COHERENT ANTI-STOKES-RAMAN SPECTROSCOPY OF SHOCK-COMPRESSED LIQUID NITROGEN/CARBON MONOXIDE MIXTURES

Vibrational spectra of liquid nitrogen/carbon monoxide mixtures, shock compressed to several high-pressure/high-temperature states, were obtained using single-pulse multiplex coherent anti-Stokes Raman scattering (CARS). 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. Up to a maximum shock pressure of 9.3 GPa, both the N2 and CO vibrational frequencies were found to increase monotonically with pressure but depended strongly on the nitrogen/carbon monoxide mixture ratio. An empirical fit of the Raman frequency shifts incorporating previously published neat nitrogen and carbon monoxide data, using a functional form dependent on pressure, temperature, and mixture ratio, accurately describes both the N2 and CO shifts. 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 and the vibrational temperatures obtained were used to improve the potential function used to calculate equation-of-state pressures and temperatures. The measured linewidths were different for N2 and CO and suggest that the vibrational dephasing time decreased to a few ps at the highest pressure shock state.