorporating previously published neat nitrogen, carbon monoxide, and oxygen data and using a functional form dependent on pressure, temperature, and mixture ratio, accurately describe the N2 , CO, and O2 vibrational frequency shifts for both the carbon monoxide–oxygen and the nitrogen–oxygen mixtures. The transition intensity and linewidth data suggest that thermal equilibrium of the vibrational levels is attained in less than 10 ns at these shock pressures. The vibrational temperatures obtained for the nitrogen–oxygen mixtures were used to improve the oxygen potential function used to calculate equation-of-state pressures and temperatures. The measured linewidths for CO, N2 , and O2 were different for the different mixtures and did not appear to depended significantly on mixture ratios. The broadening of all spectral lines suggested that the vibrational dephasing time for each species decreased to a few ps at the highest pressure shock states.