Coherent anti-Stokes Raman spectroscopy of shock-compressed liquid nitrogen/argon mixtures

Abstract

Single-pulse multiplex coherent anti-Stokes Raman scattering (CARS) was used to obtain vibrational spectra of 20%/80% liquid nitrogen/argon mixtures, shock compressed to several high-pressure/high-temperature states. A semiclassical model for CARS spectra was used to extract best fit vibrational frequencies, peak Raman susceptibilities, and Raman linewidths from the data. Up to a maximum single shock pressure of 17.1 GPa, the N2 vibrational frequency was found to increase monotonically with pressure. The vibrational frequencies measured in both the singly and doubly shocked N2/Ar mixtures correspond within experimental error to those for pure nitrogen at equivalent pressures and temperatures, implying that the influence of the interaction potential on the N2 vibrational frequency for the N2/Ar collision is not significantly different from that of a N2/N2 collision at these conditions. 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. Vibrational temperatures obtained were used to improve the potential function of argon used to calculate equation-of-state pressures and temperatures. The measured linewidths suggest that the nitrogen vibrational dephasing time decreased to about 1 ps at the highest pressure shock state.