Coherent anti-Stokes Raman spectroscopy of shock-compressed liquid nitrogen

Abstract

Vibrational spectra of liquid nitrogen shock compressed to several high pressure/high temperature states were recorded using single-pulse multiplex coherent anti-Stokes Raman scattering. Vibrational frequencies, third-order susceptibility ratios, and linewidths are presented for the fundamental and several excited-state transitions. Vibrational frequencies were found to increase monotonically up to ≈17.5 GPa single shock and ≈30 GPa double shock. Above these pressures, the vibrational frequencies were observed to decrease with further increases in pressure. The decrease in vibrational frequency occurs in a pressure regime where the shocked nitrogen is becoming optically opaque. The consequence of the decrease in vibrational frequency on the Grüneisen mode gamma and its effect on the N2 equation of state is discussed. The transition intensity and linewidth data suggest that thermal equilibrium of the vibrational levels is attained in less than 10 ns at these high pressures and temperatures. Finally, the measured linewidths exhibit an almost linear dependence on shock temperature, and also suggest that the vibrational dephasing time has decreased to less than 1 ps at the highest pressures.

Molecular response of liquid nitrogen multiply shocked to 40 GPa

“…Each fitted peak in Figure 2 represents a particular vibrational transition (e.g., 0→1, 1→2, etc.). According to previous CARS studies, [19][20][21] the Raman peaks representing higher energy states were separated by ∼30 cm −1 at all pressures and temperatures studied. Because our detection system was unable to resolve each individual peak, the peak pressure spectrum is a superposition of all peaks, and the same was fitted assuming a ∼30 cm −1 separation in those peaks.…”

“…Temperatures for shocked diatomic fluids can be determined from a knowledge of the Stokes peak intensities belonging to two neighboring vibrational transitions (e.g., 0→1 and 1→2), 19,32,33 …”

“…16,18 Using P-V Hugoniot and electrical conductivity measurements, the pressure and temperature conditions for molecular dissociation (30 GPa at 6000 K) were estimated. In the same P-T region, single and double shock compression experiments by Moore et al [19][20][21] used coherent anti-Stokes Raman spectroscopy (CARS) measurements to obtain details regarding the molecular response of liquid nitrogen and to measure temperatures. The CARS results provided additional support for the assertion that temperature governs the response of the molecular bonding in the P-T region examined in single and double shock compression studies.…”

Recent progress in dynamical spectroscopy of Raman light scattering

“…К тому же азот является важным компонентом химических технологий при синтезе многих энергетических материалов. Все это в целом подчеркивает актуаль ность рассматриваемой задачи и в конечном итоге может благоприятствовать выбору правильной физической модели, адекватной полученным результатам [93,94].…”

“…В первой работе на эту тему были достигнуты давления до 34 ГПа и температуры до 4400 К [93]. В последующей работе [94] названный диапазон удалось заметно расширить.…”